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Recently, our paper "Poststorm Thermospheric NO Overcooling?" has been published in JGR. Lei with colleagues (2012) who have proposed the "NO overcooling" concept have written Comments on this paper. Below is given our reply. Everywhere MP20 means the reference to the paper by Mikhailov and Perrone (2020). In the beginning to avoid misunderstanding, it is necessary to stress that in MP20 we did not touch on the well-documented process of the thermosphere NO cooling (e.g., Gordiets et al., 1982; Maeda et al., 1989; Mlynczak et al., 2018; Prölss, 2004, 2011; Roble, 1995; Weimer et al., 2011) which mainly takes place in the lower thermosphere. We only explained a decrease of neutral gas density at F2-layer heights during the recovery storm phase. The effect manifests seasonal dependence which is not explained by the "NO overcooling" mechanism. [ABSTRACT FROM AUTHOR]

This commentary paper presents summaries of my research work on magnetospheric reconnection and on geomagnetic storms, to which I have devoted most of my professional career in space physics. Thus, the paper summarizes: (a) my early studies on the magnetospheric reconnection-electric field, involving observations and a model of the related polar cap-electric field during events with observed interplanetary driving parameters and (b) studies on several fundamental aspects associated with the origin of geomagnetic storms, especially leading to intense and extreme levels of activity, mostly associated with the arrival of geoeffective Coronal Mass Ejections (CMEs). The latter also involves studies on recurrent geomagnetic activity caused by interplanetary high-speed streams, among which “High-Intensity, Long-Duration and Continuous Auroral Activity (HILDCAAs for short)” events were shown to represent a special class. In the final Section, I have added some comments on recent issues dealing with fundamental concepts on reconnection at the Earth's magnetopause. [ABSTRACT FROM AUTHOR]

Rapid increases in solar wind dynamic pressure, known as solar wind pressure pulses, compress the Earth's magnetosphere and can rapidly restructure the electrodynamics within. The propagation of pressure pulse effects into the magnetosphere is known as a geomagnetic sudden commencement (SC). SCs can be further subdivided into compressions which are rapidly followed by a geomagnetic storm (a sudden storm commencement, SSC) and those which are not (a sudden impulse, SI). In this paper, SSCs and SIs are compared and contrasted, and we examine in particular the differences between the pressure pulses that drive SSCs/SIs, and explore the physical conditions of the magnetosphere before pressure pulse arrival. Firstly, it is shown that SSCs are more likely to be driven by pressure pulses with higher magnitude and/or shorter rise time. Secondly, the magnetosphere is primed by stronger driving conditions and higher geomagnetic activity prior to SSCs than SIs. Finally, there is a solar cycle dependence in the occurrence and magnitude of solar wind pressure pulses. Plain Language Summary: The solar wind's dynamic pressure controls the size of the cavity the Earth's magnetic field forms in space. When the pressure increases rapidly, this is known as a solar wind pressure pulse. Pressure pulses can affect currents and magnetic fields within the Earth's space environment, and can have serious space weather implications. In this paper, the differences between pressure pulses that are followed by geomagnetic storms (large releases of energy in the Earth's system), and those that are not are investigated. The state of the Earth's system before the arrival of the pressure pulse is also considered, and its contribution to the resulting space weather effects. It is shown that larger pressure pulses are more likely to trigger a geomagnetic storm. Additionally, analysis presented shows that the Earth's system may already be active prior to the triggering of these storms. Finally, the occurrence and strength of pressure pulse events follows solar activity. Key Points: There is a solar cycle dependence to the occurrence and magnitude of sudden commencements (SCs)Sudden storm commencements (SSCs) are driven by higher magnitude and/or shorter duration pressure pulsesThe magnetosphere is primed by stronger solar wind/interplanetary magnetic field and higher geomagnetic activity for SSCs [ABSTRACT FROM AUTHOR]

Previous studies have shown that Strong Thermal Emission Velocity Enhancement (STEVE) events occur at the end of a prolonged substorm expansion phase. However, the connection between STEVE occurrence and substorms and the global high-latitude ionospheric electrodynamics associated with the development of STEVE and non-STEVE substorms are not yet well understood. The focus of this paper is to identify electrodynamics features that are unique to STEVE events through a comprehensive analysis of ionospheric convection patterns estimated from SuperDARN plasma drift and ground-based magnetometer data using the Assimilative Mapping of Geospace Observations (AMGeO) procedure. Results from AMGeO are further analyzed using principal component analysis and superposed epoch analysis for 32 STEVE and 32 non-STEVE substorm events. The analysis shows that the magnitude of cross-polar cap potential drop is generally greater for STEVE events. In contrast to non-STEVE substorms, the majority of STEVE events investigated are accompanied by with a pronounced extension of the dawn-cell into the pre-midnight subauroral latitudes, reminiscent of the Harang reversal convection feature where the eastward electrojet overlaps with the westward electrojet, which tends to prolong over substorm expansion and recovery phases. This is consistent with the presence of an enhanced subauroral electric field confirmed by previous STEVE studies. The global and localized features of high-latitude ionospheric convection associated with optical STEVE events characterized in this paper provide important insights into cross-scale magnetosphere-ionosphere coupling mechanisms that differentiate STEVE events from non-STEVE substorm events. [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]

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]

A Very Low Frequency (VLF) wave detection system has been designed at Wuhan University (WHU) and recently deployed by the Polar Research Institute of China at the Chinese Great Wall station (GWS, 62.22°S, 58.96°W) in Antarctica. With a dynamic range of ∼110 dB and timing accuracy of ∼100 ns, this detection system can provide observational data with a resolution that can facilitate space physics and space weather studies. This paper presents the first results of the wave measurements by the WHU VLF wave detection system at GWS to verify the performance of the system. With the routine operation for 3 months, the system can acquire the dynamic changes of the wave amplitudes and phases of various ground‐based VLF transmitter signals emitted in both North America and Europe. A preliminary analysis indicates that the properties of the VLF transmitter signals observed at GWS during the X‐class solar flare events are consistent with previous studies. As the HWU‐GWS path crosses the South Atlantic Anomaly region, the observations also imply a good connection in space and time between the VLF wave disturbances and the lower ionosphere variation potentially caused by magnetospheric electron precipitation during the geomagnetic storm period. It is therefore well expected that the acquisition of VLF wave data at GWS, in combination with datasets from other instruments, can be beneficial for space weather studies related to the radiation belt dynamics, terrestrial lightning discharge, whistler wave propagation, and the lower ionosphere disturbance, etc., in the polar region. Plain Language Summary: Considering the good coverage and quiet electromagnetic environment, Antarctica is an ideal place for plasma wave measurements. Various stations have been established in Antarctica, of which Palmer station is particularly noteworthy and has historically provided valuable VLF data for atmospheric, ionospheric, and magnetospheric studies. An Extremely Low Frequency/Very Low Frequency (VLF) wave detection system has been designed by Wuhan University and recently set up at Great Wall station (GWS) in Antarctica. This device can effectively record VLF signals with frequencies of 1–50 kHz, including artificial transmitter signals and natural emissions. This paper gives the broadband spectrum of 1–50 kHz signals in the north‐south and east‐west directions recorded at GWS for the first time. VLF signatures from lightning discharges, environmental disturbances, and navy transmitters in North America and Europe can be clearly identified. The overall trend of amplitude and phase of transmitter signals is consistent with the X‐ray fluxes measured by the Geostationary Operational Environment Satellite satellite. Based on the GWS observations, it is expected to reveal some significant new phenomena reflected in VLF signals and further study the distribution and propagation characteristics of VLF waves. These are of great significance for physical research and application, especially in the unique geographical location of the South Pole. Key Points: A high‐sensitivity Very Low Frequency (VLF) wave detection system with a dynamic range of ∼110 dB and timing accuracy of ∼100 ns has been designedThis system has been recently deployed by the Polar Research Institute of China in Antarctica and operated routinely for 3 monthsThe VLF wave data collected by this system can be widely used to monitor and study space weather events in the polar region [ABSTRACT FROM AUTHOR]

