Search

Although initially it was presumed that foreshock waves would propagate directly into the dayside magnetosphere, observational evidence for sinusoidal Pc3 waves in the downstream of quasi‐parallel shocks is scarce. The transmission of these waves from the foreshock into the magnetosphere remains uncertain. In this paper, we employ a 3D global hybrid simulation at a realistic scale to explore the generation and transmission of the dayside ULF waves under a radial interplanetary magnetic field. Our findings demonstrate that the Pc3 waves are self‐consistently generated in the foreshock region and then transmitted into the magnetosheath and magnetosphere. In the foreshock, the waves are excited at approximately 25 mHz and exhibit right‐handed helicity in the plasma frame, characterizing them as quasi‐parallel fast magnetosonic waves. In the magnetosphere, the fluctuating magnetic field is mainly parallel to the background magnetic field, which indicates the dominant wave modes are compressional. Fluctuations in the magnetosheath show a broader spectrum (10–100 mHz) compared to those in the magnetosphere and foreshock, potentially explaining the little observation of sinusoidal Pc3 waves in the magnetosheath. Additionally, only lower frequency compressional waves (below 30 mHz) are effectively transmitted into the dayside magnetosphere. Our simulation provides critical insights into the interactions between the solar wind and Earth's magnetosphere. Plain Language Summary: Ultralow frequency waves in the Pc3 range, with periods of 10–45 s, are frequently detected in the Earth's magnetosphere. These waves are thought to originate from ion foreshock regions upstream of Earth's quasi‐parallel bow shock. They arise from ion beam instabilities triggered by the interaction between shock‐reflected suprathermal ions and the incoming solar wind. While it was assumed that these Pc3 waves would directly enter the dayside magnetosphere, in‐situ observations of sinusoidal Pc3 waves in the magnetosheath remain rare. This scarcity of evidence has left the mechanisms of their propagation into the magnetosphere unclear. This paper employs a 3D global hybrid simulation at a realistic scale to investigate how the Pc3 waves are transmited through the bow shock, magnetosheath, and into the magnetosphere under a radial interplanetary magnetic field. Our results indicate that the Pc3 waves are self‐consistently generated in the foreshock and transmitted into the magnetosheath and magnetosphere, with only lower frequency compressional waves effectively propagating into the magnetosphere. Fluctuations in the magnetosheath exhibit a broader spectrum compared to those in the magnetosphere and foreshock. Key Points: A 3D global hybrid simulation at a realistic scale has been used to investigate the dayside ULF waves under a radial interplanetary magnetic fieldPc 3 waves are self‐consistently generated at the foreshock region and transmitted into the magnetosheath and magnetosphereFluctuations in the magnetosheath exhibit a broad spectrum, and only lower frequency compressional waves are effectively transmitted into the magnetosphere [ABSTRACT FROM AUTHOR]

The most detrimental geomagnetically induced currents (GICs) documented to date have all taken place during geomagnetic storms. Yet, the probability of GICs throughout geomagnetic storms driven by different solar wind transients, such as high‐speed streams/stream interaction regions (HSS/SIR) or interplanetary coronal mass ejection (ICME) sheaths and magnetic clouds (MC), is poorly understood. We present an algorithm to detect geomagnetic storms and storm phases, resulting in a catalog of 755 geomagnetic storms from January 1996 to June 2023 with the solar wind drivers. Using these storms and the IMAGE magnetometer network, we study the temporal and spatial evolution of spikes in the time derivative of the horizontal component of the external magnetic field, |dHext/dt| $\vert \mathrm{d}{\boldsymbol{H}}_{\mathrm{e}\mathrm{x}\mathrm{t}}/\mathrm{d}t\vert $, greater than 0.5 nT/s during geomagnetic storms driven by HSS/SIR, sheaths and MCs. Spikes occur more often toward the end of the storm main phase for HSS/SIR and MC‐driven storms, while sheaths have spikes throughout the entire main phase. During the main phase most spikes occur in the morning sector around 05 magnetic local time (MLT) and the extent in MLT is narrowest for MCs and widest for sheaths. However, spikes in the pre‐midnight sector during the main and recovery phases are most prominent for HSS/SIR‐driven storms. During the storm sudden commencement (SSC), three MLT hotspots exist, the post‐midnight at 04 MLT, pre‐noon at 09 MLT and afternoon at 15 MLT. The pre‐noon hotspot has the highest probability of spikes and the widest extent in magnetic latitude. Plain Language Summary: Geomagnetic storms can have damaging implications on technological infrastructures such as power grids, pipelines or railway systems. Those implications arise due to geomagnetically induced currents (GICs) caused by rapid changes in the Earth's magnetic field. In this paper, we present a way to automatically detect geomagnetic storms as well as the key periods of the storm's activity. The primary solar wind structures that cause geomagnetic storms are high‐speed streams and their stream interaction regions (HSS/SIR), sheaths ahead of interplanetary coronal mass ejections and magnetic clouds (MC). Using the detected geomagnetic storms we study spikes in the magnetic field changes in Fennoscandia to map the GIC activity throughout the geomagnetic storms caused by HSS/SIR, sheaths and MCs. We report on several similarities and differences in the evolution and extent of spikes during geomagnetic storms for the different solar wind structures. One such difference is for example, in the likelihood of spikes throughout the storm. For those storms caused by HSS/SIR and MCs spikes are more likely close to the time of maximum geomagnetic storm disturbance, while for storms caused by sheaths spikes are likely at any time from the beginning of the storm to the maximum storm disturbance. Key Points: This paper presents a catalog of geomagnetic storms, their phases and interplanetary drivers from 1996 to 2023During the storm sudden commencement three hotspots of spikes in |dHext/dt| exist, at 04, 09 and 15 magnetic local timeSheath storms have |dHext/dt| spikes occurring during the entire main phase, while for MC and HSS/SIR peak toward the end of the main phase [ABSTRACT FROM AUTHOR]

The plasmasphere accounts for the majority of the mass of Earth's magnetosphere and contains most of the cold ion (1 eV) population. The plasmasphere is extremely dynamic, undergoing a constant cycle of erosion and refilling. In this paper we perform a statistical study of erosion and refilling rates using 6 years of data from the Van Allen Probes from the beginning of 2013 through the end of 2018. Using in‐situ density measurements derived from the upper hybrid resonance line, we create global maps of the erosion and refilling rates over a wide range of L shells and local times. Sorting the data by L shell, magnetic local time, and distance to the plasmapause, we characterize the absolute and relative rates of erosion and refilling during a variety of geomagnetic conditions. We also examine three case studies of geomagnetic storms and compare their density evolutions during the recovery period. Our results are in agreement with refilling rates found by previous statistical studies using different methods, but somewhat lower than many of the case studies reported. We find median erosion rates of 164, 83, and 43 cm−3/day and refilling rates of 87, 42, and 27 cm−3/day at L = 3, 4 and 5, respectively when Kp ≤ ${\le} $ 3. We also find little local time dependence for both erosion and refilling rates. Plain Language Summary: The plasmasphere is the innermost region of Earth's magnetosphere, and accounts for the majority the magnetosphere's mass. During a geomagnetic storm, the plasmasphere loses mass in process called erosion where plasma is transported toward the sun until it is lost from the magnetosphere. Following the storm, material from the ionosphere slowly refills the plasmasphere in a matter of weeks. In this paper we study average erosion and refilling rates using data from the Van Allen Probes during the years 2013–2018. We create global maps of the erosion and refilling rates over a wide range of longitudes and distances to Earth. We characterize both the absolute and relative rates of erosion and refilling during storms of varying intensities. We also examine three case studies of geomagnetic storms and compare their density evolutions throughout these events. Our results are in agreement with refilling rates found by previous statistical studies, but somewhat lower than many of the case studies reported. At locations of 3, 4, and 5 Earth radii away, we find median refilling rates of 87, 42, and 27 cm−3/day for storms of the lowest intensity. We also find that the erosion and refilling rates are roughly the same for the dawn, day, dusk, and night sides of the planet. Key Points: We perform a statistical study of erosion and refilling rates in the plasmasphere over L shell, local time, and geomagnetic conditionsWe present three case studies which all show similar density evolutions during the recovery periodThe percent rate of change erosion rates are well sorted by distance to the plasmapause, while refilling rates are better sorted by L shell [ABSTRACT FROM AUTHOR]

This paper conducts a multi‐instrument and data assimilation analysis of the three‐dimensional ionospheric electron density responses to the total solar eclipse on 08 April 2024. The altitude‐resolved electron density variations over the continental US and adjacent regions are analyzed using the Millstone Hill incoherent scatter radar data, ionosonde observations, Swarm in situ measurements, and a novel TEC‐based ionospheric data assimilation system (TIDAS) with SAMI3 model as the background. The principal findings are summarized as follows: (a) The ionospheric hmF2 exhibited a slight enhancement in the initial phase of the eclipse, followed by a distinct reduction of 20–30 km in the recovery phase of the eclipse. The hmF2 in the umbra region showed a post‐eclipse fluctuation, characterized by wavelike perturbations of 10–25 km in magnitude and a period of ∼ ${\sim} $30 min. (b) There was a substantial reduction in ionospheric electron density of 20%–50% during the eclipse, with the maximum depletion observed in the F‐region around 200–250 km. The ionospheric electron density variation exhibited a significant altitude‐dependent feature, wherein the response time gradually delayed with increasing altitude. (c) The bottomside ionospheric electron density displayed an immediate reduction after local eclipse began, reaching maximum depletion 5–10 min after the maximum obscuration. In contrast, the topside ionospheric electron density showed a significantly delayed response, with maximum depletion occurring 1–2.5 hr after the peak obscuration. Plain Language Summary: On 8 April 2024, a total solar eclipse traversed across North America with a dense network of observational equipment in place, providing a great opportunity for analyzing ionospheric effects during the eclipse. This paper presents a multi‐instrument and data assimilation analysis of the three‐dimensional ionospheric electron density response to this solar eclipse, utilizing Millstone Hill incoherent scatter radar data, ionosonde observations, Swarm satellite in situ measurements, and a new TEC‐based ionospheric data assimilation system (TIDAS) over continental US and adjacent regions with the SAMI3 as the background model. The observations and SAMI3‐TIDAS data assimilation reveals the time‐evolving 3‐D spatial distribution of the ionospheric electron density during the eclipse, highlighting key features of altitude‐dependent ionospheric variation with significant discrepancies and time delays between the bottomside and topside ionosphere. Key Points: The altitude‐resolved Ne response to the solar eclipse in the 3‐D domain was effectively reconstructed by TIDAS‐SAMI3 data assimilationThe eclipse led to a substantial ionospheric Ne reduction of 20%–50%, with the maximum depletion occurring in the F region of 200–250 kmThe Ne showed a time‐delayed variation with increasing altitude, from 5 to 10 min in the bottomside to 1–2.5 hr in the topside ionosphere [ABSTRACT FROM AUTHOR]

We present a unique triangulation measurement of Strong Thermal Emission Velocity Enhancement (STEVE) observed on Sept 3rd, 2022, at Athabasca, Canada. Using two Digital single‐lens reflex (DSLR) color cameras with all‐sky fish‐eye lenses, we show the profile of STEVE altitude variation over time in 1 min resolution for the first time. We estimate the altitude variation of its visible purplish arc and green picket fence structures. We also compare the DLSR camera images with narrowband all‐sky images of an Optical Mesosphere Thermosphere Imager (OMTI) to see the correspondence of color camera images with 630 nm and OH‐band auroral/airglow emission images. The height of the purplish STEVE arc was stable at 150–170 km while present (∼0,546–0,633 UT), except for a short excursion to ∼200 km at 0,600 UT. The green picket fence structures appeared at 0,549 UT when the intensity of the STEVE arc started to intensify. They presented only for ∼7 min, and their altitude was steady at ∼110 km. The vertical movement of the STEVE arc to ∼200 km was found to be accompanied by the motion across the local magnetic field lines, suggesting a southward E × B drift underlying the westward ion drift. From the comparison with the OMTI images, we find that the purplish STEVE arc moved closer to the 630 nm arc in the all‐sky image when it rose to a higher altitude, indicating the occurrence of electron heating at a same or slightly higher altitude than the STEVE. Plain Language Summary: The Strong Thermal Emission Velocity Enhancement (STEVE) is a longitudinally extended purplish/mauve arc observed at subauroral latitudes, a region slightly equatorward of the typical aurora region. The morphological (shape and motion) characteristics of STEVE indicate that STEVE originates differently from typical aurora. Although it has been suggested that some chemical progresses are possibly triggering a STEVE, significant differences between observation and model still remain. The understanding of STEVE morphological characteristics, especially temporal features of STEVE development, will be helpful to improve our understanding about the STEVE generation. This paper presents a case study regarding the time variation of STEVE altitudes. The altitudes of STEVE are determined by triangulation using two color all‐sky cameras. We found that though the height of STEVE arc was stable for most of the time, it also rose and then descended for a short period. We also compared the images with camera detecting several wavelengths, and found that the STEVE moved closer to the red (wavelength: 630 nm, originating from oxygen) auroral arc in the all‐sky image when the STEVE elevated. This paper presents the first estimation of the temporal variation of STEVE altitudes. Key Points: First estimation of temporal variation of the altitudes of a Strong Thermal Emission Velocity Enhancement (STEVE)The estimated height of the STEVE arc was stable at ∼150–170 km for most of the time, except for a short elevation to ∼200 kmThe vertical motion of the STEVE arc entailed crossing of local magnetic field lines, and it moved closer to the 630 nm emission [ABSTRACT FROM AUTHOR]