Quantification of energetic electron precipitation caused by wave‐particle interactions is fundamentally important to understand the cycle of particle energization and loss of the radiation belts. One important way to determine how well the wave‐particle interaction models predict losses through pitch‐angle scattering into the atmospheric loss cone is the direct comparison between the ionization altitude profiles expected in the atmosphere due to the precipitating fluxes and the ionization profiles actually measured with incoherent scatter radars. This paper reports such a comparison using a forward propagation of loss‐cone electron fluxes, calculated with the electron pitch angle diffusion model applied to Van Allen Probes measurements, coupled with the Boulder Electron Radiation to Ionization model, which propagates the fluxes into the atmosphere. The density profiles measured with the Poker Flat Incoherent Scatter Radar operating in modes especially designed to optimize measurements in the D‐region, show multiple instances of close quantitative agreement with predicted density profiles from precipitation of electrons caused by wave‐particle interactions in the inner magnetosphere, alternated with intervals with large differences between observations and predictions. Several‐minute long intervals of close prediction‐observation approximation in the 65–93 km altitude range indicate that the whistler wave‐electron interactions models are realistic and produce precipitation fluxes of electrons with energies between 10 keV and >100 keV that are consistent with observations. The alternation of close model‐data agreement and poor agreement intervals indicates that the regions causing energetic electron precipitation are highly spatially localized. Plain Language Summary: Establishing how electromagnetic waves in the magnetosphere push high‐energy electrons into a funnel directed toward Earth along magnetic field lines is a critically important step to predict how electrons are lost into the atmosphere during geomagnetic storms. Wave‐electron interaction models predict the number of electrons that are funneled toward Earth from a set of in‐situ spacecraft measurements. As the electrons hurl into the upper atmosphere, ionization models predict how many electrons would be released by the neutral atmosphere due to the bombardment from funneled electrons. Electron density measurements with a radar especially tuned to optimize detection of electron densities in the upper atmosphere can be compared to predicted electron densities to determine the validity of the electron loss models. This paper reports a comparison for an interval of time when a Van Allen Probes spacecraft is measuring the waves and electrons at a location that would guarantee that the electrons would fall near the radar's location in Poker Flat, Alaska. The comparison shows that the models considered predict the correct number of electrons in multiple instances, thus establishing an important step in verifying the validity of the models of electron loss during geomagnetic storms. Key Points: Comparison between observed and modeled density for electron precipitation due to wave‐particle interactions in the magnetosphereComparison verifies the validity of D‐region electron density predicted by pitch‐angle diffusion models of wave‐particle interactionObserved electron profiles are obtained with an incoherent scatter radar mode especially designed to optimize measurements in the D‐region [ABSTRACT FROM AUTHOR]

This paper conducts a multi‐instrument analysis and data assimilation study of midlatitude ionospheric disturbances over the European and North American longitude sectors during a strong geomagnetic storm on 26–28 February 2023. The study uses a set of ground‐based (GNSS receivers, ionosondes) observations, space‐borne (DMSP, GOLD) measurements, and a new TEC‐based ionospheric data assimilation system (TIDAS). We observed a series of distinct storm‐time features with regard to storm‐enhanced density (SED) and subauroral polarization stream (SAPS) as follows: (a) Under multiple ring current intensifications, the storm‐time subauroral ionosphere produced long‐lasting duskside SAPS for ∼36 hr along with considerable dawnside SAPS for several hours. (b) Associated with long‐lived SAPS, strong SED occurred consecutively in the European longitude sector near local noon during a positive ionospheric storm and later in the North American longitude sector near local dusk during a negative ionospheric storm. (c) The 3‐D morphology of SED in multiple longitude sectors was reconstructed using TIDAS data assimilation technique with fine‐scale details, which revealed a narrow ionospheric plasma channel with electron density enhancement and layer uplift. Key Points: The storm‐time subauroral ionosphere produced long‐lasting duskside subauroral polarization stream (SAPS) for ∼36 hr and noticeable dawnside SAPS for several hoursAssociated SED occurred in European longitudes during a positive storm period and then in North America during a negative storm periodThe 3‐D SED morphology in multiple longitude sectors was reconstructed using a new total electron content (TEC)‐based ionospheric data assimilation system [ABSTRACT FROM AUTHOR]

Traveling atmospheric disturbance (TAD) plays an important role in the energy and momentum transfer from the lower atmosphere to the upper atmosphere, and from high‐ to low‐latitudes. It is common to observe TADs propagating toward low latitudes because of enhanced Joule heating and/or the Lorentz force at the high‐latitude ionosphere during storm time. However, energy or momentum variation associated with their equatorward propagation remains unclear. Two geomagnetic storms occurred on 7–8 September 2017 and upper atmospheric disturbances are observed by the Swarm satellites and the Global Navigation Satellite System Total Electron Content network. We conduct a model simulation and term analysis of the energy equation to investigate the dominant terms of TAD. Adiabatic heating, conduction heating, and advection heating dominate the energy budget of TAD. Adiabatic heating plays an important role in the energy budget by transferring the most energy of TAD. An anti‐phase relationship between adiabatic and conduction heating is found in the propagation of these TADs. An in‐phase relationship between adiabatic and advection heating is also found. Physical processes behind these anti‐phase and in‐phase relationships are illustrated with a schematic. Finally, based on the dominant terms and the relationships between them, the whole process of generation, propagation, and dissipation of TAD is given. Plain Language Summary: Wave is a prevalent phenomenon in the atmosphere. They can transfer the energy and momentum from where they are generated to where they arrived. Traveling atmospheric disturbance (TAD) accompanied by gas expansion and compression is a specific wave type in the atmosphere. TAD often has a huge spatial scale of hundreds or even thousands of km, and it can propagate from one hemisphere to another hemisphere or even travel an entire circle around the earth. Many observations about TAD rise a challenge in explaining the mechanism of TAD's generation, propagation, and dissipation. This paper uses a physically based model to study TADs. We use the parameters calculated by the energy equation of the model to study the relatively effective heating processes in the energy budget of TAD. The relatively effective processes are found and relationships between them are shown. Further analysis and explanations are given to understand the relationships between these physical processes. Finally, the whole process from the generation of TAD to the dissipation of TAD is given. Key Points: Model‐data comparisons are used to understand the behavior of traveling atmospheric disturbance (TAD) during a geomagnetic storm on 7–8 September 2017A continental‐scale TAD/traveling ionospheric disturbance pair is observed simultaneouslyThe roles the dominant terms played during the propagation of TAD are given [ABSTRACT FROM AUTHOR]

The fast Van Allen radiation belt electron dynamics during geomagnetic storms have not yet been fully explained, in part due to limitations of standard satellite missions in both orbit and the number of spacecraft. Here we overcome these limitations using measurements from the Global Positioning System (GPS) constellation during an acceleration event on 26 August 2018. We show that the acceleration of relativistic electrons occurs in two distinct bursts, each dominated by a different acceleration mechanism. The first burst enhances the radiation belt electrons by four orders of magnitude in 2 hr and is consistent with ULF‐wave radial diffusion. The second burst is likely caused by the local acceleration and delivers an order‐of‐magnitude increase in 20 min. This work demonstrates how distributed, operational measurements can be used to resolve phenomena not observable with previous capabilities, and that rapid energization of the radiation belt can occur much faster than previously reported. Plain Language Summary: In this paper, we present a detailed analysis of terrestrially‐trapped electron space radiation during the August 2018 geomagnetic storm. This event is characterized by a very fast enhancement in the trapped electron population that increases particle counts by more than a factor of a thousand over only 6 hr. Such fast dynamics cannot be resolved by typical survey missions due to their long orbital periods. We instead use measurements from 20 satellites in the Global Positioning System (GPS) constellation, which allows us to perform an analysis of the space radiation dynamics on much shorter timescales. These GPS data reveal that the fast enhancement during the August 2018 storm occurred in two distinct bursts. By introducing a novel technique for GPS particle data analysis, we also determine that each of the bursts is governed by different physical processes that act on different timescales. The revealed fast dynamics of near‐Earth trapped radiation point toward a need to reevaluate the classic paradigm that the changes in the radiation levels are slow and can be revealed by surveys with a low number of spacecraft. Indeed, we foresee a critical role for constellation measurements, such as from GPS, in the future of radiation belt science. Key Points: Constellation measurements of the Van Allen radiation belt electrons can be used to reveal fast nonadiabatic changes at sub‐orbit timescalesElectron acceleration during the August 2018 storm consists of two distinct acceleration bursts governed by different physical processesULF‐wave radial diffusion and local acceleration can significantly alter radiation belt electron content on timescales of minutes to hours [ABSTRACT FROM AUTHOR]

The impact of geomagnetic storms on the mesosphere temperature has been controversial and lacks direct observational evidence. The intricate chemical and physical processes in the mesosphere, combined with the scarcity of observations, pose challenges to achieving a thorough comprehension of storm‐induced turbulences in this region. Currently, some investigations have characterized temperature responses during geomagnetic storms and the focus has largely been on changes above mesopause (∼90 km). In this work, the responses of temperature in the mesosphere (75–95 km) to the storms on September 8 and 15, 2003 and its causes are studied using observations from the Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) instrument onboard the Thermosphere Ionosphere Mesosphere Energetics and Dynamics (TIMED) satellite. It is found that (a) temperature responses for the moderate storm on September 15 manifest as increases within the latitude range of 80°S to 65°S, with peak values decreasing from approximately 15 K to around 7 K as latitude decreases, while for the minor strom of September 8 temperature changes only occur at ∼80°S with peaks of 7 K and −10 K, (b) the temperature responses can be transmitted down to 76–81 km, depending on the latitude and storm magnitude, and (c) there are significant fluctuations in both ozone (exceed 45%) and atomic oxygen (exceed 90%) after storm onset, highly correlated with temperature temporal and spatial variations. We suggest that the increases in ozone caused by the increases in atomic oxygen concentrations are the major contributor to rising temperature. Plain Language Summary: Geomagnetic storms may have a non‐negligible effect on the atmospheric circulation, neutral temperature, winds, and composition of the mesosphere and lower thermosphere. However, there is a lack of understanding of the perturbations and its chemical and physical processes in the mesosphere during geomagnetic storms. In this paper, we analyze temperature, ozone, and atomic oxygen observations acquired from the SABER instrument aboard the TIMED satellite. Our work demonstrate that variations in mesosphere temperature occur at high latitudes and are correlated with the magnitude of the storm. Additionally, significant disturbances in ozone and atomic oxygen are observed from 80°S to 65°S latitude. We propose that the variations in mesosphere temperature at high latitudes are dominated to changes in ozone caused by the heightened concentrations of atomic oxygen. Key Points: Temperatures of the mesosphere observed by SABER have remarkable responses to the geomagnetic storms of 8 and 15 September 2003The mesosphere temperature variations occur at high latitudes and can be conveyed to 76–81 km, depending on the latitude and storm magnitudeOzone variations are responsible for the mesosphere temperature variations at high latitudes during storms [ABSTRACT FROM AUTHOR]

The paper underlines the view that the appearance of beading and its nonlinear growth in the onset arc occurs independently from the onset of reconnection in the tail at about 20 RE. Both events follow from an extreme thinning of the central current sheet of the tail at the end of the growth phase. Subsequently, we concentrate on the processes connected with the onset arc breakup. Its origin lies in the instability of a high‐beta plasma layer building up at the outer boundary of the dipolar magnetosphere during the substorm growth phase, the growth phase arc (GPA) being the ionospheric trace. The observation of auroral streamers triggering the onset arc instability lets us analyze what is known about auroral streamers with strong support from high‐resolution videos of two substorm onsets. We conclude that they may be low‐entropy content bubbles with a balanced field‐aligned current system, framing a flow channel. However, there are unresolved questions. The visible streamer is identified as an Alfvénic arc. In searching for a mechanism by which a streamer bubble lining up along the GPA can trigger the instability, we are led to the recognition that an entirely new non‐MHD process must be at work. Taking also into account the surprising fact that the beads are moving oppositely to the convection in GPA and auroral streamer, we postulate the appearance of a new current system in the gap between the two. What happens can be described as the mating of two current sheets, which were completely separated before. It breaks the stability of the high‐beta plasma layer and channels the release and conversion of free internal energy. For this reason, we name the process mating instability. A physical analysis of this process shows consistency with detailed features exhibited by the two videos Plain Language Summary: The substorm begins with two independent events of common origin. The first one is normally the brightening of the growth phase arc, which is the trace of a hot plasma layer forming at the inner border of the tail. The second one is due to flow bursts, emerging from reconnection in the tail, arriving at the inner edge of the tail, the very reason for the substorm. The common origin is the thinning of the tail current sheet due to the stretching by the solar wind. The paper is devoted to the understanding of the first event. As observed about 10 years ago, the brightening of the growth phase arc with beadlike structures and growth of instability is often triggered by a weak arc, an auroral streamer, arriving from high latitudes and lining up with the growth phase arc. The paper proposes that the trigger process involves the formation of a new current circuit between the two arcs by the mating of the neighboring current sheets, which involves a non‐MHD process. It generates the sudden appearance and motion of the beads and constitutes a channel for the outflow of internal energy of the high‐energy plasma. Key Points: Growth phase arc brightening and instability and reconnection in the near‐Earth tail are completely separate processesAuroral streamers may be low‐entropy content bubbles with a flow channel attached and manifested by an Alfvénic arcMating of two unconnected current sheets by a non‐MHD process creates a channel for outflow of energy from the high‐beta plasma layer [ABSTRACT FROM AUTHOR]

In this paper, the ionospheric quiet‐time disturbances otherwise known as Pre‐Magnetic Storm Signatures (PMS) have been studied using the F2‐layer peak electron density (NmF2) data obtained from 12 Digisonde/ionosonde stations distributed across equatorial, low and mid‐latitudes. The datasets used spans the years 2010–2012. Results from this study reveals strong PMS in NmF2 with percentage deviations (ΔNmF2) ranging from −91% to 500% at the equatorial, low and mid‐latitudes, with maxima occurring at the equatorial region. Significant effects on the peak height of the F2‐layer (hmF2) were also observed, and they are correlated with the variations in NmF2 particularly at the equatorial station during the PMS. The duration of a PMS is found to be 12–48 h. Although, it was difficult to state clearly the connection between the PMS and the geomagnetic storm that usually follows within 24–48 h; but the NmF2 and hmF2 responses during the PMS were quite similar to those observed during geomagnetic storms. A slight increase in the Solar‐wind‐plasma speed (>20 km/s) was also observed during PMS. The PMS occur under a southward IMF‐Bz, moderate aurora activity (AE ranging from 114 to 560 nT) and quiet ring current (Dst >−10 nT). Therefore, it is pertinent to consider a certain threshold of the aurora indices (AE, AL, and AU) in addition to the Dst, ap, and Kp in the definition of a geomagnetically quiet day. This may eliminate the ambiguity in explaining the ionospheric variability that occurs few days before Sudden Storm Commencement (SSC). Key Points: Pre‐Magnetic Storm Signatures (PMS) on NmF2 were observed to be maximum at the equatorial ionosphereIonospheric Responses during PMS were observed to be similar to the effect of geomagnetic stormsThere is a need to consider a certain threshold of the aurora indices (AE, AL, and AU) in the definition of a geomagnetic quiet day [ABSTRACT FROM AUTHOR]