Journals occasionally solicit manuscripts for special collections, in which all papers are focused on a particular topic within the journal's scope. For the Journal of Geophysical Research: Space Physics, there have been 51 special collections from 2005 through 2018, with a total of 1,009 papers out of the 8,881 total papers in the journal over those years (11%). Taken together, the citations to special collection papers, as well as other metrics, are slightly higher than papers not in special collections. Several paper characteristics were examined to assess whether they could explain the higher citation and download values for special collection papers, but they cannot. In addition, indirect methods were conducted for assessing self‐citations as an explanation for the increased citations, but no evidence was found to support this hypothesis. It was found that some paper types, notably Commentaries and Technical Reports, have lower average citations but higher average downloads than Research Articles (the most common type of paper in this journal). This implies that such paper types have a different kind of impact than "regular" science‐result‐focused papers. In addition to having higher average citations and downloads, special collections focus community attention on that particular research topic, providing a deadline for manuscript submissions and a single webpage at which many related papers are listed. It is concluded that special collections are worth the extra community effort in organization, writing, and reviewing these papers. Plain Language Summary: Journals sometimes focus the attention of the research community by having a special collection, even an entire issue, devoted to a single topic. A reasonable question to ask is whether the extra effort of organizing, promoting, and maintaining the special collection is worthwhile. This paper examines paper impact in this journal, the Journal of Geophysical Research Space Physics, separating the special collection papers from those not in special collections. The short answer is, on average, yes, at least based on the metric of citations. Some characteristics of the paper were also assessed, such as the use of a colon in the title, the average author team size, the average number of references in each paper, and the paper type of the articles. None of these factors explains the higher average citations and downloads for papers in special collections. In this analysis, though, it was found that several paper types have lower‐than‐average citations but higher‐than‐average downloads, including Commentaries (personal perspectives articles) and Technical Reports (describing new methods or data sets). This implies that such papers are being read but perhaps not heavily referenced (yet). The overall conclusion is that special collections are worth the extra work. Key Points: JGR Space Physics published 51 special collections from 2006 to 2018, totaling 1,009 papers out of 8,881Average citations and downloads are slightly higher for papers in special collections compared to those not in collectionsPaper attributes thought to influence citations were analyzed, finding no statistically significant effect for special collection papers compared to others [ABSTRACT FROM AUTHOR]

Data products relating to auroral arc systems are often sparse and distributed while ionospheric simulations generally require spatially continuous maps as boundary conditions at the topside ionosphere. Fortunately, all‐sky auroral imagery can provide information to fill in the gaps. This paper describes three methods for creating electrostatic plasma convection maps from multi‐spectral imagery combined with plasma flow data tracks from heterogeneous sources. These methods are tailored to discrete arc structures with coherent morphologies. The first method, "reconstruction," builds the electric potential map (from which the flow field is derived) out of numerous arc‐like ridges that are then optimized against the plasma flow data. This method is designed for data from localized swarms of spacecraft distributed in both latitude and longitude. The second method, "replication," uses a 1D across‐arc flow data track and replicates these data along a determined primary and secondary arc boundary while simultaneously scaling and rotating to keep the flow direction parallel to the arc and the flow shear localized at the arc boundaries. The third, "weighted replication," performs a replication on two data tracks and calculates a weighted average between them, where the weighting is based on data track proximity. This paper shows the use of these boundary conditions in driving and assessing 3D auroral ionospheric, multi‐fluid simulations. Plain Language Summary: The aurora, or northern and southern lights, are embedded within a complicated system of interacting electric fields, magnetic fields, and charged particles, the more energetic of which produce the lights themselves by exciting the neutral atmosphere. This brings about a 3D electric current system. These currents enter and exit the atmosphere along the Earth's magnetic field lines, and can only close their circuit between 80 and 150 km. This paper outlines the importance of simulating auroral arc systems in 3D and thus the need for generating continuous horizontal top‐boundary drivers for these simulations. This is difficult as the available data products are limited. This paper provides three methods for creating these boundary conditions using multi‐color, all‐sky auroral imagery in conjunction with approximately across‐arc plasma flow data tracks provided by spacecraft, sounding rockets, and/or radar measurements. Key Points: We provide three methods for developing ionospheric convection flow maps from limited data tracks in conjunction with auroral imageryMethods for generating distributed plasma flow surrounding auroral arcs can benefit from auroral arc information provided by all‐sky imageryModeling realistic auroral current closure needs drivers that vary along‐arc, across‐arc, and in altitude via electron precipitation spectra [ABSTRACT FROM AUTHOR]

In this paper, using a three‐dimensional multifluid MHD model, we studied the effects of the interplanetary magnetic field (IMF) clock angle and the latitude position of the intense crustal magnetic field (ICMF) on the escape of ions O+, O2+ ${\mathrm{O}}_{\mathrm{2}}^{+}$, and CO2+ ${\mathrm{C}\mathrm{O}}_{\mathrm{2}}^{+}$ at Mars. The main results are as follows: (a) The IMF clock angle affects the ion escape at Mars. When the ICMF is on the dayside, the ion escape rate reaches a maximum at the IMF clock angles close to 60°–90° and a minimum at the IMF clock angles close to 120°–150°, because the ICMF can change the topology of the magnetic field and affect the interaction between the solar wind and Mars. The difference between the maximum and minimum ion escape rates due to the IMF clock angle can reach over 50%. (b) Compared with the −ESW hemisphere, the escape flux of O2+ ${\mathrm{O}}_{\mathrm{2}}^{+}$ and CO2+ ${\mathrm{C}\mathrm{O}}_{\mathrm{2}}^{+}$ in the +ESW hemisphere is more significant. However, O+ generally has a larger escape flux in the −ESW hemisphere. The different results in the ±ESW hemispheres might be due to the larger distribution of the hot oxygen corona, which changes the flow pattern of O+. (c) The latitude location of the ICMF can also affect the ion escape. When the ICMF is on the dayside, as the subsolar point varies from 25°S to 25°N, that is, the intense crustal magnetic field position keeps shifting southward, the ion escape rate shows a gradual increase. Plain Language Summary: Ion escape on Mars refers to the process by which ions in the Martian atmosphere are carried away into space by the solar wind. Generally, the solar activity, the solar wind, the interplanetary magnetic field (IMF), and the remaining crustal magnetic field can affect the ion escape. The effect of the IMF clock angle on the ion escape at Mars under a real crustal magnetic field model has not been investigated. In addition, the influence of the latitude location of the intense crustal field on the ion escape at Mars has not been reported. In this paper, using a three‐dimensional multifluid MHD model, we study the effects of the IMF clock angle and the latitude location of the intense crustal magnetic field on the escape of ions (O+, O2+ ${\mathrm{O}}_{\mathrm{2}}^{+}$, and CO2+ ${\mathrm{C}\mathrm{O}}_{\mathrm{2}}^{+}$) at Mars. The results show that the IMF clock angle affects the ion escape at Mars. Contrary to O2+ ${\mathrm{O}}_{\mathrm{2}}^{+}$ and CO2+ ${\mathrm{C}\mathrm{O}}_{\mathrm{2}}^{+}$ (O+ generally has a larger escape flux in the −ESW hemisphere. The ion escape rate increases when the intense crustal magnetic field is on the dayside and keeps shifting southward. Key Points: The IMF clock angle affects the ion escape at MarsContrary to O2+ ${\mathrm{O}}_{\mathrm{2}}^{+}$ and CO2+ ${\mathrm{C}\mathrm{O}}_{\mathrm{2}}^{+}$, O+ generally has a larger escape flux in the −ESW hemisphereWhen the intense crustal magnetic field is on the dayside and keeps shifting southward, the ion escape rate increases [ABSTRACT FROM AUTHOR]

Pc5 ULF waves play an important role in transporting energy and particles in the coupled magnetospheric and ionospheric system. They are known to be initiated by dynamic pressure fluctuations upstream of the magnetopause, including those induced by hot flow anomalies (HFAs). However, the role of HFAs in generating magnetospheric and ground magnetic Pc5 ULF oscillations has not been investigated statistically yet. Thus, in this paper, we investigate the contribution of HFAs to ground magnetic Pc5 ULF oscillations and analyze how the characteristics of HFAs influence these oscillations, based on the coordinated observations between the THEMIS probes and the ground magnetometers at high latitudes during the years 2008, 2009 and 2019. We find that HFAs can serve as a notable source of ground magnetic Pc5 ULF oscillations, with about 18.9% of Interplanetary Magnetic Field (IMF) discontinuity‐induced HFAs associated with discernible enhancements in Pc5 ULF wave power, whereas spontaneous HFAs play a comparatively minor role in generating these oscillations. Furthermore, we observe that the cores of HFAs are likely to contribute more significantly to modulating the induced ground magnetic Pc5 ULF oscillations than their compressed boundaries. More dynamic pressure reductions within HFA cores correspond to stronger ground magnetic Pc5 ULF oscillations. Additionally, HFAs can propagate with the IMF discontinuity along the bow shock, continuously generating ground magnetic Pc5 ULF oscillations during their propagation. This research sheds light on the mechanisms underlying Pc5 ULF wave generation and underscores the role of HFAs in driving magnetospheric‐ionospheric interactions. Plain Language Summary: Ultra Low Frequency (ULF) waves, specifically Pc5 waves, are important for transporting energy and particles between the Earth's magnetic field and its upper atmosphere. These oscillations start because of changes in pressure outside the Earth's magnetosphere, such as hot flow anomalies (HFAs). However, it has not been studied how HFAs contribute to ground Pc5 oscillations in detail yet. So, in this paper, we look at how HFAs make Pc5 oscillations in the Earth's geomagnetic field and how the features of HFAs affect these oscillations. We studied data from THEMIS probes and ground magnetometers in 2008, 2009, and 2019. We found that about 18.9% of triggered HFAs caused noticeable increases in ground magnetic Pc5 wave power. Spontaneous HFAs didn't have as big of an effect. We also found that the core of HFAs is more important for making Pc5 waves than the edges. When the pressure reduction inside HFAs increases, Pc5 waves get stronger. Also, we noticed that HFAs move together with discontinuities in the solar wind, creating Pc5 oscillations as they move. This research helps us understand how Pc5 oscillations are made and shows that HFAs are important for connecting different parts of Earth's space environment. Key Points: HFAs can be an external source of ground magnetic Pc5 ULF oscillations, while SHFAs have weaker effects on generating these oscillationsThe cores of HFAs contribute more to modulating their induced intensity of ground magnetic Pc5 ULF oscillations than their boundariesHFA‐induced ground magnetic Pc5 ULF oscillations can propagate following the propagation direction of HFAs [ABSTRACT FROM AUTHOR]

Statistically ion and electron densities are enhanced above strong crustal magnetic field regions according to measurements made by the Mars Atmosphere and Volatile Evolution (MAVEN) spacecraft. Plasma created by ionization of neutrals in the lower ionosphere (where chemistry dominates) flows upward and becomes trapped on closed magnetic field loops. Enhanced ion density in the ionosphere (particularly O2+) is associated with enhanced photochemical escape of atomic oxygen. This paper presents a quasi‐1D multi‐fluid time‐dependent model of the Martian ionosphere for nine ion species. Ionospheric temperatures are adopted but ion densities and velocities (along the field lines) are determined using a numerical solution of the continuity and momentum equations. Diurnal effects are explored by varying photoionization rates. Three crustal field cases are considered: a low altitude closed, a high altitude closed, and a high altitude open field line. Additionally, a case with no crustal field is modeled to provide a comparison between regions with and without crustal fields in the upper Martian ionosphere. Model results show higher ion and electron densities in the crustal field cases than in the purely induced field case. Additionally, we find that densities are generally higher on the closed field lines than on the open field lines, and ion velocities are generally up the field lines, away from the Martian surface. We also find that velocities are larger on the open field line case. We compare modeled density results to MAVEN data and find general agreement. Implications for atmospheric escape, particularly photochemical escape of O, are also discussed. Plain Language Summary: Mars may not have a global magnetic field like the Earth, but it does have magnetic sources in its crust, giving rise to localized crustal magnetic fields. Previous spacecraft missions have measured increased ion and electron densities over regions where these crustal fields are strongest. This paper presents a model of ion flow along crustal fields at Mars, to try to explain the enhancement seen in data. Ion densities and velocities along crustal field lines are calculated. A diurnal (day to night) cycle is implemented to see how densities and velocities change throughout a Mars day. Multiple crustal field cases are modeled, as well as a generic "non‐crustal" ionosphere case. Modeled results show higher ion and electron densities in the crustal field line cases when compared to the non‐crustal case. Differences between modeled cases are compared with data results, and general agreement is evident. The effects of the modeled crustal fields on atmospheric escape are also discussed. Key Points: Plasma flows along Martian crustal magnetic field lines are modeled for nine ion speciesPlasma densities are higher for closed crustal magnetic fields than for open or induced fieldsUpward ion flow speeds are greater on open than on closed crustal magnetic fields [ABSTRACT FROM AUTHOR]

The sporadic E‐layer (Es) exhibits unique opportunity for exploring the coupling from lower to upper atmosphere. It was found that the East Asia is with the highest intensity and occurrence probability of Es. By using the long‐term data of 21 ionosonde stations in China and Japan over the past 60 yrs, this paper explores the probable control on the Es layer from the lower layers. It is found that the intensity of the Es layer is strongly correlated with the surface atmospheric temperature, terrain, and land‐sea boundary. The correlation coefficient of the intensity of Es with surface temperature is as high as 0.8204, while that with the terrain and land‐sea boundary is up to 0.6668. Based on the coupling between the lower and upper atmosphere, this paper reveals the probable controls from lower layers on the intensity of the Es in East Asia. Plain Language Summary: As an occasional ionized layer of the ionosphere, the sporadic E‐layer (Es) exhibits unique opportunity for exploring the coupling from lower to upper atmosphere. Despite extensive research on both behavior and mechanism of the Es, none have adequately explained the strong inhomogeneous of its global morphology. It is found in fact that the East Asia is with the highest intensity and occurrence probability of Es. By using the long‐term data of 21 ionosondes in China and Japan over the past 60 yrs, this letter explores the formation mechanism of the strongest Es regions in the world. It is found that the intensity of the Es layer is strongly correlated with the surface atmospheric temperature, terrain, and land‐sea boundary. The correlation coefficient between the intensity of Es and surface temperature is highest, while that with the terrain and land‐sea boundary is also high. By analyzing the data as long as six decades, this letter reveals the probable controls from lower layers on the intensity of the Es in East Asia. Key Points: By analyzing the six‐decade data of 21 ionosondes in East Asia, it reveals probable controls on the strongest Es layer from lower layersFor the first time, it is found that the intensity of Es layer is highly correlated with the surface atmospheric temperatureAn interesting coupling from lower to upper atmosphere may be found by taking advantage of observations longer than half of a century [ABSTRACT FROM AUTHOR]

The ability to understand and model ionospheric plasma flow on all spatial scales has important implications for operational space weather models. This study exploits a recently developed method to statistically separate large‐scale and meso‐scale contributions to probability density functions (PDFs) of ionospheric flow vorticity measured by the Super Dual Auroral Radar Network (SuperDARN). The SuperDARN vorticity data are first sub‐divided depending on the Interplanetary Magnetic Field (IMF) direction, and the separation method is applied to PDFs of vorticity compiled in spatial regions of size 1° of geomagnetic latitude by 1 hr of magnetic local time, covering much of the high‐latitude ionosphere in the northern hemisphere. The resulting PDFs are fit by model functions using maximum likelihood estimation (MLE) and the spatial variations of the MLE estimators for both the large‐scale and meso‐scale components are presented. The spatial variations of the large‐scale vorticity estimators are ordered by the average ionospheric convection flow, which is highly dependent on the IMF direction. The spatial variations of the meso‐scale vorticity estimators appear independent of the senses of vorticity and IMF direction, but have a different character in the polar cap, the cusp, the auroral region, and the sub‐auroral region. The paper concludes by discussing the sources of the vorticity components in the different regions, and the consequences for the fidelity of ionospheric plasma flow models. Plain Language Summary: The ability to accurately model the flow of ionized gases (plasma) in the Earth's ionosphere (the ionized region of the Earth's upper atmosphere) has important implications for operational space weather models. Large‐scale variations in this plasma flow are modeled well, but fluctuations on the meso‐scale and small scale are typically ignored in these models. This study uses measurements from a high‐latitude ground‐based radar network to measure the ionospheric plasma flow in terms of its vorticity (how straight or curved the flow is). The measured vorticity is separated into components related to both the large and meso‐scale fluctuations. The paper discusses the origins of the meso‐scale component, and what needs to be done before it can be confidently added to ionospheric plasma flow models. Key Points: The spatial variation of meso‐scale ionospheric flow vorticity is independent of the prevailing IMF By directionMeso‐scale ionospheric flow vorticity is strongest in the cusp and the auroral regionMeso‐scale ionospheric flow vorticity is most intermittent in the polar cap and the sub‐auroral region [ABSTRACT FROM AUTHOR]

Ultra‐low frequency (ULF) waves transfer energy and momentum into the ionosphere‐thermosphere system. To quantify this energy, this paper first presents a new method to quantitatively detect ULF waves in Incoherent Scatter Radar (ISR) data based on 2D fast‐Fourier transforms and subsequent reconstruction of the wave. In parallel with other data sets, including optical, magnetometer, satellite, and models, we present the first full ionospheric energy dissipation rates for a ULF wave, split into electromagnetic (EM) and kinetic fluxes. The EM energy deposition is calculated from the use of the Poynting theorem, looking at Joule and frictional heating rates, where both rates show the same order of magnitude (1.24 × 1013 and 7.3 × 1012 J) respectively when integrated over the wave lifetime of 2 hr 15 min and an area of 4° magnetic latitude × 74° magnetic longitude. However, contrary to the common assumption that the EM flux is dominant, we determined the kinetic flux, to be almost equal in magnitude (8.7 × 1012 J). This indicates that previous papers might have underestimated the total energy dissipation by ULF waves. Compared to the substorm energy budget, we find that locally, the ULF wave event studied here makes up approximately 10% of a typical substorm cycle budget. Plain Language Summary: The Earth's magnetic field lines can move back and forth regularly, like waves on a guitar string. When this happens on the time scale of minutes, we call these ultra‐low frequency (ULF) waves. ULF waves play an important role in the transfer of energy from the Earth's magnetic field to the Earth's upper atmosphere. In this study, we developed a new method to detect ULF waves in radar data to obtain information about the wave. By combining several data sets, such as radar, optical, satellite, and magnetic field measurements, we were then able to determine how much energy is being transferred in one ULF wave event. Both electromagnetic and kinetic energy are important in this transfer of energy. Key Points: We present a new method to detect and reconstruct ULF waves in ISR data with a 2D fast‐Fourier transformWe estimate ionospheric energy dissipation rates due to EM and kinetic energy sources using the new methodWe find that kinetic and EM energy rates are of similar magnitude and should both be included in ionospheric energy deposition by ULF waves [ABSTRACT FROM AUTHOR]

In this paper, we study the ionization‐recombination processes in the lower ionosphere during solar flares of various classes in June 2014 and September 2017. For the first time, ionization and recombination rates of the ionospheric D‐region were estimated using the experimental data on variations in the amplitude and phase characteristics of very low frequencies (VLF) signals and two‐channel data from the GOES satellite (0.05–0.4 nm and 0.1–0.8 nm). The empirical two‐parameter Wait‐Ferguson model was used to calculate temporal changes in the electron concentration Ne during solar flares of different classes. GOES satellite data and black body model were used to estimate the X‐ray fluxes in the wavelength spectral range λ ≤ 0.3 nm. Joint analysis of the temporal evolution of the Ne vertical profile in the lower ionosphere and X‐ray fluxes in different wavelength ranges was carried out. As a result, the values of the ionization and recombination rate coefficients were obtained, and the spectral ranges of radiation that have the greatest impact on Ne at given heights were determined. Calculated ionization and recombination rate coefficients and ranges were successfully verified using VLF data during different solar flares. The results obtained in this work can be used in future studies for a more accurate assessment of the response of lower ionosphere to solar flares of various classes. Plain Language Summary: Solar flares have a significant impact on the state and dynamics of the lower ionosphere and, as a consequence, on the conditions for the radio waves propagation in a wide frequency range. Changes occurring in the ionospheric D‐region are mainly due to variations in the amplitude and phase of VLF‐signals. Analysis of the variation of these parameters together with the X‐ray fluxes recorded by the GOES satellite during solar flares of June 2014 and September 2017 allowed us to study the processes occurring in the ionosphere. In this paper, we empirically estimated the ionization and recombination rates and the spectral ranges of radiation that have the greatest impact on the ionization processes of the lower ionosphere during solar flares. The results obtained can be used in future studies for a more accurate assessment of the response of lower ionosphere to solar flares of various classes. Key Points: VLF signals data and GOES flux data were used to calculate the ionization and recombination rates in the ionospheric D‐regionThe spectral ranges of radiation with the greatest influence on the ionization processes were estimatedObtained values of ionization and recombination rates can be used to estimate time variations of electron concentration during flares [ABSTRACT FROM AUTHOR]

We investigated the amplitude of medium scale traveling ionospheric disturbances (MSTIDs, with periods 25–100 min) and their dependence on the solar activity using 16 years data of the rapid run‐ionosonde operating at high latitudes (67° $67{}^{\circ}$N, Sodankylä, Finland). A deep learning neural network was applied to ionograms to extract critical frequency of the F2 region (foF2) with a 1 min time resolution. Then, we analyzed the relative amplitude of MSTIDs (i.e., 2δ $2\delta $foF2/foF2), which corresponds to the amplitude of atmospheric gravity waves (AGWs) causing MSTIDs. The amplitude of AGWs propagating upward increases with height due to the decreasing density of the air, and hmF2 varies depending on local time, seasonal and solar activity conditions. To account for this effect, we calculated a corrected MSTID amplitude by normalizing the relative amplitude for the air density at the hmF2. The corrected amplitudes show no clear dependence on F10.7 during winter (0–12 UT), equinox (20‐01 UT) and summer (19‐01 UT), while a positive dependence of corrected amplitudes on F10.7 was observed during winter and equinox, in 14–22 UT and 15–19 UT, respectively. Corresponding to the dependence behaviors of corrected and relative amplitudes, two likely mechanisms of MSTIDs, AGWs from the lower atmosphere and auroral sources, are inferred. Their subsequent roles in the solar activity dependence of MSTID amplitudes were separately discussed, although in reality, the observed dependence is complex and often involves several mechanisms together. Plain Language Summary: HF radio frequencies used for ground and/or space‐ground communications and navigation systems often face challenges due to traveling ionospheric disturbances (TIDs), which can degrade the propagation of radio signals. Employing deep learning for rapid‐run ionosonde data enabled us to monitor TIDs at Sodankylä Geophysical Observatory (SGO) effectively. Daytime TID occurrences are often associated with disturbances triggered from the lower atmosphere (AGWs). However, the increase in hmF2, which reduces the likelihood of AGWs initiated from the lower atmosphere reaching the region of peak ionization, and the expansion of the auroral oval due to frequent substorms, shift the nighttime triggers to auroral electrodynamics in high latitude regions. In this paper, we examine the dependence of MSTID amplitudes on solar activity after applying a new correction approach. Key Points: foF2 obtained from 1 min ionograms through deep learning enabled us to conduct long‐term MSTID investigations in high latitudesCorrection of MSTID amplitudes to the neutral density level at different hmF2 altitudes properly describes the solar activity dependenceCorresponding to the causing mechanisms, the high‐latitude corrected MSTID amplitudes infer different types of dependence on solar activity [ABSTRACT FROM AUTHOR]

The Martian ionosphere has many similarities and differences compared to Earth, attracting significant attention. This paper utilizes radio occultation data from the Mars Global Surveyor (MGS), Mars Atmosphere and Volatile Evolution Mission (MAVEN), and ESA Mars Express mission (MEX), associated with the measurements from Mars Advanced Radar for Subsurface and Ionosphere Sounding (MARSIS) aboard MEX, to investigate the seasonal variation of the Martian dayside ionosphere. Data preprocessing is performed first to isolate the influence of solar zenith angle and solar radiation, the seasonal variation of the Martian dayside ionosphere explored in this study. The results suggest that the peak electron density reaches its minimum and maximum at LS = 70.2° and LS = 250.2°, respectively, with an amplitude of 16.5% concerning the average (LS is referred to as the solar longitude of Mars). The peak altitude experiences its minimum at LS = 78.4° and maximum at LS = 258.4°, and its relative amplitude is 12.1%. The outcomes suggest that the seasonal variations in the Martian ionosphere are primarily attributed to the Mars‐Sun distance change, which is different from the situation on Earth. This study will benefit both Martian ionosphere modeling and Earth's ionosphere understanding in the future. Plain Language Summary: The Martian ionosphere shares many similarities with Earth's, which has attracted considerable attention. The Martian ionosphere is primarily driven by solar radiation, including the solar zenith angle and solar radiation strength. So far, missions of MGS, MEX, and MAVEN have collected thousands of measurements about the Martian ionosphere over decades. Based on data preprocessing and systematic error correction, this study explored the seasonal variation in the dayside Martian ionosphere's peak electron density and altitude. The primary results suggest that the seasonal variation in the Martian ionosphere is mainly driven by the Sun‐Mars distance changing, and the mechanisms are distinguished for peak electron density and altitude. In particular, the peak electron density is directly driven by solar radiation, while the peak altitude is due to the lower boundary perturbation. These findings are quite different from Earth and shed light on further understanding in the future. Key Points: The seasonal variability of the Martian dayside ionosphere is studied using multi‐source dataBoth the peak electron density and peak altitude exhibit notable seasonal variationsThe mechanisms are different between the seasonal variations in the peak electron density and altitude [ABSTRACT FROM AUTHOR]

The G‐condition (NmF2 ≤ NmF1) was observed by ground‐based ionosondes at Mohe and Beijing during the geomagnetic storm occurred on 23 and 24 April 2023. We studied exospheric temperature (Tex) responses during the G‐condition. Tex was derived from electron density (Ne) profiles (∼150–200 km) by the method proposed by Li et al. (2023, https://doi.org/10.1029/2022ja030988). The retrieved Tex showed obvious enhancements, with relative deviation of ∼10%–35% and ∼3%–20% at Mohe and Beijing, respectively. Additionally, chemical reaction rate increased by ∼15%–100% and ∼13%–30%, and O/N2 decreased by ∼17%–35% and ∼23%–30% at Mohe and Beijing, respectively. Under photochemical equilibrium assumption, peak Ne is inversely proportional to chemical reaction rate and proportional to O/N2. Increased chemical reaction rate and decreased O/N2 indicate a decrease in peak Ne. Compared to the increased Tex, the relative enhancement in Tex is more significantly associated with the G‐condition, with relative deviation above ∼10% during the G‐condition. Plain Language Summary: Normally, the peak Ne in the F2 layer (NmF2) is the largest in the full Ne profile. G‐conditions are special events in ionospheric observations, when NmF2 is smaller than or equal to NmF1 (i.e., NmF2 ≤ NmF1). Due to shielding effect of the F1 layer, no information above hmF1 can be obtained from ground‐based ionosondes, and Ne peak height is typically below 200 km. In previous works, incoherent scatter radar (ISR) observations are usually used to extract thermospheric parameters (Tex, neutral composition and thermospheric wind). In this paper, ground‐based ionosonde observations (∼150–200 km) at Mohe and Beijing prior to, during and after the G‐condition (from 22 to 26 April 2023) were selected to derive Tex using the method proposed by Li et al. (2023, https://doi.org/10.1029/2022ja030988). Furthermore, corresponding neutral temperature (Tn) and densities were further calculated by replacing the Tex module in the NRLMSISE‐00 model with the retrieved Tex. In our analysis, the retrieved Tex and chemical reaction rate showed significant enhancements and O/N2 showed obvious decrease compared to quiet reference days. This is consistent with the analysis of the mechanism of G‐conditions formation in previous studies. Key Points: The G‐condition (NmF2 ≤ NmF1) was observed by ground‐based ionosondes at Mohe and Beijing during the G4 storm on 24 April 2023Relative enhancements in Tex during the G‐condition were greater than that in other periods, with relative deviation above ∼10%Enhanced Tex with corresponding increased chemical reaction rate and decreased O/N2 collectively contributed to a decrease in Ne [ABSTRACT FROM AUTHOR]

Global Ionosphere Thermosphere Model (GITM) results have been compared with measurements from Global‐scale observations of the limb and disk (GOLD) and Sounding of the Atmosphere using Broadband Emission Radiometry (SABER). For the first time, GOLD‐derived exospheric temperature and column‐integrated O/N2 ${\mathrm{O}/\mathrm{N}}_{2}$ ratio measurements have been used to validate GITM model results. We examine two geomagnetic storm events for which we drive GITM with space weather conditions to understand how well the model reproduces the thermospheric responses to geomagnetic activity. In this paper, a recently developed auroral model, the Feature Tracking of Aurora (FTA) model, has been employed to calculate auroral electron precipitation in GITM (GITM w/FTA), and results are compared with the OVATION prime (OP) driven GITM model (GITM w/OP). GITM w/FTA simulated temperature, neutral density, and nitric oxide (NO) density are generally higher compared to the GITM w/OP model. During the geomagnetic storm, the GITM model and GOLD‐derived exospheric temperature agree between ∼0° ${\sim} 0{}^{\circ}$ and 5° $5{}^{\circ}$N latitude in the equatorial region. GOLD measurements show strong O/N2 ${\mathrm{O}/\mathrm{N}}_{2}$ peaks on either side of the equator during the geomagnetic storm period, which is also observed in our model results. The NO cooling peaks estimated by GITM models are ∼ ${\sim} $20 km lower than SABER observations during geomagnetic storms. GITM w/FTA better matches SABER observations than GITM w/OP except at low latitudes during storm time. Our model‐model/data comparisons show that improved auroral models are needed to better capture the thermospheric variations during geomagnetic storms. Key Points: Global Ionosphere Thermosphere Model was run with the OVATION Prime and the Feature Tracking of Aurora models, and results compared with satellite observations of exospheric temperature, O/N2 ${\mathrm{O}/\mathrm{N}}_{2}$, and NO emissionOur model results show strong O/N2 ${\mathrm{O}/\mathrm{N}}_{2}$ peaks at ±30° $\pm 30{}^{\circ}$ latitudes during storm time, which is consistent with Global‐scale observations of the limb and disk observationsGITM w/FTA model reproduces the SABER NO emission more accurately at high latitudes during geomagnetic storms [ABSTRACT FROM AUTHOR]