This paper reports a large‐scale magnetospheric line radiation (MLR) event during a moderate geomagnetic storm on 11 September 2018, which was well recorded by the China‐Seismo‐Electromagnetic Satellite (CSES) in the upper ionosphere. The event shows a symmetrical propagation feature at the conjugated locations between the two hemispheres, exhibiting a large spatial extension roughly from the latitudes 54°N to 53°S. The parallel structures are visible both in the electric and magnetic fields at a frequency band ranging from the local proton cyclotron frequency to ∼1.6 kHz. The wave intensity of parallel spectral lines was primarily enhanced in high latitude regions, gradually weakening at mid‐low latitudes, and then got absorbed in the equatorial region, presenting a distinct V‐shaped structure. The frequency spacings between neighboring spectral lines roughly vary from ∼80 to 110 Hz at the high latitudes and ∼80–130 Hz at the low latitudes, suggesting a slight variation feature with latitude. The parallel spectral structures of MLR drift between ∼0.39 and 0.57 Hz/s at high latitudes and ∼0.18–0.19 Hz/s at low latitudes. The wave vector analysis shows that the MLR waves are right‐hand polarized, obliquely propagating toward the Earth and in the azimuthal direction, where the Poynting flux is primarily oriented perpendicular to the ambient magnetic field. The other large‐scale MLR events all exhibit similar parallel structures and polarization characteristics, suggesting the universality of such a phenomenon. However, the azimuthal angles differ among different events, showing complex features. Plain Language Summary: Magnetospheric line radiation (MLR) is a unique electromagnetic wave distinguished by parallel spectral lines. This study reports a large‐scale MLR event that occurred in the dayside ionosphere. The event shows a symmetrical propagation feature, with a large spatial extension between latitudes 54°N and 53°S. The parallel structures are visible both in the electric and magnetic spectrogram, ranging from the local proton cyclotron frequency to ∼1.6 kHz. The MLR structures were primarily enhanced in high latitude regions, gradually weakening at mid‐low latitudes, and then got absorbed in the equatorial region, presenting a distinct V‐shaped structure. The frequency spacings of MLR roughly vary from ∼80 to 110 Hz in the high latitudes and from ∼80 to 130 Hz in the mid‐low latitude region, slightly varying with latitude. The MLR structures drift between ∼0.39 and 0.57 Hz/s at high latitudes and ∼0.18–0.19 Hz/s at low latitudes. This MLR event is right‐hand polarized, obliquely propagating toward the Earth and in the azimuthal direction, and the Poynting flux is primarily oriented perpendicular to the ambient magnetic field. However, the azimuthal angles differ among different events, indicating the complexity of the wave propagation feature. Key Points: A large‐scale magnetospheric line radiation (MLR) event shows a symmetrical propagation feature in two hemispheres, presenting a distinct V‐shaped structureBoth the frequencies of the parallel spectral lines and their frequency spacings slightly drift with latitudesThe MLR waves are right‐hand polarized, obliquely propagating toward the Earth and azimuthal direction [ABSTRACT FROM AUTHOR]

This paper conducts a multi‐instrument analysis of a latitudinal plasma density peak at the middle latitudes during the early recovery phase of the April 2023 geomagnetic storm. The total electron content (TEC), peak density of the F layer, and the in situ plasma density from Swarm and Defense Meteorological Satellite Program (DMSP) satellites all capture this peak feature. This narrow latitudinal peak structure appeared around 50°N and extended from 40°E to 150°E in longitude with a prolonged duration of about 8 hr from sunset to midnight. This mid‐latitude peak reveals a noticeable equatorward motion and a slight westward shift. According to the plasma composition observations from DMSP satellites, this peak structure shows an O+ ions dominance, which means that this peak is more likely to be formed by an internal rather than an external source from the plasmasphere. Meanwhile, the middle latitude Fabry–Perot interferometer (FPI) observed strong equatorward thermospheric winds, and the peak height of the F layer presented a visible elevation, which suggests that the equatorward wind lifting caused the plasma density enhancement. Besides, the O/N2 ratio significantly decreased at lower and middle latitudes, and ion drift observations showed a distinct subauroral westward channel. Based on these simultaneous measurements, this structure's sharp equatorward and poleward boundaries might be related to the O/N2 ratio change and the subauroral polarization stream (SAPS) flow separately. Plain Language Summary: The ionospheric response to geomagnetic storm disturbances exhibits diverse plasma density structures. During the early recovery phase of the April 2023 geomagnetic storm, a distinct latitudinal plasma density peak is observed at middle latitudes. The formation of the mid‐latitude plasma density peak structure is not settled yet, even though some homologous structures have been reported. This study investigates the spatial and temporal features of this mid‐latitude peak structure and the dominant ion composition. The investigation is achieved by utilizing the total electron content, F layer parameters, and in situ plasma observations. Furthermore, the observations of neutral wind, thermospheric composition, and ion drift observations show a good correlation with the spatial‐temporal characteristics of this peak structure. These clues suggest that the formation of the mid‐latitude peak structure during the April 2023 geomagnetic storm may be attributed to a combined effect involving equatorward neutral winds, thermospheric composition change, and subauroral polarization stream flow. Key Points: A mid‐latitude peak is present in total electron content, F layer peak density, and in situ plasma density during the storm recovery phaseThe mid‐latitude peak covers 40°–150°E in longitude with a noticeable equatorward motion and slight westward shiftThe peak structure may be associated with the thermospheric equatorward wind, composition change, and subauroral polarization stream [ABSTRACT FROM AUTHOR]

The recalibrated FAST/TEAMS data is used to study the response of O+ and H+ outflow to energy inputs in the nightside aurora during the 24–25 September 1998 geomagnetic storm, the same storm studied by Strangeway et al. (2005), https://doi.org/10.1029/2004JA010829. In contrast to the cusp, the Poynting flux and electron precipitation energy input are not as well correlated on the nightside, so their effects on outflow can be differentiated. The O+ outflow shows a strong correlation with both the Alfvénic Poynting flux (r = 0.71) and the soft electron precipitation (r = 0.69), while the H+ outflow only correlates well with the electron number flux (r = 0.74). This indicates that the auroral H+ outflow is close to its limiting flux without additional wave acceleration, while the outflow for the heavier O+ ion is increased by additional wave acceleration. Plain Language Summary: Geomagnetic activity can cause electrons to precipitate into the ionosphere in the nightside auroral region. It can also deliver electromagnetic wave energy to the same region. The electron precipitation can both heat and further ionize the ionosphere, leading to ions moving up along the field line. The wave energy can further accelerate the ions. If the ions are accelerated enough by these processes, they will flow out along the magnetic field, escaping the ionosphere. This paper finds that the H+ outflow increases with increased precipitating electrons in the nightside aurora. The O+ outflow increases with both electron precipitation and wave acceleration. Key Points: Electron precipitation and Poynting flux are less correlated on the nightside than the cusp, so their effects on outflow can be distinguishedO+ outflow is correlated with both Poynting flux and electron precipitation, while H+ is only correlated with electron precipitationParameterization of the outflow dependence is consistent between the dayside cusp and the nightside auroral regions [ABSTRACT FROM AUTHOR]