The Weddell Sea Anomaly (WSA) is a phenomenon of unique intensity and geographic extent that occurs in December (Southern Hemisphere summer) over the southeastern Pacific and southwestern Atlantic oceans regions. Historically, the classic definition of the WSA refers to a situation in which the midnight NmF2 (or TEC) values are greater than the noon NmF2 (or TEC) values. However, several articles published in the last decades have shown that the WSA is a much more complex phenomenon, and its definition might need to be reformulated. This paper presents a phenomenological description of the WSA using a novel type of vertical Total Electron Content (TEC) maps, obtained from altimeter satellite TEC data. The focus of this study is on the possible connection between the WSA and the unexpected expansion and contraction periods observed on the Equatorial Ionospheric Anomaly (EIA) throughout the Southern Hemisphere. The data analysis revealed that the WSA is only one of a number of observed anomalies. Furthermore, we show a significant correlation between the behavior of the EIA and the Y component of the geomagnetic field, which maximizes in the WSA region. We then present a possible hypothesis for interpreting these results. Plain Language Summary: This contribution presents the findings of an investigation about the Weddell Sea Anomaly, an intriguing ionospheric phenomenon occurring between South America and Antarctica during December. Despite the several decades since the initial reports, the Weddell Sea Anomaly remains a topic of scientific research. The study is based on a novel self‐developed technique to produce global ionospheric maps using TEC measurements from satellite altimeter missions. The results presented provide support for the hypothesis that the Weddell Sea Anomaly may be connected to the Equatorial Ionospheric Anomaly. Furthermore, our findings indicate that the Equatorial Ionospheric Anomaly also displays anomalous behavior over the entire southern hemisphere, with the Weddell Sea Anomaly as the most contrasting of them. We also demonstrate the significant correlation that exists between the Equatorial Ionospheric Anomaly regions and the Y component of the geomagnetic field in relation to the WSA. We then present a possible hypothesis for interpreting these results. Key Points: The study of the Weddell Sea Anomaly with ULIC maps provides a new conceptual framework supporting and extending published theoriesThe new framework considers the anomaly as a mark of a more complex phenomenon that correlates significantly with the geomagnetic fieldThe manuscript presents an innovative technique for producing TEC maps that are suitable to study the climatology of the ionosphere [ABSTRACT FROM AUTHOR]

The paper contains a detailed analysis of the formation of an auroral spiral based on hitherto not published observations by the all‐sky camera in Kilpisjärvi, Northern Finland. We conclude that spirals appearing during a substorm form by a modification of the interface between tail and magnetosphere, the location of the generator current of the westward electrojet. Driven by the arriving flow bursts, this current is subject to ruptures by the appearance of a sequence of hook‐like structures. These structures can move eastward with speeds up to 3 km/s. The propagation is attributed to a constructive magnetic fracture process driven from behind by the power of the arriving flow bursts. Poleward bending and extension of a hook‐like structure, followed by a turning to the west and then equatorward, is the first step in spiral formation. It becomes the primary spiral arm, if a poleward arm grows out of weaker auroral structures, poleward and eastward of it. We suggest that the upward field‐aligned currents related to the bright spiral arms are largely balanced by adjacent downward currents. The electric fields associated with the connecting Pedersen currents are consistent with the counter‐clockwise motion. An important additional ingredient in the observed configuration is an eastward directed flow field, which is the generator of an additional upward current and possibly crucial for the spiral formation. Electric field data from literature throw confusing light on the propagation of a spiral, whether like a vessel in the ocean or by incorporating the magnetic flux ahead of it. Plain Language Summary: Auroral spirals are a fascinating phenomenon in the night sector during substorms. An event, observed by the all sky camera in Kilipisjärvi, is interpreted in the light of the physical processes at substorm onset. The forces exerted by an arriving flow burst on the magnetic field of the near‐dipolar magnetosphere generate an eastward directed current, which is the generator of the auroral electrojet (AEJ). This current can be broken into sections by an instability suggested to occur at strong flow gradients. Out of such sections, a spiral may form in two steps. First, a hook‐like structure, arising from the eastern end of the section, will extend poleward, then westward and finally equatorward thus forming the primary spiral arm. The poleward arm grows out of residues of the rupture process by draping themselves around the upper end of the primary arm. The luminous spiral arms, generated by upward currents, must be closely accompanied by downward currents located in the dark regions of the spiral. The electric fields, connected with the current closure in the ionosphere, are consistent with the counter‐clockwise sense of spiral formation. The creation of convolved auroral forms such as spirals serves an enhanced dumping of the entering energy. Key Points: Ruptures of the generator current lead to the formation of hook‐like structures out of which a spiral can evolveA primary spiral arm extending poleward in a counter‐clockwise sense is completed into a spiral by reformation of residues from the ruptureThe upward field‐aligned currents, which are associated with the luminous structures, are largely balanced by adjacent downward currents [ABSTRACT FROM AUTHOR]

In astrophysics and space, supercritical shock is generated when an object interacts with an incoming supersonic plasma stream. Its downstream plasmas are highly turbulent, containing abundant vortices on all scales from magnetohydrodynamic to electron gyroscales. Understanding the production of these vortices is at the forefront, especially on the electron scale. Using ultrafast measurements of NASA's Magnetospheric Multiscale spacecraft, we report on the fortunate multi‐spacecraft observation of the formation of an electron vortex directly generated inside the Earth's quasi‐parallel bow shock transition and propagated to the downstream turbulent magnetosheath. The vortex is generated inside the shock transition by anisotropic ∼100–600 eV electrons trapped in an ion‐scale magnetic hole which could show a tornado‐like magnetic morphology. Our results demonstrate that the electron vortex can develop not only as a product of the forward cascade but also from the shock transition into its downstream turbulence, which adds to the short‐scale turbulence and dissipation. Plain Language Summary: Turbulence is a ubiquitous process in fluids where nonlinear interactions take place to generate dynamically evolving vortex structures across a broad range of scales. It plays a leading role in the cascade of mass and energy transport to increasingly small scales until it is dissipated as heat. In ionized plasmas, turbulence is more complex when considering electromagnetic fields, different kinds of charged particles, waves, and dissipation mechanisms. For studying plasma turbulence, a readily available natural laboratory is the near‐Earth environment. The solar wind plasma encounters the Earth's magnetosphere, generating a supercritical bow shock. The plasma downstream of the bow shock is highly turbulent and contains abundant plasma vortices from the macroscale to the electron gyroscale. Therefore, understanding the bow shock's role in vortex generation is crucial and essential to understanding the development of turbulence in collisionless magnetized plasmas. This paper presents a precise origin of electron‐scale vortex for unpredictable and random plasma turbulence. The vortex can directly start inside the collisionless shock as a small‐scale vortex before being transported downstream turbulence, instead of the popular belief that developing from the downstream turbulence and larger‐scale vortices. Key Points: Electron vortex can directly start inside the supercritical shock as a small‐scale vortex before being transported downstream turbulenceEvidence for its generation inside the shock transition is provided by identifying the energetic shock electron population as its sourceThe vortex could show a tornado‐like magnetic morphology, coupling with a magnetic hole and propagating to the downstream magnetosheath [ABSTRACT FROM AUTHOR]

During magnetospheric substorms, plasma from magnetic reconnection in the magnetotail is thought to reach the inner magnetosphere and form a partial ring current. We simulate this process using a fully kinetic 3D particle‐in‐cell (PIC) numerical code along with a global magnetohydrodynamics (MHD) model. The PIC simulation extends from the solar wind outside the bow shock to beyond the reconnection region in the tail, while the MHD code extends much further and is run for nominal solar wind parameters and a southward interplanetary magnetic field. By the end of the PIC calculation, ions and electrons from the tail reconnection reach the inner magnetosphere and form a partial ring current and diamagnetic current. The primary source of particles to the inner magnetosphere is bursty bulk flows (BBFs) that originate from a complex pattern of reconnection in the near‐Earth magnetotail at xGSM=−18RE ${x}_{\text{GSM}}=-18{R}_{\mathrm{E}}$ to −30RE ${-}30{R}_{\mathrm{E}}$. Most ion acceleration occurs in this region, gaining from 10 to 50 keV as they traverse the sites of active reconnection. Electrons jet away from the reconnection region much faster than the ions, setting up an ambipolar electric field allowing the ions to catch up after approximately 10 ion inertial lengths. The initial energy flux in the BBFs is mainly kinetic energy flux from the ions, but as they move earthward, the energy flux changes to enthalpy flux at the ring current. The power delivered from the tail reconnection in the simulation to the inner magnetosphere is >2×1011 ${ >} 2\times 1{0}^{11}$ W, which is consistent with observations. Plain Language Summary: During intervals of increased solar activity, the magnetic field in Earth's stretched night‐side tail undergoes intense reconfiguration that can energize particles. This process is called magnetic reconnection. Ions and electrons from reconnection can reach the inner magnetosphere. In this paper, we simulate this process using a novel model that includes Earth's global magnetic field configuration and self‐consistently models particle physics for both electrons and ions. We find that the particles are accelerated significantly near the sites of magnetic field reconfiguration with ions gaining 10 s of keV energy. As they propagate earthward, they end up contributing energetically to the formation of a ring current system partially encircling Earth. Key Points: Ions and electrons accelerated by 10–50 keV near the magnetotail reconnection can reach the inner magnetosphereA partial ring current is formed during the simulation, with highest energy flux between midnight and duskThe ion and electron energy fluxes are mostly kinetic in the reconnection region but change to enthalpy flux earthward [ABSTRACT FROM AUTHOR]

Using peak electron density data from the Global‐scale Observations of the Limb and Disk (GOLD) imager, equatorial plasma bubbles (EPBs) from October 2018 to December 2022 are identified in this paper. The occurrence characteristics of EPBs is statistically analyzed. The results show that EPBs have strong seasonal and longitudinal variations in the range of longitude −60°–0° and magnetic latitude 25° to −20°: (a) The occurrence of EPBs is highest during the spring and autumn equinoxes and lowest during the summer. (b) Equinox asymmetry is found, that the occurrence of EPBs is much higher in autumn than in spring. (c) A peak in the longitudinal distribution of EPBs is observed, with the highest occurrence occurring between −10° and 0° longitude. Additionally, a second peak is evident at −50° longitude in autumn. The GOLD imager is capable of conducting prolonged observations of EPBs in the same region from space, thereby offering a novel perspective on EPBs. Plain Language Summary: EPBs are depletion regions of the plasma in the equatorial ionosphere, which frequently occur in the bottom of F layer to the upside of ionosphere. Using peak electron density data from the Global‐scale Observations of the Limb and Disk (GOLD) imager, we identify and statistically study EPBs between October 2018 and December 2022. The image subspace analysis with the singular value decomposition and the binarization with magnetic latitude are used for the identification of EPBs. The climatological variations of the EPBs are then statistically analyzed and some results are obtained. Seasonal variations in the occurrence of EPBs, asymmetries in the occurrence of spring equinoxes and autumn equinoxes, variations in longitudes are found. Key Points: We identify EPBs in GOLD peak electron density data for the period October 2018 to December 2022 and analyzed them statisticallySignificant seasonal variations in the occurrence of EPBs were observed. The highest was observed in autumn, with the lowest in summerObservation of equinox asymmetry and longitude variability in the occurrence of EPBs [ABSTRACT FROM AUTHOR]

A negative ionospheric storm occurred over the North American sector on 4 November 2021. By the integration of hemispheric power (HP), Ionospheric Connection Explorer (ICON), Global‐scale Observations of the Limb and Disk and global navigation satellite system total electron content (TEC) observations, a complete observation chain is obtained for the first time. At 07:11 UT, prominent energy was input into high latitudes. From 11:51 to 15:49 UT, strong southwestward neutral wind was observed by ICON over the south of North America, followed by significant ∑O/N2 depletion (60%) and temperature enhancement (∼300 K at ∼150 km). TEC depletion started at 12:00 UT in the northeastern North America and expanded to most of the United States. The TEC reduction showed the latitudinally tilted structure similar to that of ∑O/N2 depletion and temperature enhancement, which was shaped by the southwestward neutral wind. The thermosphere‐ionosphere‐electrodynamics general circulation model (TIEGCM) reproduced the observations qualitatively and was utilized to investigate the details of the negative storm generation and evolution processes. Strong equatorward wind and ∑O/N2 depletion reached the North America at midnight, much earlier than the TEC depletion, which began at sunrise. At 90°W, the time delay between TEC and ∑O/N2 depletions increased from ∼3.3 hr at 30°N to ∼11 hr at 60°N. Several factors contributed to the time delay including the time difference between midnight and sunrise, ∑O/N2 transport direction, westward wind and the variation of solar elevation angle with latitude. Plain Language Summary: During geomagnetic storms, the increase/decrease of ionospheric plasma density during geomagnetic storms relative to geomagnetic calm period is called positive/negative ionospheric storms. Due to the lack of observations of thermospheric properties, there are very few studies on negative storms over middle latitudes using direct neutral wind and temperature observations. This paper investigates the negative storm over the North American sector on 4 November 2021. A complete observation chain of the negative storm generating processes is presented by using hemispherical power index, neutral wind, neutral temperature and composition, and total electron content (TEC) measurements. Strong southward and westward neutral wind were observed by Ionospheric Connection Explorer satellite over middle and low latitudes. Global‐scale Observations of the Limb and Disk satellite also recorded prominent thermospheric composition changes and temperature enhancement. Subsequently, TEC depletion occurred was consistent with that of neutral temperature and composition variations. A numerical model is employed to uncover the details of the processes. Thermospheric composition changes occurred between 40° and 60°N firstly at 06:00 UT in the North America sector while TEC depletion appeared at 12:00 UT. There were four factors contributed to the time lag between TEC and composition changes including the time difference between midnight and sunrise, composition transport, westward wind and solar elevation angle. Key Points: During a negative storm, strong southwestward neutral winds and prominent ∑O/N2 depletion were observed by Ionospheric Connection Explorer and Global‐scale Observations of the Limb and Disk, respectivelyA complete observation chain of the negative ionospheric storm generation at middle latitudes is first established for the first timeThe time delay between TEC and ∑O/N2 depletions increased from 3.3 hr at 30°N to ∼11 hr at 60°N [ABSTRACT FROM AUTHOR]