The magnetopause is treated as a boundary where the pressure of the incoming solar wind is balanced by the pressure of the geomagnetic field and the plasma pressure inside the magnetopause is often neglected. However, published studies of pressure balance at the magnetopause reveal an excess of the magnetosheath pressure. Moreover, our survey of about 50,000 THEMIS magnetopause crossings shows that about 1% of them exhibits even larger magnetic field in the magnetosheath than in the magnetosphere. A subsequent analysis of crossings observed in the subsolar region under a southward interplanetary magnetic field shows the pressure of the dense cold ion population as an important component of the total magnetospheric pressure. This component is too cold to be registered by standard ion analyzers but its density can reach 300 cm−3. The second effect connected with a presence of cold plasma population is a reduction of the reconnection rate that slows down the transport of the magnetic flux down the tail and leads to magnetic pile‐up in the subsolar magnetosheath. Plain Language Summary: The magnetopause is usually treated as a boundary where the pressure of the incoming solar wind is balanced by the pressure of the geomagnetic field and the plasma pressure inside the magnetopause is often neglected. However, we have found a large number of subsolar magnetopause crossings exhibiting larger magnetic field in the magnetosheath than in the adjacent magnetospheric region. Taking into account the contribution of the magnetosheath plasma, the pressure balance at the magnetopause seems to be violated. Statistical analysis revealed that these events are observed predominantly during geomagnetic storms when the cold plasma from the plasmasphere comes toward the magnetopause. This plasma is too cold to be registered by the ion spectrometers but its density is sufficient to keep the pressure balance across the magnetopause as we demonstrate in two case studies. Our analysis explains contradiction that can be found in several already published papers pointing out a lack of the magnetospheric pressure at the magnetopause. Key Points: About 1% of subsolar magnetopause crossing exhibits larger magnetic field in the magnetosheath than in the magnetosphereSuch magnetopause configuration is typical for periods of geomagnetic stormsThe cold dense plasma of plasmaspheric origin was identified as a component keeping the pressure balance across the magnetopause [ABSTRACT FROM AUTHOR]

This paper analyses magnetosphere‐ionosphere (M‐I) coupling from a perspective that is independent of inertial reference frame, and delineates how physical theories of M‐I coupling are affected by the principle of relativity. For the first time in the context of M‐I coupling, we discuss the literature from the 1970s on how the low‐velocity limit of the theory of special relativity is applied to electrodynamics. In most M‐I coupling theories, a particular low‐velocity limit applies, known as the "magnetic limit." Two important consequences of this literature are: (a) significant displacement currents in Maxwell's equations break the Galilean invariance of the equations and (b) magnetic fields are not generated by currents created by a net charge density in motion. We show how reference frame‐independent descriptions of M‐I coupling require that ion‐neutral relative velocities and ion‐neutral collisions are key drivers of the physics. Currents are independent of reference frame whereas electric fields depend on reference frame. Starting with the same momentum equations that are typically used to derive Ohm's law, it is possible to express the perpendicular ionospheric current as depending on collisions between ions and neutrals, and electrons and neutrals, without reference to electric fields. Ignoring the relative motion between ions and neutrals results in errors exceeding 100% for estimates of high latitude Joule heating during significant geomagnetic storms, when ion‐neutral velocity differences are largest near the initiation of large‐scale ion convection. Plain Language Summary: Interactions between the magnetized and ionized solar wind, the magnetospheric cavity surrounding Earth, and Earth's ionized upper atmosphere (ionosphere) can create rapid (∼1 km/s) large‐scale motions of the ionosphere during periods known as geomagnetic storms. We use the principle of relativity (PR) to gain insight into the complex physics of these interactions. Relativity states that the physics governing geospace must be independent of the velocity of an observer making measurements of the system. We write key equations governing interactions of the magnetosphere‐ionosphere system in terms of quantities that do not depend on the observer's motion. In doing so, we find that some previous theories had over‐emphasized the importance of a large‐scale electric field that grows during storms, while neglecting important physics related to collisions between the ionized portion of the atmosphere and the un‐ionized "neutral" component that contains much more mass. These collisional interactions create upper atmospheric heating and expansion, and cause large‐scale currents to flow between the ionosphere and magnetosphere, resulting in a multitude of impacts to our technological society. Using the PR, and isolating the physics that is independent of observer motion, led us to a deeper understanding of key physical processes during storms. Key Points: Relativistic transformations applied to electrodynamics are analyzed in the context of magnetosphere‐ionosphere (M‐I) couplingWe present an "Ohm's law" relating horizontal ionospheric currents to quantities that do not vary with inertial reference frameElectrodynamic theories of M‐I coupling that do not account for the relative motion of ions and neutrals are not quantitatively accurate [ABSTRACT FROM AUTHOR]

Mirror modes (MMs) are ubiquitous in space plasma and grow from pressure anisotropy. Together with other instabilities, they play a fundamental role in constraining the free energy contained in the plasma. This study focuses on MMs observed in the solar wind by Solar Orbiter (SolO) for heliocentric distances between 0.5 and 1 AU. Typically, MMs have timescales from several to tens of seconds and are considered quasi‐MHD structures. In the solar wind, they also generally appear as isolated structures. However, in certain conditions, prolonged and bursty trains of higher frequency MMs are measured, which have been labeled previously as MM storms. At present, only a handful of existing studies have focused on MM storms, meaning that many open questions remain. In this study, SolO has been used to investigate several key aspects of MM storms: their dependence on heliocentric distance, association with local plasma properties, temporal/spatial scale, amplitude, and connections with larger‐scale solar wind transients. The main results are that MM storms often approach local ion scales and can no longer be treated as quasi‐magnetohydrodynamic, thus breaking the commonly used long‐wavelength assumption. They are typically observed close to current sheets and downstream of interplanetary shocks. The events were observed during slow solar wind speeds and there was a tendency for higher occurrence closer to the Sun. The occurrence is low, so they do not play a fundamental role in regulating ambient solar wind but may play a larger role inside transients. Plain Language Summary: Plasma strives to be in equilibrium with little to no free energy. However, this is often not the case, especially in close proximity to complex structures such as shock waves and interplanetary coronal mass ejections (ICMEs). The latter is an eruption of plasma from the Sun that propagates outward into the solar system. In the presence of some free energy, instabilities will arise to remove it, one example is the mirror mode (MM) instability. Instabilities such as these are of extremely high importance to plasma physics as they act as a feedback mechanism to the plasma. Nevertheless, there are many open questions regarding the MM instability, especially when their properties are different from the most common scenarios. Typically, MMs in the solar wind appear as dips that are isolated structures. However, this paper investigates MMs when they appear as sudden bursts of magnetic peaks and dips and typically have smaller temporal scales. These kinds of MMs have been called MM storms. This study aims to address at what distances from the Sun they arise, what types of solar wind structures they are associated with, quantify their physical properties, and understand what local plasma conditions are important. Key Points: Mirror mode (MM) storms predominantly occurred during slow solar windHeliospheric plasma sheet crossings were effective at setting up MM unstable conditionsSpatial scales of MM structures approached and were smaller than ion‐scales [ABSTRACT FROM AUTHOR]

The occurrence and intensity of the geomagnetic storms/activity in the weak solar cycle 24 (SC24) were studied mainly for low latitudes, with intensity being the maximum value of the activity. The impulsive strength of the activity giving its mean value during the main phase, which can better indicate the effect of the activity on utility systems, has not received much attention at any latitude. In this paper, we investigate the intensity and impulsive strength of the 179 and 85 clear geomagnetic activities (DstMin ≤ −50 nT) identified in the low, mid and high latitude indices (SymH, Kp, and AE) and corresponding solar wind velocity V, IMF Bz, and the product V × Bz in solar cycles 23–24 (1996–2019) for the first time. Compared to SC23, the total intensity and total impulsive strength in SC24 in all latitudes reduce by nearly equal amounts (∼60%) as the reduction in the number of activities (∼53%). The average intensity and average impulsive strength, however, reduce by nearly equal and largest amounts in low latitudes (∼23%), which is close to the reduction in the combination <−(V × Bz)MP > by ∼30%. At mid and high latitudes, the average intensity and average impulsive strength reduce by only small amounts (∼7.5%). Only the impulsive strength of the geomagnetic activity at low latitudes (IpsSymH) and the combination <−(V × Bz)MP> identify the two power outages happened in SC23. The correlation between IpsSymH and <−(V × Bz)MP> is also high (0.83) in SC23. [ABSTRACT FROM AUTHOR]