We study the dynamic evolution of dayside magnetopause reconnection locations and their dependence on the interplanetary magnetic field (IMF) cone angle via 3‐D global‐scale hybrid simulations. Cases with finite IMF Bx and Bz but IMF By = 0 are investigated. It is shown that the dayside magnetopause reconnection is unsteady under quasi‐steady solar wind conditions. The reconnection lines during the dynamic evolution are not always parallel to the equatorial plane even under purely southward IMF conditions. Magnetopause reconnection locations can be affected by the generation, coalescence, and transport of flux ropes (FRs), reconnection inside the FRs, and the magnetosheath flow. In the presence of an IMF component Bx, the magnetopause reconnection initially occurs in high‐latitude regions downstream of the quasi‐perpendicular bow shock, followed by the generation of multiple reconnection regions. In the later stages of the simulation, a dominant reconnection region is present in low‐latitude regions, which can also affect reconnection in other regions. The global distribution of reconnection lines under a finite IMF Bx is found to not be limited to the region with maximum magnetic shear angle. Plain Language Summary: The channel of solar wind energy transfer into the magnetosphere is widely believed to be controlled by magnetic reconnection at the dayside magnetopause. Understanding the location of dayside magnetopause reconnection is crucial to comprehending the energy coupling process between the solar wind and the magnetosphere. Magnetopause reconnection has been intensively investigated by numerical simulations and space observations. Nevertheless, the locations and dynamic evolution of reconnection sites in the 3‐D magnetopause reconnection under different interplanetary magnetic field (IMF) conditions are barely scrutinized. In this paper, we investigate the effects of IMF Bx on the motion of magnetopause reconnection locations by using 3‐D global hybrid simulations under a southward IMF Bz. We find that the reconnection is highly variable due to local magnetopause processes associated with the generation, coalescence, and transport of flux ropes (FRs), as well as reconnection inside the FRs. Under southward IMF conditions with IMF Bx, the magnetopause reconnection initially occurs downstream of the quasi‐perpendicular bow shock but eventually dominates in low‐latitude regions, including the magnetopause downstream of the quasi‐parallel bow shock. Key Points: Dayside magnetopause reconnection is highly variable but the reconnection locations eventually maintain quasi‐steady in the low latitudeIn the presence of interplanetary magnetic field Bx, high‐latitude dayside reconnection can be inhibited by reconnection near the equator and magnetosheath flowVariations of reconnection locations are also controlled by the generation, coalescence, and transport of flux ropes (FRs), and reconnection inside FRs [ABSTRACT FROM AUTHOR]

In this study, we investigate the intricate electrodynamics of the Earth's horizontal component of the geomagnetic field (ΔH) in response to two significant solar flares (SF) occurring on 03 July and 28 October 2021. These flares are classified as X1.59 and X1.0, respectively. It is noted that the ΔH follows the X‐ray variation during the SF, but there is a time lag of a few minutes between the X‐ray and ΔH. A possible explanation for the time lag is the neutral atmosphere and ionosphere coupling, via ion drag. Plain Language Summary: The ionospheric electron density and Earth's magnetic field are strong disturbed during solar flares (SFs) X‐class. In this paper we show that horizontal component of the magnetic field (ΔH) follows the X‐ray variation during the SF, but there is a time lag of a few minutes, between the X‐ray and ΔH. The time lag between the X‐ray and ΔH peaks vary from 1 to 8 min, the ion drag force between the neutral atmosphere and ionosphere may explain. Key Points: The synchronization of ΔH peaks and valleys from equatorial‐low‐mid latitudes indicates that the X‐ray and ΔH variations are relatedA visible time lag exists between the X‐ray burst peak and ΔH peaks and valleysIt is proposed the coupling between the neutral upper atmosphere and the ionosphere as a possible explanatory framework for this time lag [ABSTRACT FROM AUTHOR]

Led by the Institute of Geology and Geophysics, Chinese Academy of Sciences, we have built a brand‐new modular active digital phased array, with all solid‐state transmission and digital receiving incoherent scatter radar (ISR) in Sanya (18.3°N, 109.6°E), a station in low latitude China called Sanya ISR (SYISR) since 2015. The development of SYISR involved the indoor design and development of key components, an outdoor prototype test, and the production and debugging of the entire array. The unique features of SYISR include a single‐channel directly connected T/R unit and antenna, a radar array monitoring and calibration network, environmental adaptability design and open architecture. The entire radar has 4,096 channels and 5,930 modules in total. All the technical indices, mainly including a >2 MW peak power, a 43 dBi antenna gain and a <120 K noise temperature, either meet or are superior to the designed value through an independent evaluation. Waveforms of single pulse, linear frequency modulation, Barker code, long pulse and alternating code have been implemented to meet multiple purposes. Four observational modes for ionospheric experiments, including zenith stare, perpendicular to geomagnetic field, meridian scan, and all sky scan, have been developed. We have implemented time domain decoding, frequency domain decoding, and statistical inversion methods in calculating the autocorrelation function and power spectra in signal processing. The preliminary experimental results on ionospheric parameters, plasma lines, irregularities and hard targets are reasonable and encouraging, which greatly enhances our confidence in achieving our scientific goals in the future. Plain Language Summary: The Earth's ionosphere (∼60–1,000 km above the sea level) is composed of dense plasma, which shows complicated variations versus altitude, geographic location, and solar activity level. When a ground‐based radio wave is transmitted onto these plasmas, it will generate stimulated radiation, with a small portion of them radiating back to the ground. If this backscattered radiation is collected by a ground‐based radar, a variety of ionospheric parameters, including plasma density, temperature, and movement speed, can be derived through a complicated signal processing algorithm. An ionospheric monitoring method called incoherent scatter radar (ISR) was therefore developed. However, since this backscattered radiation is very weak, the ISR should be built with sufficiently large power (usually megawatts) and apertures (usually hundreds of square meters). This leads to the high cost of building an ISR, with only a few having been built in the world thus far. In this paper, we describe a newly built ISR, called Sanya ISR (SYISR), in low latitude China by applying modern radar technologies, such as modular active digital phased arrays, all solid‐state transmitting and digital receiving. The development of SYISR followed the principle of a gradual and orderly progress, including indoor design, outdoor prototype testing and the entire radar construction and debugging. The radar technical indices have been evaluated, and all meet the designed value. We also developed a signal processing algorithm and a variety of observational modes. Finally, preliminary experimental results are shown, which are reasonable and encouraging. In the future, we will gradually achieve our scientific goals via effective data accumulation and analysis. Key Points: Sanya incoherent scatter radar (SYISR) is a newly built incoherent scatter radar in low latitude China with modular active electronic scanning, digital phased array, solid‐state transmitting, and digital receivingThis paper details the hardware, key parameters, data processing and preliminary experimental results of SYISRThe key parameters meet or are superior to the original design, and the preliminary observations are encouraging [ABSTRACT FROM AUTHOR]

Auroral particle precipitation is usually assumed to be equally strong for both signs of the By component of the interplanetary magnetic field (IMF). This is also the case in most currently used precipitation models, which parameterize solar wind driving by empirical coupling functions. However, recent studies have showed that geomagnetic activity is significantly modulated by the signs and amplitudes of IMF By and the Earth's dipole tilt angle Ψ. This so called explicit By dependence is not yet included in any current precipitation models. In this paper, we quantify this By dependence for auroral electron precipitation for the first time. We use precipitation measurements of the Defense Meteorological Satellite Program (DMSP) Special Sensor J instruments from years 1995–2022. We show that the dawnside electron precipitation at energies 13.9–30 keV is greater at auroral latitudes for opposite signs of By and Ψ in both hemispheres, while the dusk sector is mostly unaffected by By and Ψ. For energies below 6.5 keV the By dependence is strong poleward of the auroral oval in the summer hemisphere, also exhibiting a strong dawn‐dusk asymmetry. We also show that By dependence of precipitation modulates ionospheric conductance. Plain Language Summary: Electron precipitation into the Earth's ionosphere creating the aurora borealis is driven by the magnetic field carried by the solar wind. The electron precipitation is known to increase the most, when the magnetic field of the solar wind points to south. The currently used models assume equally strong electron precipitation for eastward and westward solar wind magnetic fields. However, in this paper we show that the electron precipitation is different for duskward and dawnward magnetic fields, and that this magnetic field dependence varies with the season. We show that the electron precipitation is stronger on the dawnside in both hemispheres during the northern hemisphere winter when the solar wind magnetic field points duskward. In northern hemisphere summer the dependence on the magnetic field direction is opposite. We show that the solar wind magnetic field modulates the ionospheric conductivity in a similar way. Key Points: IMF By and Earth's dipole tilt angle Ψ (season) modulate auroral electron precipitationPrecipitation is stronger for 13.9–30 keV at auroral latitudes on the dawnside for both hemispheres, when By and Ψ have opposite signsBy drives a strong dawn‐dusk difference for energies 1.4–6.5 keV in the summer hemisphere [ABSTRACT FROM AUTHOR]

We analyze the local dynamics of magnetotail reconnection onset using Magnetospheric Multiscale (MMS) data. In conjunction with MMS, the macroscopic dynamics of this event were captured by a number of other ground and space‐based observatories, as is reported in a companion paper. We find that the local dynamics of the onset were characterized by the rapid thinning of the cross‐tail current sheet below the ion inertial scale, accompanied by the growth of flapping waves and the subsequent onset of electron tearing. Multiple kinetic‐scale magnetic islands were detected coincident with the growth of an initially sub‐Alfvénic, demagnetized tailward ion exhaust. The onset and rapid enhancement of parallel electron inflow at the exhaust boundary was a remote signature of the intensification of reconnection Earthward of the spacecraft. Two secondary reconnection sites are found embedded within the exhaust from a primary X‐line. The primary X‐line was designated as such on the basis that (a) while multiple jet reversals were observed in the current sheet, only one reversal of the electron inflow was observed at the high‐latitude exhaust boundary, (b) the reconnection electric field was roughly five times larger at the primary X‐line than the secondary X‐lines, and (c) energetic electron fluxes increased and transitioned from anti‐field‐aligned to isotropic during the primary X‐line crossing, indicating a change in magnetic topology. The results are consistent with the idea that a primary X‐line mediates the reconnection of lobe magnetic field lines and accelerates electrons more efficiently than its secondary X‐line counterparts. Key Points: Magnetotail reconnection onset was triggered by electron tearing during a solar wind pressure pulseOnset was characterized by the rapid collapse of the current sheet thickness and kinetic‐scale flux rope formationA primary X‐line was established within minutes of the onset [ABSTRACT FROM AUTHOR]

Magnetosonic (MS) waves are common plasma waves in the Earth's magnetosphere. The self‐consistent excitation of MS waves has been studied by 2D particle‐in‐cell simulations in the meridian and equatorial planes of a dipole magnetic field. However, the direction of wave propagation is artificially limited in the previous 2D simulations. Therefore, the 3D simulation of MS waves needs to be investigated. In this paper, we investigate the excitation and evolution of MS waves in the Earth's dipole magnetic field based on a 3D general curvilinear particle‐in‐cell simulation. We find that the MS waves are excited primarily within 3° of the equator when the thermal velocity of the ring distribution is much less than the ring velocity of the ring distribution. These waves propagate along both the radial and azimuthal directions nearly perpendicular to the background magnetic field. In the linear stage, the growth rates of MS waves are almost equal in the radial and azimuthal directions. Compared with the waves propagating along the radial direction, the waves propagating along the azimuthal direction can grow for a longer time, resulting in a larger wave amplification in this direction after saturation. The simulation results provide a valuable insight to understand the self‐consistent evolution of MS waves in the Earth's dipole magnetic field, and the findings are useful for understanding the plasma wave‐particle interaction in the Earth's radiation belts. Plain Language Summary: The Earth's magnetosphere is a natural plasma laboratory. Since the plasma in the magnetosphere is collisionless, electromagnetic waves are significant agents for the transport of energy and momentum among different particles. The MS waves are important electromagnetic waves in the magnetosphere, which have a frequency range from several Hertz to several hundred Hertz. Recently, these waves have been receiving an increasing interest due to their potential importance in accelerating radiation belt electrons and heating plasmaspheric ions. The 2D particle‐in‐cell simulations have been used to investigate MS waves in the meridian and equatorial planes of a dipole magnetic field. However, the direction of wave propagation is artificially limited in the previous 2D simulations. Thus, the 3D simulation of MS waves still needs to be studied. In this paper, we perform a 3D particle‐in‐cell simulation to investigate the excitation of the MS waves in a dipole magnetic field. A larger wave amplification is found in the azimuthal direction because they can grow for a longer time in this direction. Our results can contribute to the understanding of the spatial distribution of MS waves in the Earth's magnetosphere. Key Points: Three‐dimensional particle‐in‐cell simulation of magnetosonic (MS) wave excitation in a dipole magnetic field has been studied for the first timeThe linear growth rates of MS waves are almost equal in the radial and azimuthal directionsA larger wave amplification is observed in the azimuthal direction since the waves can grow for a longer time in this direction [ABSTRACT FROM AUTHOR]

Using an automated novel approach we conduct a reproducible systematic survey of electromagnetic ion cyclotron wave activity detected by Van Allen Probe B during the time period 2013 January 1–2019 July 15. We identify approximately 500 hr of EMIC wave activity, an occurrence rate of ∼ 0.85%. Accounting for satellite dwell time, we find that EMIC waves preferentially occur on the dayside, between 9 and 15 magnetic local time. This is true for both the H+ and He+wavebands. Higher amplitude waves are found at higher values of L shell, while weaker waves occur at low L. The highest amplitudes are concentrated at high L near dawn and dusk. It is also found that EMIC wave occurrence is enhanced during periods of strong geomagnetic activity, with an occurrence rate of 2.7%. During storm times, waves preferentially occur in the afternoon and early evening sectors. The full list of electromagnetic ion cyclotron wave detection times and their properties is made publicly available to the community. This provides a reference catalog for comparison with other magnetospheric phenomena and other wave databases. Plain Language Summary: Electromagnetic Ion Cyclotron (EMIC) waves are found throughout Earth's magnetosphere and the solar system. These waves in Earth's magnetosphere interact with the ring current and radiation belt population and push these particles into our atmosphere. Thus, it is useful to know when and where these waves occur. In this paper, we present a new approach toward identifying these waves in large data sets. Using this new approach, we identified 500 hr of EMIC waves from the Van Allen Probe B data set between Jan 2013–July 2019. Our catalog of events follows similar statistics found for EMIC waves by others, validating our methodology. Like others, we found that the waves occur more frequently on the dayside of the Earth. Higher amplitude waves were found at greater distances from the Earth, close to the edge of the magnetosphere. It was also found that the wave activity was greater during periods of geomagnetic activity than during quiet conditions. And in the interest of open science, we have made both the detection code and the list of waves available to the public. Key Points: We conduct a survey of electromagnetic ion cyclotron waves detected by Van Allen Probe B between 2013 January 1–2019 July 15The overall EMIC wave occurrence rate was ∼0.85% with a dayside preference. Stronger waves are found at higher L shells and during stormsThe full list of EMIC wave detection times and their properties is made publicly available to the community, along with the methodology [ABSTRACT FROM AUTHOR]

This paper introduces a Triangle Network‐Joint Slope (TN‐JS) approach to characterize the spatial and temporal dynamics of Equatorial Plasma Bubbles (EPBs) during geomagnetic storms. To collaboratively determine the EPB drift directions from multiple stations, a Delaunay triangle network is constructed, utilizing the distribution of Ionospheric Piercing Points (IPPs). The Time Difference of Arrival (TDOA) is extracted through cross‐correlating the Rate of Total Electron Content (ROT). The EPB drift direction can be approximately calculated by considering TDOA and IPP distances within each individual triangle of the network. This calculation is then refined through a joint statistical analysis. Using a reference station as the origin, the remaining stations within the network are projected along the estimated EPB drift direction. A spatial‐temporal color map illustrating regional ionospheric anomaly ROT observations is constructed. The EPB drift velocity among multiple stations can be collectively estimated by fitting the slope of this map, facilitating outlier exclusion. Accounting for satellite dynamic effects and the diverse orbit characteristics of GPS and BDS, corresponding IPP scan velocity compensation is performed and analyzed for EPB dynamic estimation. Using the geomagnetic storm event that occurred on September 8 as a case study, the spatial‐temporal kinetic properties of EPBs is characterized by analyzing Global Navigation Satellite System (GNSS) observations from 17 Hong Kong monitoring stations with the proposed TN‐JS approach. The results indicate during this magnetic event, that EPBs exhibit a westward drift trend with velocities ranging from a few tens to hundreds of meters per second in GPS and BDS observations. Plain Language Summary: Total Electron Content (TEC) is a path integrated electron density and its rate (ROT) of change reflect the ionospheric disturbance during magnetic storms. This article introduces a new method called Triangle Network‐Joint Slope (TN‐JS) to study the movement of Equatorial Plasma Bubbles (EPBs). TN‐JS uses a network of GNSS monitoring stations to determine the drift velocity of EPBs. By resampling ROT correlation using triangulation along the drift direction, TN‐JS transforms traditional EPB dynamic estimation into image processing of the color‐coded ROT maps. The TN‐JS algorithm is tested with data collected from 17 monitoring stations around Hong Kong during a geomagnetic storm on 8 September 2017 to show EPBs drifting westward at speeds ranging from tens to hundreds of meters per second. Key Points: A Delauny Triangle Network is built for statistically inferring EPB drift velocity by cross‐correlating and slope fitting multi‐sites' ROTThe orbit diversity offered by GPS MEO and BDS GEO/IGSO satellites provides measures of ionospheric irregularities inhomogeneityThe analysis unveiled a significant EPB westward drift event with a speed exceeding 500 m/s during the 2017 geomagnetic storm over Hong Kong [ABSTRACT FROM AUTHOR]

In this study, based on the OI135.6 nm night airglow data of the FY‐3D Ionospheric Photometer (IPM) during the 2018–2021 geomagnetically quiet period, the global wavenumber 4 longitudinal structure of the equatorial ionization anomaly (EIA) at 2:00 local time was discovered, and the component of the wavenumber 4 was extracted from these structures. Compared with the OI135.6 nm night airglow data of the Special Sensor Ultraviolet Spectrographic Imager (SSUSI) F18 during 2011–2014, there were significant differences in the variation pattern of the relative amplitude of the two versus solar activity and the seasonal variation in the proportion of the component of the wavenumber 4. Based on the neutral wind speed observation results of the Michelson Interferometer for Global High‐Resolution Thermospheric Imaging on board the Ionospheric Connection Explorer (ICON) from 2020 to 2021, the longitudinal structures of the 4 ionospheric waves after midnight are related to the cross‐equatorial meridional wind. We believe that the wavenumber 4 longitudinal structures after midnight originate from the semidiurnal eastward‐propagating with zonal wavenumber 2 (SE2) nonmigrating tide in the cross‐equatorial neutral wind rather than the diurnal eastward‐propagating with zonal wavenumber 3 (DE3) nonmigrating tide in from the zonal wind, which modulates the daytime wavenumber 4 longitudinal structures. Plain Language Summary: The longitudinal structures of EIA have been extensively studied by using satellite data. However, there are few observations and studies, due to the weak ionosphere near midnight. In this paper, we studied the longitudinal structures of EIA at 02:00 local time during geomagnetically quiet period, benefitted from the satellite orbits and high sensitivity of FY‐3D IPM. We found that the wavenumber 4 longitudinal structures of EIA still exist at 02:00 local time and are obvious at equinoxes. Compared with SSUSI F18 data, FY‐3D IPM data showed different characteristics of wavenumber 4 component of EIA longitudinal structures. Because of the different local time of data between SSUSI F18 and FY‐3D IPM, we consider that the longitudinal wavenumber 4 structures of EIA after midnight originated from the cross‐equatorial neutral wind rather than the electric field modulated by zonal neutral wind in daytime. Key Points: The wavenumber 4 longitudinal structures of equatorial ionization anomaly (EIA) still exist after midnight and are more obvious at equinoxesThe wavenumber 4 component of EIA after midnight is positively correlated with the solar activity levelThe origin of the wavenumber 4 longitudinal structures of EIA after midnight is the modulation of the cross‐equatorial neutral wind [ABSTRACT FROM AUTHOR]

Two solar eclipse events in 2023 appeared to produce considerable enhancements in the thermospheric column density ratio of monatomic oxygen to molecular nitrogen (ΣO/N2) as measured by TIMED GUVI. We quantify potential sources for eclipse‐induced ΣO/N2 changes and find that the observed enhancements arise from the ionospheric O+ radiative recombination contribution to the OI 135.6 nm emission from which ΣO/N2 is derived. Variations in the solar Extreme Ultra Violet (EUV) and X‐ray spectrum, due to the difference between the disk spectrum and the coronal spectrum, are also considered but shown to have negligible contributions to the ΣO/N2 enhancements. After accounting for the radiative recombination contribution, we constrain the real thermospheric compositional change to the uncertainty level of the measurements of 5%–10%. These results are valuable for the interpretation of eclipse‐induced ΣO/N2 changes that will further first‐principle model comparisons and lead to a better understanding of the response of the thermosphere to localized variations in solar EUV and X‐ray forcing. Plain Language Summary: The Thermosphere Ionosphere Mesosphere Energetics and Dynamics' Global Ultraviolet Imager provides an estimate of the column ratio of monatomic oxygen (O) to molecular nitrogen (N2) in the upper atmosphere (ΣO/N2) as a data product under normal, sunlit conditions. Eclipses are special conditions. In this paper, we model the measured ultraviolet emissions from O and N2 that go into the retrieval algorithm to understand why this increase in the radiance ratio occurs within the totality of the eclipse. There are two components to the O emission, the glow created by electrons (photoelectrons) that result from the ionization of the upper atmosphere and the glow created by recombination of ionospheric O+ ions. We find that the photoelectron source decreases significantly during the eclipse, but near the magnetic equator, the ionospheric recombination emission does not, which causes the retrieval algorithm to overestimate the ΣO/N2 column density ratio during eclipses. The observations and models are consistent with this explanation. Key Points: The apparent thermospheric ΣO/N2 enhancements (10%–30%) for two eclipse events are found to be predominantly of ionospheric originVariations in the solar spectrum due to wavelength‐dependent attenuation are found to have a negligible impact on ΣO/N2 during the eclipseThe true eclipsed‐induced thermosphere compositional changes are determined to be within 5%–10% of the pre‐eclipse values [ABSTRACT FROM AUTHOR]

There are growing concerns about the effect of solar flares on the ionosphere, mainly due to possible deterioration or damage to our communication and navigation satellite systems. On 3 July 2021, and 28 October 2021, there were solar flares (SFs) classified as X1.59 and X1.0, respectively. These two SFs were the only ones of X‐class that occurred during the last low solar activity (LSA:2018–2021). Data from magnetometers and Global Positioning System (GPS)—Total Electron Content (TEC) are used to investigate the spatial‐temporal electrodynamics of the ionosphere from pole‐to‐pole in the American sector. Employing ∆H and vertical TEC, along with the ROT (rate of change of VTEC) parameter. Rapidly ∆H disturbances closely follow the X‐ray variation and the ∆H valleys and peaks are well‐synchronized during the SFs, indicating that they are linked. Major disturbances in the ∆H are noticed in the mid‐low‐equatorial latitudes. However, minor disturbances were seen at high latitudes. Also, |ROT| is a good indicator of the electron density changes during the SFs, especially when the X‐ray intensity rises to the peak. Plain Language Summary: There are growing concerns about the effect of solar flares on the ionosphere, mainly due to possible deterioration or damage to our communication and navigation satellite systems. This kind of space weather event known as solar flare, releases energy in the form of radiation in the entire electromagnetic spectrum. However, the UV, EUV, and X‐ray radiation burst penetrates deeper into the Earth's atmosphere and is absorbed in the D and E regions (lower ionosphere) and F‐region. In this investigation the ionospheric disturbances are investigated, from pole‐to‐pole, using a magnetometer and GPS‐TEC networks. Key Points: The primary objective of this paper is to investigate the ionospheric response to solar flares across the entire latitudinal rangeThe ∆H exhibited synchronized peaks and valleys, during SF. The ROT is the most effective parameter to study electron density disturbancesThe methodology employed in this study involves the calculation and analysis of ∆H, VTEC, and ROT (Rate of TEC change) [ABSTRACT FROM AUTHOR]

This paper investigates the impact of a powerful gamma ray burst (GRB) that occurred on 9 October 2022, on the Earth's environment using a very low frequency receiver (VLF) to probe the lower ionospheric region (the D region). In addition to the VLF data analysis, we employ numerical simulation through the Long Wavelength Propagation Capability code (LWPC) to derive the increase in the D− region electron density. Our results revealed discernible perturbations in amplitude and phase across all transmitter paths (NAA, DHO, ICV, and NSC) to the Algiers receiver persisting for 40 min. At the maximum of the signal perturbation, the LWPC simulation results showed a decrease in the mean new reference height h′ from 74 to 65.71 km, along with an increase in the sharpness factor β from 0.3 to 0.4875 km−1. Under these new conditions, the electron density increased from its ambient value (216.10 cm−3) to 33.7 103 cm−3. Plain Language Summary: Gamma rays are good indicator of the activity that is taking place in the universe. As they travel freely in the space, these phenomena can cause strong disturbances in the Earth's environment. In 9 October 2022, a powerful burst of gamma rays was recorded by many satellites in the near Earth space. This radiation burst was responsible of the ionospheric disturbance resulting in a modification of the propagation conditions of the radio waves. The intensity of the disturbance as well as its spatial extent can be determined through the analysis of different radio signals propagation and the level of the ionization increases. These results give us insight about the coupling between the gamma rays and the ionized layer of the Earth atmosphere. Key Points: The impact of GRB221009A on the VLF transmitter signal dynamics captured by measurements from the Algiers VLF receiver is reportedA numerical simulation showcases the changes in electron density dynamics in the ionosphere's D‐region during the GRB221009A phenomenonThe model accurately replicates the variations in VLF signal parameters (amplitude and phase) recorded during the GRB221009A event [ABSTRACT FROM AUTHOR]

This paper presents the results of a nearly 2‐year long campaign to detect and analyze meteor persistent trains (PTs)—self‐emitting phenomena which can linger up to an hour after their parent meteor. The modern understanding of PTs has been primarily developed from the Leonid storms at the turn of the century; our goal was to assess the validity of these conclusions using a diverse sample of meteors with a wide range of velocities and magnitudes. To this end, year‐round observations were recorded by the Widefield Persistent Train camera, 2nd edition (WiPT2) and were passed through a pipeline to filter out airplanes and flag potential meteors. These were classified by visual inspection based on the presence and duration of trains. Observed meteors were cross‐referenced with the Global Meteor Network (GMN) database, which independently detects and calculates meteor parameters, enabling statistical analysis of PT‐leaving meteors. There were 4,726 meteors codetected by the GMN, with 636 of these leaving trains. Among these were a large population of slow, dim meteors that left PTs; these slower meteors had a greater train production rate relative to their faster counterparts. Unlike prior research, we did not find a clear magnitude cutoff or a strong association with fast meteor showers. Additionally, we note several interesting trends not previously reported, which include PT eligibility being primarily determined by a meteor's terminal height and an apparent dynamical origin dependence that likely reflects physical meteoroid properties. Key Points: New observations of meteor persistent trains (PTs) are not consistent with many of the previous assumptionsMost PTs were left by relatively slow and dim meteors which are traditionally not expected to produce themWe found that meteor properties such as terminal altitude and dynamical origin affect the likelihood that PTs develop [ABSTRACT FROM AUTHOR]