The gravity‐driven and pressure‐gradient currents coexist in the ionosphere, and their effects are significant in there, rather than at the outside of the ionosphere; and can be important while studying the ionospheric currents using low‐Earth‐orbiting (LEO) satellite measurements. Maute and Richmond (2017, https://doi.org/10.1002/2017JA024841) (MR17) have demonstrated that above the F region peak, directions of these two coexisting currents are opposite and the net magnetic effects along the ambient magnetic field are nonsignificant. In the view of the diamagnetic corrections being applied to the LEO magnetic field measurements to account for the pressure‐gradient currents, it is imperative to examine the proposition of MR17. In the present paper, we have estimated the gravity and pressure‐gradient currents, using altitude profiles of electron density obtained from the Constellation Observing System for Meteorology, Ionosphere, and Climate (COSMIC)‐1 satellite cluster. In order to get the latitudinal profiles of magnetic field variations at a fixed local time (LT) using COSMIC data, it is required to combine either different days at a fixed longitude or all the longitudes on a fixed day, thus compromising with either days or longitudes. It is found that the net magnetic field is significant in the low‐latitude region, which increases with solar flux and decreases with altitude. The magnetic field effects show strong LT dependence and are significant in the noon to evening sector. The comparison of the present estimates with the diamagnetic corrections emphasizes that correcting for only one current can introduce unviable errors and thus supports the suggestion of MR17. Plain Language Summary: In addition to the ionospheric‐dynamo currents, the currents due to gravity and plasma pressure‐gradients flow in the Earth's ionosphere, whose contribution in the ground magnetic field measurements is negligible compared to that of ionospheric‐dynamo. However, considering the magnetic field due to gravity and pressure‐gradient currents present in the ionosphere may be important while studying the ionospheric currents using low‐Earth‐orbiting (LEO) satellite measurements. Above the F region peak where LEO satellites generally fly, the directions of these two coexisting currents are opposite and the net magnetic field along the ambient magnetic field is nonsignificant. In the view of the diamagnetic corrections being applied to the LEO magnetic field measurements to account for the pressure‐gradient currents, it is imperative to compute the magnetic field effects of these currents using actual observations of ionospheric electron densities. In the present paper, we have estimated the gravity and pressure‐gradient currents, using altitude profiles of electron density obtained from the Constellation Observing System for Meteorology, Ionosphere, and Climate (COSMIC)‐1 satellite cluster. The magnetic variations due to these currents are estimated at different heights, latitudes, solar fluxes, and local times. Key Points: COSMIC data have limitations in obtaining good latitudinal coverage of plasma densities on a given day at a fixed local time and longitudeThe magnetic field variations due to gravity and pressure‐gradient currents increase with solar flux and reduce with altitudeThese effects are stronger at equator to low‐latitude region and in the noon to evening sector and negligible in other local time sectors [ABSTRACT FROM AUTHOR]

The main question addressed in this study is whether particles with relativistic energy can be injected directly into the inner magnetosphere by very near‐Earth reconnection during magnetic storms. We study a sequence of events observed in the solar wind, in the magnetotail, at geosynchronous orbit (GEO), in the inner magnetosphere, and in the ionosphere during the main phase of a geomagnetic storm on 16 June 2012–17 June 2012. The storm was caused by a magnetic cloud with a high dynamic pressure and a strong southward IMF lasting about 10 hr. These conditions caused an extreme compression of the magnetosphere (SymH reached ∼150 nT) and an enhancement of the lobe magnetic field strength to ∼90 nT at R ∼ 10 Earth radii (RE). We focus on an hour‐long interval between 1050 and 1150 UT on 17 June 2012 when the Time History of Events and Macroscale Interactions during Substorms (THEMIS) satellites were at apogee at geocentric distances R ≈ 12 RE near midnight. At that time a thin current sheet formed between cis‐GEO distances and THEMIS. This thin current sheet reconnected between GEO and THEMIS. A strong lobe magnetic field enabled ion and electron energization to energies E ≈ 600 keV. Fluxes of high energy (up to relativistic) particles at GEO increased within ≈20 s after reconnection onset detected by THEMIS. Fluxes of relativistic electrons at L ∼ 4 in the morning sector increased within about 600 s after reconnection onset. We interpret these observations as signatures of direct injection of reconnected particles into the inner magnetosphere. Plain Language Summary: Solar flares, coronal mass ejection events may cause severe disturbances in the near‐Earth space environment called magnetic storms. During a magnetic storm the Earth magnetosphere experiences quite dramatic changes that include rapid reconfiguration of magnetic field lines that leads to conversion of the magnetic energy to kinetic and thermal energy of the space plasma (reconnection) and acceleration of charged particles. The accelerated particles and plasmas from the outer magnetosphere may penetrate deeper into the near‐Earth space, where the magnetic field is close to a dipole. Particles become trapped in the dipole field and form an electric current that flows around the Earth (ring current) and reduce the magnetic field observed on the ground. Details of the ring current formation are not well understood yet. In this paper we study a magnetic storm during which reconnection happened very close to the dipole field and particles, energized via reconnection were directly injected into the inner magnetosphere. Events like this are potentially hazardous for satellites on geosynchronous orbit and in the inner magnetosphere. Key Points: We studied space‐ and ground‐based observations during a storm caused by magnetic cloud impactOur observations suggest that a thin current sheet formed and reconnected at L about 10Reconnection caused relativistic particle injections into the inner magnetosphere [ABSTRACT FROM AUTHOR]

This paper examines the response of dynamical complexity in traveling ionospheric disturbances (TIDs) across Eastern Africa sector during major geomagnetic storms. Detrended total electron content derived from eight stations of Global Positioning System receivers across Eastern Africa was used to unveil the transient features of dynamical complexity response in TIDs. Neural network entropy (NNetEn) was applied to the detrended TEC time series data to capture the degree of dynamical complexity. The NNetEn track the distinct features associated with the occurrence of TIDs. The results show that as the signatures of TIDs begin to emerge, low values of NNetEn signifying reduction in the degree of dynamical complexity response were observed while high values of NNetEn were depicted as the signatures of TIDs subsides signifying increase in the dynamical complexity response. Reduction in dynamical complexity response associated with the occurrence of TIDs is more evident in the Southern Hemisphere compared to Northern Hemisphere. Furthermore, we found that the propagation of TIDs is more prominent at Equinoctial season compared to solstitial season. The latitudinal observation of NNetEn revealed higher degree of dynamical complexity response in ADIS and NEGE signifying that the development of TIDs is minimal in ADIS and NEGE. Finally, the reduction in dynamical complexity associated with the occurrence of TIDs were obvious during all the phases of geomagnetic storms. In particular, the dynamical complexity response at initial and recovery phases of geomagnetic storm depicts more TIDs features. Key Points: As the signatures of traveling ionospheric disturbances (TIDs) begin to emerge, the dynamical complexity reduces while dynamical complexity increases as TIDs signatures subsidesDynamical complexity associated with TIDs features is more evident in the Southern Hemisphere compared to the Northern HemisphereThe propagation of TIDs is more prominent at Equinoctial season compared to the solstitial season [ABSTRACT FROM AUTHOR]