When solar wind and interplanetary magnetic field (IMF) disturb, thermospheric winds change accordingly. Among the momentum forces driving high‐latitude thermospheric winds, ion drag is supposed to greatly affect wind variations through ion‐neutral coupling when abrupt and strong changes in ion drifts occur. However, due to the great inertia of thermospheric winds it needs a certain period of time for the wind changes to be prominent both in speed and direction. How long the neutral winds take to change from one steady state to another through the ion‐neutral coupling process is currently still a controversial issue. In this paper, we examine the high latitudinal ion‐neutral coupling time scale based on the Thermosphere Ionosphere Electrodynamics General Circulation Model simulations, which can determine whether wind variations are dominantly driven by ion drag by analyzing the relative contribution of each momentum force. It is found that the spatial variation of ion‐neutral coupling time scale is primarily determined by local electron density, but also varies with neutral density and ion‐neutral collision frequency. Simulations during periods of medium solar activity at ∼250 km altitude show that the ion drag‐dominated region is generally located at the dayside convection inverse boundary and the coupling time scale (e‐folding time) is ∼1 hr when IMF By is the dominant component of the IMF and changes direction. Meanwhile, the southward component of IMF Bz enlarges the ion drag‐dominated region. When IMF Bz is southward with a large magnitude, ion drag‐dominated region is primarily located in the nightside auroral oval with ∼2 hr coupling time scale. Key Points: The spatial variation of ion‐neutral coupling time scale depends primarily on local electron densityThe ion drag dominant region is located at the dayside convection cell boundary (nightside auroral oval) when interplanetary magnetic field (IMF) By (negative Bz) disturbsWhen wind variation is greatly affected by IMF By, the concurrent negative shift of IMF Bz obviously enlarges the ion drag dominant region [ABSTRACT FROM AUTHOR]

We present broadband ionospheric scintillation observations of highly defined symmetric quasi‐periodic scintillations (QPS: Maruyama, 1991, https://doi.org/10.1029/91rs00357) caused by plasma structures in the mid‐latitude ionosphere using the LOw Frequency ARray (LOFAR: van Haarlem et al., 2013, https://doi.org/10.1051/0004‐6361/201220873). Two case studies are shown, one from 15 December 2016, and one from 30 January 2018, in which well‐defined main signal fades are observed to be bounded by secondary diffraction fringing. The ionospheric plasma structures effectively behave as a Fresnel obstacle, in which steep plasma gradients at the periphery result in a series of decreasing intensity interference fringes, while the center of the structures largely block the incoming radio signal altogether. In particular, the broadband observing capabilities of LOFAR permit us to see considerable frequency dependent behavior in the QPSs which, to our knowledge, is a new result. We extract some of the clearest examples of scintillation arcs reported in an ionospheric context, from delay‐Doppler spectral analysis of these two events. These arcs permit the extraction of propagation velocities for the plasma structures causing the QPSs ranging from 50 to 00 m s−1, depending on the assumed altitude. The spacing between the individual plasma structures ranges between 5 and 20 km. The periodicities of the main signal fades in each event and, in the case of the 2018 data, co‐temporal ionosonde data, suggest the propagation of the plasma structures causing the QPSs are in the E‐region. Each of the two events is accurately reproduced using a thin screen phase model. Individual signal fades and enhancements were modeled using small variations in total electron content (TEC) amplitudes of order 1 mTECu, demonstrating the sensitivity of LOFAR to very small fluctuations in ionospheric plasma density. To our knowledge these results are among the most detailed observations and modeling of QPSs in the literature. Plain Language Summary: Quasi‐periodic scintillations (QPS) are repeated variations in radio signals received on the ground from radio sources beyond the Earth's atmosphere. As these signals pass through plasma structures in the ionosphere they undergo refractive lensing which is seen at the receiver as a distinct signal fade, bounded on both sides by diffraction fringing. Analysis of these QPS using 2D Fourier transforms produced very clear examples of "scintillation arcs" which were used to extract the velocity of the ionospheric plasma producing these patterns. These symmetric quasi‐periodic oscillations form a distinct category of radio signals which were reproduced in this study using a phase screen model with very small changes in amplitude, corresponding to variations in ionospheric plasma density along the raypath of <<1% of ionospheric plasma density along the raypath. These features are seen across the wide bandwidth from 24 to 64 MHz, with frequency dependent variation in the width of the signal fade and fringing all being visible. Consequently we see how even these very small changes in ionospheric plasma density are nonetheless able to produce quite distinct variations in received signal strength. The observations in this paper thus demonstrate some of the fundamental physical processes of radio scattering in the ionosphere. Key Points: These are the first reported broadband ionospheric scintillation observations of symmetric quasi‐periodic scintillations (QPS)The QPS are very highly defined showing the clearest reported examples of ionospheric scintillation arcs in delay‐Doppler spectraThe observations are successfully reproduced using a phase screen model in which plasma density is varied by <1 mTECu [ABSTRACT FROM AUTHOR]

The paper presents the effects of the storm‐time prompt penetration electric fields (PPEF) and traveling atmospheric disturbances (TADs) on the total electron content (TEC), foF2 and hmF2 in the American sector (north and south) during the geomagnetic storm on 23–24 April 2023. The data show a poleward shift of the Equatorial Ionization Anomaly (EIA) crests to 18°N and 20°S in the evening of 23 April (attributed to eastward PPEF) and the EIA crests remaining almost in the same latitudes after the PPEF reversed westward. The thermospheric neutral wind velocity, foF2, hmF2, and TEC variations show that TADs from the northern and southern high latitudes propagating equatorward and crossing the equator after midnight on 23 April. The meridional keograms of ΔTEC show the TAD structures in the north/south propagated with phase velocity 470/485 m/s, wave length 4,095/4,016 km and period 2.42/2.30 hr, respectively. The interactions of the TADs also appear to modify the wind velocities in low latitudes. The eastward PPEF and equatorward TADs also favored the development of a clear/not so clear F3 layer in northern/southern regions of the equator. Plain Language Summary: The thermosphere‐ionosphere‐magnetosphere system is largely affected during events of geomagnetic storms. Its dynamics, mainly in the ionized environment, may impact modern lives by degradations in the satellite signals affecting, for example, all applications involving Global Navigation Satellite System. The thermosphere and ionosphere responses to the large geomagnetic storm of 23–24 April 2023 are analyzed here and the main discoveries are the unexpected spatial plasma density distribution over Boa Vista (MLat ≈ 8°N) due to resulting effects of a disturbed eastward electric field and traveling atmospheric disturbance (TAD). The interactions/interferences of TADs were able to explain the unexpected moment of the peak in thermospheric neutral wind speed measurements over the equatorial station, as well as all variations in ionospheric parameters (foF2, hmF2, and total electron content) recorded in pairs by seven Digisondes and GNSS receivers spread across Brazil. Key Points: Interactions of traveling atmospheric disturbances (TADs) cause unexpected thermospheric neutral wind speeds in the American equatorial and low‐latitude sectorsThe effects resulting from a disturbed electric field and TADs were able to produce anomalous electron density distribution at low latitudesThe variations in the ionospheric observational measurements recorded around 4 UT in Brazil on 24 April 2023 were explained by the passages of TADs [ABSTRACT FROM AUTHOR]

Interplanetary (IP) shocks are one of the dominant solar wind structures that can significantly impact the Geospace when impinge on the Earth's magnetosphere. IP shocks severely distort the magnetosphere and induce dramatic changes in the magnetospheric currents, often leading to large disturbances in the geomagnetic field. Sudden enhancements in the solar wind dynamic pressure (PDyn) during IP shocks cause enhanced high‐latitude convection electric fields which penetrate promptly to equatorial latitudes. In response, the equatorial electrojet (EEJ) current exhibits sharp changes of magnitudes primarily controlled by the change in PDyn and the local time. In this paper, we further investigated the influence of shock impact angle on the EEJ response to a large number (306) of IP shocks that occurred during 2001–2021. The results consistently show that the EEJ exhibits a heightened response to the shocks that head‐on impact the magnetosphere (frontal shocks) than those with inclined impact (inclined shocks). The greater EEJ response during the frontal shocks could be due to a more intensified high‐latitude convection electric field resulting from the symmetric compression of the magnetosphere. Finally, an existing empirical relation involving PDyn and local time is improved by including the effects of impact angle, which can quantitatively better predict the EEJ response to IP shocks. Plain Language Summary: Solar Wind, a continuous stream of high‐energy particles emanating from the Sun, is ubiquitous in interplanetary (IP) space. In unison, the energetic and/or transient eruptions on the Sun often release bursts of fast solar wind. When this fast solar wind interacts with the ambient solar wind in the IP space, a shock front is formed known as IP shock. These IP shocks (if Earth‐directed) can impinge on the Earth's magnetosphere and transfer tremendous amounts of energy and momentum. As a result, the Earth's magnetic field is often severely disturbed. Severe geomagnetic disturbances are known to cause a myriad of space weather effects from the loss of satellites in space to damage of electrical power grids and transmission lines on ground. Historically, geomagnetic field disturbances are known to be more cataclysmic at high latitudes and lessen at latitudes below the auroral region. However, these disturbances again dramatically enhance at equatorial latitudes due to a unique ionospheric current system, known as equatorial electrojet (EEJ). This study provides new insights into the significant role of angle of shock impacting the magnetosphere and the resultant disturbances in EEJ. This study also derives an empirical relation to accurately estimate the EEJ disturbances during IP shocks. Key Points: The angle of interplanetary (IP) shock impact on the magnetosphere plays a significant role in controlling the equatorial electrojet (EEJ) responseThe magnitude of EEJ change due to the impact of frontal IP shocks is significantly larger than that for the inclined shocksAn improved empirical relation including the effects of impact angle is derived which provide better estimates of EEJ responses to IP shocks [ABSTRACT FROM AUTHOR]

Calculating the magnetic flux transfer across the open‐closed boundary (OCB) per unit time and distance—the reconnection electric field—is an important means of remotely monitoring magnetospheric dynamics. Ground‐based measurements of plasma convection velocities together with velocities of the OCB are commonly used to infer reconnection rates. However, this approach is limited by spatial coverage and often lacks robust uncertainty quantification. In this paper, we assimilate Super Dual Auroral Radar Network convection measurements, ground magnetometer data, and estimates of the conductance derived from the Imager for Magnetopause‐to‐Aurora Global Exploration satellite imagers, using the Local mapping of polar ionospheric electrodynamics (Lompe) framework over a region in North America. We present a new method to assess various contributions to uncertainties in the derived reconnection electric fields, including a novel approach to estimate uncertainties in conductance from global auroral imaging. Our method is demonstrated on a substorm event with an associated pseudobreakup during a period of favorable observational coverage. In this case study, the uncertainties in the reconnection electric field are ∼5–10 mV/m at the peak of substorm expansion, roughly 15% of the peak reconnection electric field. We find that the main contributor to the reconnection electric field estimates after substorm onset is the OCB motion, whereas during the pseudobreakup the main contributor is ionospheric plasma convection. Plain Language Summary: The solar wind is a stream of particles and magnetic field flowing away from the Sun. Under certain orientations of the solar wind magnetic field, magnetic reconnection can occur between the magnetic fields of the solar wind and Earth. Magnetic reconnection is the process by which oppositely directed magnetic field lines "break," then merge into a new configuration. Magnetic reconnection is the primary process responsible for coupling the solar wind energy to the Earth's magnetic field, and is ultimately the principal driver of space weather. In the Earth's magnetosphere, reconnection is associated with topological changes at both dayside and nightside, where stored magnetic energy is released through a global reconfiguration of the magnetosphere which is referred to as magnetospheric substorms. We can remotely monitor this reconnection from the ground by studying the transfer of magnetic flux across the boundary between open and closed magnetic field lines. In this study, we combined data from ground‐based radars, ground magnetometers, and space‐based auroral imaging to monitor the reconnection rate across the entire nightside auroral oval for a full substorm event. We additionally derived uncertainties in this reconnection rate stemming from uncertainties in the ionospheric velocities, the ionospheric conductances, and the open‐closed boundary motion. Key Points: We estimate Hall and Pedersen conductance uncertainties using global auroral satellite images from Imager for Magnetopause‐to‐Aurora Global Exploration (IMAGE) for an isolated substormWe calculate uncertainties in ionospheric reconnection electric fields related to conductance, open‐closed boundary (OCB) motion, and convectionWe find that the main contributor to the reconnection electric field estimates after onset is the OCB motion [ABSTRACT FROM AUTHOR]

The relationship between auroral, ground, and plasma sheet signatures in the late growth phase is crucial for understanding the sequence of events during a substorm expansion phase onset. Here we show conjugate ground‐auroral‐satellite observations of a substorm that occurred on 18 September 2021, between 04:45 and 05:00 UT, where four auroral activations were detected in the all‐sky imagers. An initial activation showed the brightening of an equatorward arc within the cutoff of the 630 nm emissions, indicating activity on closed field lines well inside the open‐closed field line boundary (OCFLB). During a second activation, auroral beads were observed on a brightening arc, equatorward and within the OCFLB, followed by the transformation from small‐scale to large‐scale vortices. The tail current sheet was highly disturbed during the auroral vortex evolution, including pressure and magnetic disturbances, an apparent broadening of a previously thin current sheet, and a breakdown of the frozen‐in condition. Our observations clearly show late growth phase dynamics, including arc brightenings, the formation of auroral beads, and auroral vortex development, can occur well in advance of fast Earthward flows in the tail. Indeed, it is only during that later activity that auroral breakup and strong Earthward flows, which we associate with magnetic reconnection further down the tail, are observed together with strong magnetic bays on the ground. The sequence of events is consistent with an inside‐to‐outside model at substorm expansion phase onset, most likely via a shear‐flow ballooning instability in the transition region from dipole to tail‐like fields in the near‐Earth plasma sheet. Plain Language Summary: Substorm onset is associated with the explosive release of stored magnetic energy, which can be visualized as auroral activity in the ionosphere, magnetic‐field disturbances in the ground‐based magnetometers, and plasma sheet disturbances in the magnetosphere. Even though the processes that lead to energy storage are well known, the exact sequence of events and the triggering factors that lead to the release of this stored energy are poorly understood. In this study, we show conjugate auroral‐ground‐satellite observations of a substorm event that occurred on 18 September 2021. Four auroral activations were observed in the all‐sky imagers, all of which can be associated with plasma sheet disturbances observed in the satellites. Our observations show an initial activation and bead‐like structures on a brightening arc, followed by the formation and expansion of vortex‐like auroral forms, all of which can be associated with magnetic field and pressure fluctuations in the near‐Earth nightside magnetosphere on closed field lines. Auroral breakup and strong magnetic bays on the ground are only observed after the arrival of fast Earthward flows in the magnetosphere. Overall, this paper identifies disturbances in the near‐Earth plasma environment which are the counterparts to the evolving auroral forms seen leading up to the substorm expansion phase onset. Key Points: Plasma sheet dynamical counterparts are reported for an evolving sequence of late growth phase auroral formsPlasma sheet current disruption and ion kinetic scale perturbations occur in advance of fast Earthward flows and magnetic reconnectionOne‐to‐one correspondence between plasma sheet disturbances and auroral forms implies ballooning instability in advance of auroral breakup [ABSTRACT FROM AUTHOR]