Electron injections are critical processes associated with magnetospheric substorms, which deposit significant electron energy into the ionosphere. Although wave scattering of <10 keV electrons during injections has been well studied, the link between magnetotail electron injections and energetic (≥100 keV) electron precipitation remains elusive. Using conjugate observations between the Electron Loss and Fields Investigation (ELFIN) and Magnetospheric Multiscale (MMS) missions, we present evidence of tens to hundreds of keV electron precipitation to the ionosphere potentially driven by kinetic Alfvén waves (KAWs) associated with magnetotail electron injections and magnetic field gradients. Test particle simulations adapted to observations show that dipolarization‐front magnetic field gradients and associated ∇B drifts allow Doppler‐shifted Landau resonances between the injected electrons and KAWs, producing electron spatial scattering across the front which results in pitch‐angle decreases and subsequent precipitation. Test particle results show that such KAW‐driven precipitation can account for ELFIN observations below ∼300 keV. Plain Language Summary: Energetic electron precipitation from magnetospheric injections has a major impact on magnetosphere‐ionosphere coupling. This energy deposition is largely in the form of electron precipitation driven by wave‐particle interactions in the magnetotail. Although wave‐driven precipitation with energies less than approximately ∼10 keV has been studied extensively, the link between energetic electron precipitation (≥∼100 keV) and electron injections remains elusive. Combining observations and simulations, this paper provides evidence of such precipitation driven by kinetic Alfvén waves, which have been previously observed to be ubiquitously associated with magnetospheric electron injections but have not been considered as an important driver for precipitation of such electrons. Key Points: Conjugate Electron Loss and Fields Investigationand Magnetospheric Multiscale observations reveal evidence of tens to hundreds of keV electron precipitation associated with kinetic Alfvén wavesDipolarized magnetic field gradients and perpendicular magnetic drifts allow Landau resonance between waves and injected electronsAdiabatic transport and spatially varying E × B and grad‐B drifts induce perpendicular momentum scattering and electron losses [ABSTRACT FROM AUTHOR]

The geomagnetic field is composed of a variety of sources that act on a wide range of timescales and amplitudes. The separation of magnetic storm effects from quiet variations is needed to accurately quantify impacts of space weather events. The extraction of such quiet contributions within geomagnetic measurements is achieved by the determination of baselines, which, ideally, is done by a simple algorithm which captures quiet sources suitably well, while being applicable to an extensive network of magnetic observatories independent of the period of time. In this work, we apply signal filtering techniques on the horizontal components of geomagnetic field measurements from low‐ and mid‐latitude observatories to determine baselines. The variations within the baseline are investigated for magnetically quiet periods between 1991 and 2019, focusing on long‐term trends, seasonal and local time dependencies, and day‐to‐day variability. The analysis confirms that the contributing quiet sources include the secular variation and the solar quiet (Sq) current system. The non‐negligible day‐to‐day variability, that is typical for Sq in low‐ and mid‐latitudes, is embedded within the baseline. Thus, the filter approach extracts quiet magnetic field variations well. Comparisons with other baseline methods show good agreements. We conclude that the filter approach can be used to determine baselines automatically during magnetically quiet periods without the need of further apriori information and is applicable on a wide network of magnetic observatories. It marks the first step for deriving magnetic indices for (near) real‐time space weather applications. Plain Language Summary: The Earth's intrinsic magnetic field is generated by the motion of molten rock within its interior and interacts with the constant flow of charged particles coming from the Sun. Measurements of the geomagnetic field strength on the surface not only include the intrinsic magnetic field but also phenomena that arise due to this interaction. Some of these phenomena show regular variations without major effects and some, like solar storms, are able to disrupt the geomagnetic field, affecting technological systems. In order to quantify how harmful disruptive events are, it is important to determine the regular variations first. In this paper, we determine the regular variations within the signal (baselines) by applying signal filtering techniques on geomagnetic field measurements. Our analysis shows that regular variations during undisturbed days in low‐ and mid‐latitude ranges are captured accurately. Key Points: A basic signal filtering approach is used to determine geomagnetic baselines during quiet periodsThe baselines capture the secular variation and solar quiet current systems accuratelyThe baseline method is applicable for low‐ and mid‐latitude magnetic observatories and does not require any apriori information [ABSTRACT FROM AUTHOR]

The objective of this paper is the development of a kinetic model for plasmasphere refilling following geomagnetic storms. The kinetic model is based on the "particle‐in‐cell" method, a method based on the simulation of particle motion and thus well suited to high altitude, low‐density regimes, where the plasma transport equations are not valid. The model was validated with exact, analytical benchmarks, which are provided in this paper. The refilling results obtained from the kinetic model were then compared with results from a recently developed hydrodynamic solution methodology based on the "flux‐corrected transport" method, and the limitations of hydrodynamic modeling for low‐density flow at high altitudes were explored. Plain Language Summary: The objective of this paper is the application of recently developed hydrodynamic and kinetic models to the plasmasphere refilling problem following geomagnetic storms. The refilling results for H+ ions were compared with a view to understanding the underlying physics. Key Points: Developed a kinetic model for the plasmasphere refilling problem following geomagnetic stormsCompared the kinetic results with results from a previously developed hydrodynamic modelThe limitations of hydrodynamic modeling at high‐altitude, low‐density regimes are explored [ABSTRACT FROM AUTHOR]

In this paper, currents causing the sudden commencement (SC), the AU/AL indices, and the positive bay during the substorm are identified from the global simulation and Biot‐Savart's law. Candidate currents assumed as causes of these ground magnetic variations are the ionospheric Hall current, the ionospheric Pedersen current, the field‐aligned current (FAC), and other magnetospheric currents than the FAC. In general, FAC effect and Pedersen current effect cancel out each other under the restriction of Fukushima's theorem. During the SC, for instance, the midlatitude preliminary positive impulse appears in the prenoon and midlatitude preliminary reverse impulse (PRI) appears in the postnoon, due to the remaining effect of the Hall current. However, violations of the Fukushima's theorem are also common such as in the cases of the equatorial PRI, the auroral electrojet, and the positive bay. The equatorial PRI caused by the Pedersen current appears both in the prenoon and postnoon regions. In the auroral region, the Hall current effect prevails over other currents so much and determines the AU/AL indices only from it regardless other currents. The midlatitude positive bay on the nightside is generated by the effect of the FAC. From these diverse reproduction of ground magnetic variations, a further verification is given for the global simulation in reproductions of the magnetosphere‐ionosphere coupling process. Key Points: Ground magnetic variations in the SC and the substorm are decomposed to four components by the simulation and Biot‐Savart's lawFukushima's theorem is violated for the equatorial PRI, the AU/AL indices, and the positive bay of the substormThe conductivity model of the global simulation is verified by comparisons between reproduced and observed ground magnetic variations [ABSTRACT FROM AUTHOR]

Interplanetary coronal mass ejections are at the center of the research on geomagnetic activity. Sheaths, highly fluctuating structures, which can be found in front of fast interplanetary coronal mass ejections, are some of the least known geoeffective solar transients. Using Morlet transforms, we analyzed the magnetic fluctuations in a list of 42 well‐identified and isolated magnetic clouds driving a sheath and shock (Masías‐Meza et al., 2016, https://doi.org/10.1051/0004-6361/201628571. We studied the fluctuations inside sheaths by defining two quantities: the power and the anisotropy. With a simple statistical approach we found that sheaths, in particular, those driven by a fast magnetic cloud, encountering a highly turbulent solar wind, and forming a high Alfvén Mach number shock have high levels of turbulent energy (∼10 times compared with the solar wind) as well as a low anisotropy (approximately halved compared with the solar wind) of their fluctuations. On the other hand, the effect of the shock angle and plasma beta in the solar wind are less straightforward: If the shock is quasi‐parallel or the beta in the solar wind is high, both the turbulent energy in the sheaths and the anisotropy of the fluctuations are reduced; but for quasi‐perpendicular shocks or low beta solar wind the turbulent energy and anisotropy can take any value. Plain Language Summary: Solar flares are sometimes linked with the emission of interplanetary structures, which may collide with Earth. When this happens, it can lead to temporary changes in the magnetic field of Earth and possibly affect human technology. These effects are a subset of what is known as geoeffectiveness. We do have an idea of which types of structures may or may not have consequences on Earth, and, for example, magnetic clouds are quite well known for their large impact on the Earth magnetic field; however, we still struggle to understand the consequences of some puzzling interplanetary structures called sheaths. These can often be found preceding a magnetic cloud when the latter is fast enough to generate a shock wave. We think that one of the reasons these sheaths keep having surprising effects on the Earth's magnetic field is because they, themselves, are not yet very well known. The present paper aims at characterizing one of the key properties of sheaths: their magnetic fluctuations, that is, the rapid temporal variation of the magnetic field. We found that those fluctuations are indeed quite particular in sheaths: They have markedly more energy than in the usual solar wind (about 10 times more) and tend to change direction all the time. Conversely, in the solar wind, some directions seem to be privileged and the energy is relatively low. In this paper, we also show that these particular properties of the magnetic fluctuations are all the more pronounced when the magnetic cloud driving the sheath is moving faster and when the solar wind in front of the sheath already has strong fluctuations and magnetic pressure. This work gives us a better insight into the dynamics of the sheath, which may eventually improve our understanding of their geoeffectiveness. Key Points: The fluctuations in sheaths have increased power (∼10 times) and compressibility (∼2 times) compared to the solar wind'sThose characteristics depend on magnetic clouds' speed, preexisting fluctuations in the solar wind, and shock's parameters [ABSTRACT FROM AUTHOR]