This paper investigates the formation and segmentation of the tongue of ionization into two consecutive polar cap patches using multi‐instrument observations from 27 February 2014. We provide insights into how the interplanetary magnetic field (IMF) variations influence the formation and segmentation of these patches. Our findings reveal that the entry of dayside dense plasma into the polar cap is predominantly driven by the modified convection near the cusp region, which is controlled by the transition of IMF By or the sudden drop of IMF Bz. Furthermore, we observe a rapid north‐westward plasma flow within the patch segmentation region, accompanied by equatorward‐expanded and enhanced convection near the cusp region. This fast‐moving flow, approximately 1.5 km/s, is characterized by low density and high electron temperature and shows a signature of a Subauroral Polarization Stream. This suggests that the fast‐westward flow, in conjunction with the expansion and contraction of ionospheric convection, plays a crucial role in the segmentation of polar cap patches from the dayside plasma reservoir. This study provides a comprehensive observation of the evolution of polar cap patches, thereby advancing our understanding of the dynamic mechanisms governing patch formation and segmentation. Plain Language Summary: Polar cap patches are highly dense structures in the polar cap region, typically forming in the cusp region and moving toward the nightside due to ionospheric convections. To better understand the factors that govern the formation and segmentation of these patches, a combination of satellite and ground‐based observations is used to investigate two consecutive polar cap patches on 27 February 2014. We found that the configuration of IMF By and Bz significantly influences cusp region convection, causing dayside sunlit dense plasma to enter the polar cap when cusp convection expands equatorward and the flow shifts northward, a process that leads to patch formation. Additionally, we observe strong north‐westward flows of low‐density, high‐electron‐temperature plasma in the subauroral region during periods of geomagnetic disturbance, displaying characteristics of Subauroral Polarization Streams (SAPS). This flow is observed in the patch segmentation region along with the equatorward‐expanding, enhanced cusp convection. These observations indicate that SAPS, along with the expansion and contraction of ionospheric convection, play a key role in the segmentation of polar cap patches from their equatorward plasma reservoir. This study significantly advances our understanding of the dynamic mechanisms governing the formation and segmentation of polar cap patches. Key Points: The transient change of IMF By and Bz leads to the formation of the tongue of ionization and segmentation of polar cap patchesA tongue of ionization is segmented into two polar cap patches by low electron density, high electron temperature, fast westward flowsThe expansion/contraction of the ionospheric convection also plays a crucial role in the segmentation of dayside dense plasma [ABSTRACT FROM AUTHOR]

In this paper, we study Titan's magnetotail using Cassini data from the T122‐T126 flybys. These consecutive flybys had a similar flyby geometry and occurred at similar Saturn magnetospheric conditions, enabling an analysis of the magnetotail's structure. Using measurements from Cassini's magnetometer (MAG) and Radio and Plasma Wave System/Langmuir probe (RPWS/LP) we identify several features consistent with reported findings from earlier flybys, for example, T9, T63 and T75. We find that the so‐called 'split' signature of the magnetotail becomes more prominent at distances of at least 3,260 km (1.3 RT) downstream of Titan. We also identify a specific signature of the sub‐alfvenic interaction of Titan with Saturn, the Alfvén wings, which are observed during the T123 and T124 flyby. A coordinate transformation is applied to mitigate variations in the upstream magnetic field, and all the flybys are projected into a new reference frame—aligned to the background magnetic field reference frame (BFA). We show that Titan's magnetotail is confined to a narrow region of around ∼4 RT YBFA. Finally, we analyze the general draping pattern in Titan's magnetotail throughout the TA to T126 flybys. Key Points: During the T123 and T124 Titan flybys an Alfvén wing‐like structure was detectedAn analysis of all tailside flybys reveals a clear bipolar draping of the magnetic field around TitanMagnetic field and plasma density are analyzed during T122‐T126 to estimate the escape rate in an magnetohydrodynamics regime [ABSTRACT FROM AUTHOR]

To ensure the robustness of both civilian and military infrastructure, it is important to protect electric power grids, smart grids, and other electrotechnologies from known and possibly as‐of‐yet unknown space weather hazards. Space weather can generate intense geoelectric fields at the surface of the Earth, as well as large voltage gradients across long distances of the Earth. These voltage gradients can lead to geomagnetically induced currents (GICs), which are known to produce hazards to electric power grids. The finite‐difference time‐domain (FDTD) method is a powerful and versatile method that has already been applied to the study of geoelectric fields. The advantages of FDTD over other methods are that it can account for more geometrical complexities and realistic time waveforms and that it directly solves for geoelectric fields. Snell's Law predicts that any electromagnetic waves incident on the ground should essentially propagate straight downwards into the low resistivity ground. For this reason, vertical FDTD grid resolutions of 1/3 of a skin depth were usually chosen, while the horizontal grid resolution was relaxed. We find, however, that there is another important consideration for choosing an FDTD grid resolution applied to real‐world scenarios: localized field variations due to currents generated by ground features. It turns out the grid resolution requirements are much stricter when taking this physics into account. Key Points: Previous FDTD models limited the vertical resolution to <13 ${< } \frac{1}{3}$ of a skin depth but relaxed the horizontal resolution for efficiencyThis paper shows that there is another important consideration: localized field variations due to currents generated by ground featuresWe find that the required vertical and horizontal resolution requirements are higher for realistic scenarios than previously thought [ABSTRACT FROM AUTHOR]

The thermospheric density and its variations are crucial to aerospace activities as well as space weather research and operation. However, due to the difficulties in observing the thermosphere, there has been a lack of effective descriptions for the general characteristics of thermospheric density. In this paper, the Two‐Line‐Element data sets (TLEs) from multi‐target low Earth orbit satellites are used to derive a proxy of the daily average atmospheric density in the thermospheric shell located in the vicinity of LEOs' orbital altitude. It captures the overall characteristics of the thermosphere and exhibits good correlations (∼0.9) with modeled and observed thermospheric density. By applying the spectral whitening method to this proxy, a new index JsT ${J}_{s}^{T}$ is derived to describe non‐periodic perturbation of the density where the specific satellite passed by. The fact that the JsT ${J}_{s}^{T}$ obtained from different satellites within the same thermospheric shell presents significant consistency to each other means that the new index is a good indicator for the overall feature of the variations of thermospheric density, and it is possible to define a unified regional index JrT ${J}_{r}^{T}$ to describe density disturbances for the thermospheric shell where these satellites fly through. Moreover, the JrT ${J}_{r}^{T}$ at different altitudes also present good consistency suggesting the possibility of defining a global index JpT ${J}_{p}^{T}$, capable of describing the density variation of the entire thermosphere. Plain Language Summary: Due to limited in situ observation, the thermospheric density is not easy to obtain. Herein, we have extracted the density information of the thermosphere based on the Two‐Line‐Element data sets and developed a new index using the spectral whitening method, which describes aperiodic disturbances of the density. The construction of this index series can address the limitations of existing quantitative or graded descriptions of space weather disturbances and the problem of mismatched or inconsistent parameters/indices. Ultimately, this will contribute to the monitoring and warning of space weather causal chains. Key Points: A proxy of daily average thermospheric mass density was extracted from Two‐Line‐Element data setsBy applying the spectral whitening method to this proxy, a new index is derived to describe non‐periodic perturbations in the thermosphereA unified regional index was defined to describe density disturbances for the thermospheric shell where these satellites fly through [ABSTRACT FROM AUTHOR]

In this paper, the equatorial thermosphere anomaly (ETA) is investigated using accelerometer measurements to determine whether the feature is density‐dominated, wind‐dominated, or some combination of the two. An ascending‐descending accelerometry (ADA) technique is introduced to address the density‐wind ambiguity that appears when interpreting the ETA in atmospheric drag acceleration analyses. This technique separates ascending and descending acceleration measurements to determine if a wind's directionality influences the interpretation of the observed ETA feature. The ADA technique is applied to accelerometer measurements taken from the Challenging Minisatellite Payload mission and has revealed that the ETA is primarily density‐dominated from 9:00 to 16:00 local time (LT) near 400 km altitude, with the acceleration perturbations behaving similarly between 2003 and 2004 across all seasons. This finding suggests that the perturbations in the acceleration due to in‐track wind perturbations are small compared to the perturbations due to mass density, while indicating that the formation mechanisms across these local times are similar and persistent. The results also revealed that in the terminator region at 18:00 LT the acceleration perturbations deviate appreciably between ascending and descending passes, indicating different or multiple processes occurring at this local time compared to the 9:00–16:00 LT ascribed to the ETA. These results help constrain ETA formation theories to specific local times and thermospheric property responses without the use of supplemental wind measurements, while also indicating regions where in‐track winds cannot always be neglected. Plain Language Summary: Earth's thermosphere, a layer in the upper atmosphere, is a highly variable region that contributes to satellite drag in low Earth orbit. The equatorial thermosphere anomaly (ETA) is a perturbation feature that produces increases and decreases in drag accelerations across the lower latitudes of the thermosphere. Direct measurements of the thermosphere are often limited, so satellite instruments, such as accelerometers, have been used to analyze the acceleration that a satellite experiences due to atmospheric drag to indirectly measure thermospheric properties. However, the measured drag acceleration is dependent on both mass density and wind, making it difficult to determine whether a change in drag across the ETA is due to either property. This has implications on what mechanisms are responsible for its formation. This work introduces an ascending‐descending accelerometry technique to exploit the fact that while acceleration perturbations produced by mass density are independent of satellite direction, those arising from winds can either be positive or negative depending on the relative spacecraft motion. By separating satellite orbits into ascending (south‐to‐north) and descending (north‐to‐south) passes, it can be determined whether drag acceleration perturbations are due to mass density, wind, or a mixture of the two, helping constrain possible formation mechanisms. Key Points: Accelerometer observations of the Equatorial Thermosphere Anomaly (ETA) cannot distinguish between in‐track density and wind perturbationsA new technique is applied to observations determining that the ETA, occurring between 9 and 16 LT, is a density‐dominated featureThe ETA demonstrates repeatable annual behavior for each season, suggesting a common and persistent formation mechanism [ABSTRACT FROM AUTHOR]

From the global simulation, we reproduce the solar wind‐magnetosphere‐ionosphere (S‐M‐I) interaction under the northward interplanetary magnetic field (IMF) with negative By. Reconnection structures, the plasma sheet, and lobes are formed in magnetospheric convection, while lobe/round‐merging/reciprocal/nightside cells appear in the ionosphere. Associated with the S‐M interaction, northern open field is generated at the evening open‐closed (O/C) boundary, due to successive cusp and interchange reconnections (in round‐merging cell) or by Dungey‐type reconnection (in nightside cell). Corresponding interchange and Dungey‐type reconnections occur at the southern null. Dungey‐type reconnection at the same time generates southern open field on the outer‐most magnetopause. Open field injected into the northern polar cap/void/lobe constructs the open field part of the round‐merging and nightside cells. After open‐field convection in the lobe, reclosures occur by again successive cusp and interchange reconnections on the dayside separator, or separator reconnection on the nightside separator. Former closed field line proceeds toward the evening O/C boundary through the dayside closed‐field convection in the round‐merging cell, while latter closed field line through the nightside closed‐field convection in the nightside cell. Shear that causes the large‐scale sun‐aligned arc is generated by the process of injecting open magnetic field into the void and the conjugate of process of connecting return flux from the plasma sheet to the nightside cell in counter hemisphere. Key Points: Under the northward IMF, the S‐M interaction has in front and off‐front components exciting the round‐merging cell and the nightside cellIn the nightside cell, closed flux is formed at nightside separator and transported along the nightside cell to the flank where reopenedThe HCA is explained by shear structure presented in this paper, without adopting evening‐morning symmetrical convection as in the DLR [ABSTRACT FROM AUTHOR]

We study electromagnetic ion cyclotron (EMIC) waves based on observations from the ionosphere, magnetosphere, and ground during a geomagnetic storm recovery phase on 28 August 2018. In this case, multiple ducting EMIC waves in the ionosphere show higher frequencies in the post‐midnight than those in the pre‐midnight. Ionospheric EMIC wave frequency differences in magnetic local time (MLT) are consistent with MLT frequency differences in the equatorial magnetosphere, which are mainly caused by different background magnetic field at different L‐shells. Moreover, we report the first observation of frequency range selections in ionospheric ducting EMIC waves and find that frequency selections depend on the magnetic field intensity in the main part of the ionospheric waveguide, with higher frequency corresponding to larger magnetic field. This study reveals the important role of background magnetic field in regulating ducting EMIC wave frequencies in the ionosphere. Plain Language Summary: Electromagnetic ion cyclotron (EMIC) waves are in the frequency range of 0.2–5 Hz, and they may occur in three different bands, which are named as H+, He+, and O+ bands from high to low frequencies. In this paper, He+ band EMIC waves in the nightside during a geomagnetic storm are observed by satellites in the ionosphere, and their frequencies are different, that is, frequencies are higher in the post‐midnight than those in the pre‐midnight. Ionospheric EMIC wave frequency differences in local time have been noticed before but have not yet been explained. We discover that ionospheric EMIC wave frequency differences in local time agree well with frequency differences in the magnetosphere. Besides frequency discrepancies in local time, in the post‐midnight, ionospheric EMIC waves tend to select higher or lower frequency range relative to frequencies in the magnetosphere. We find that these frequency selections mainly depend on the magnetic field intensity in the ionosphere. The frequency selections in ionospheric EMIC waves have not been identified before, and it is important to understand variations of EMIC waves in the ionosphere. Key Points: Ionospheric EMIC wave frequency differences in magnetic local time (MLT) are consistent with MLT frequency differences in the equatorial magnetosphereHigher/lower wave frequencies in the equatorial magnetosphere are at lower/higher L‐shells with larger/smaller background magnetic fieldFrequency selections of ducting EMIC waves depend on local magnetic field, and frequency is higher when the magnetic field is larger [ABSTRACT FROM AUTHOR]