The geospace plume, referring to the combined processes of the plasmaspheric and the ionospheric storm‐enhanced density (SED)/total electron content (TEC) plumes, is one of the unique features of geomagnetic storms. The apparent spatial overlap and joint temporal evolution between the plasmaspheric plume and the equatorial mapping of the SED/TEC plume indicate strong magnetospheric‐ionospheric coupling. However, a systematic modeling study of the factors contributing to geospace plume development has not yet been performed due to the lack of a sufficiently comprehensive model including all the relevant physical processes. In this paper, we present a numerical simulation of the geospace plume in the 31 March 2001 storm using the Multiscale Atmosphere‐Geospace Environment model. The simulation reproduces the observed linkage of the two plumes, which, we interpret as a result of both being driven by the electric field that maps between the magnetosphere and the ionosphere. The model predicts two velocity channels of sunward plasma drift at different latitudes in the dusk sector during the storm main phase, which are identified as the sub‐auroral polarization stream (subauroral polarization streams (SAPS)) and the convection return flow, respectively. The SAPS is responsible for the erosion of the plasmasphere plume and contributes to the ionospheric TEC depletion in the midlatitude trough region. We further find the spatial distributions of the magnetospheric ring current ions and electrons, determined by a delicate balance of the energy‐dependent gradient/curvature drifts and the E × B drifts, are crucial to sustain the SAPS electric field that shapes the geospace plume throughout the storm main phase. Key Points: The first whole geospace simulation to demonstrate coherent storm‐time evolution of plasmaspheric and total electron content (TEC) plumesThe model demonstrates plasmasphere erosion and TEC depletion by the subauroral polarization streams (SAPS)SAPS is sustained by magnetospheric ion and electron distributions formed by a delicate balance of energy‐dependent and E × B drifts [ABSTRACT FROM AUTHOR]

We present a new model of the near‐Earth magnetospheric field produced by electric currents in the inner magnetosphere and the associated induced magnetic field. The model is designed to track hourly variations of these fields and accounts for their local time asymmetries. It is built by applying spherical harmonic analysis to vector measurements from the ground observatory network at low and mid‐latitudes. The primary and induced fields are separated with an approach in the time domain that uses a a priori radially‐symmetric electric conductivity model of the Earth. The model coefficients are computed at one‐hour time steps between 1997 and 2022. This model is shown to be consistent to within a few nT with previously developed indices which track the magnetospheric ring current. It is also validated against data from the Swarm, CHAMP and Øersted satellites. The fit to satellite data is comparable to that of the CHAOS‐7.15 model for geomagnetically quiet times, and improved by up to 20% on some components for geomagnetically moderate and active times. We attribute these differences mostly to a better representation of local time asymmetries, both on average and during individual geomagnetic storms. This model can be used in various applications, such as investigating the properties of the magnetospheric field and its sources and separating the magnetospheric field from the fields of other sources in geomagnetic field modeling. Plain Language Summary: Geomagnetic field modeling aims at building data‐based mathematical representations, or models, of the various contributions to the total Earth's magnetic field measured at or near the Earth's surface. One of such contributions is the magnetic field generated by electric currents in the inner magnetosphere, including the so‐called ring current. In this paper, we present a new model of the near‐Earth magnetic field generated in the inner magnetosphere based on data collected in ground magnetic observatories. The model covers the 1997–2022 time span, includes improved representations of the local time asymmetries of the field and of the effect of electrical induction in the Earth's mantle, and is validated against data collected in low Earth orbits by the Swarm, CHAMP and Oersted satellites. We find that the model provides an improved representation of the magnetic field generated in the inner magnetosphere during periods of moderate and high geomagnetic activity, including magnetic storms. Key Points: A new ground‐data based model of hourly variations of the primary inner near‐Earth magnetospheric and associated induced field is presentedComparison with satellite data shows up to 20% performance improvement compared to the CHAOS‐7 model for moderate and active timesThe model accounts for variations of the field with local time and can be used to study geomagnetic storms [ABSTRACT FROM AUTHOR]

Ionospheric responses to geomagnetic storms can cause irregular plasma structures and scintillation of Global Navigation Satellite System (GNSS) signals. In this paper, we investigate time lags between the detection of GNSS signal scintillation at northern hemisphere auroral latitudes and the onset of 15 geomagnetic storms that occurred in 2015–2017. The results show that the time lags between the detection of ground‐based GNSS scintillations and the observed sudden change in solar wind parameters are between tens of minutes and 15 hr. This time lag consists of two segments. The first segment is about 30–80 min, which is the lag between observed disturbances in the geomagnetic field and solar wind disturbances detected by orbiting spacecrafts around the L1 point. The second segment is between the observed GNSS signal scintillation and the storm sudden commencement (SSC). This second lag segment varies in the range of about 10–830 min and highly depends on the storm onset time and geomagnetic locations of GNSS signal propagation paths. Longer time lags over 450 min were observed with the signal ionospheric piercing point at 60–70° geomagnetic latitudes (MLAT) on the dayside, while shorter lags of 10∼450 min were observed with the signal IPPs at 68–81° MLAT and on the nightside at the time of SSC. The lag time variations can be explained by ionospheric irregularity production and transport processes associated with a variety of auroral and polar cap phenomena in response to solar wind coupling to the magnetosphere and ionosphere. Key Points: Time lags between storm sudden commencement and scintillation detections by ground‐based Global Navigation Satellite System receivers at northern auroral latitudes were surveyedThe time lags varied with geomagnetic latitude and magnetic local time of satellite‐receiver line‐of‐sight signal paths assumed at ionospheric piercing pointsShorter time lag was detected by the observations close to aurora cusp as well as those on the nightside at the storm onset [ABSTRACT FROM AUTHOR]

Numerical simulations using the National Center for Atmospheric Research (NCAR) thermosphere‐ionosphere‐electrodynamics general circulation model (TIE‐GCM) are performed to elucidate the effects of the interplanetary magnetic field (IMF) on the middle thermosphere composition during a "geomagnetically quiet" period from the day of year (DOY) 110–111 in 2019 (when the Auroral electrojet (AE) index never exceeded 300 nT and the Kp never exceeded 2). In particular, this paper aims to examine how the Global‐scale Observations of the Limb and Disk (GOLD) mission observed daytime thermospheric O and N2 column density ratio (∑O/N2) depletion at mid‐latitudes originated under such a "geomagnetically quiet" condition. A comparison of electric potential, Joule heating rate per unit mass, ion velocity, neutral temperature and winds in the middle thermosphere (∼160 km) between real IMF and without the IMF east‐west component (By) indicates that a By dominant condition can enhance their strengths under this "geomagnetically quiet" condition. Consequently, ∑O/N2 depletion with a stronger magnitude (30% compared with ∼8% without By) and larger disturbed area was introduced in the post‐midnight sector at high‐latitudes due to strong and localized upwelling associated with the enhanced Joule heating rate per unit mass. The ∑O/N2 depletion was transported equatorward and corotated from local post‐midnight to early morning, and was observed by GOLD at middle latitudes during daytime. Plain Language Summary: This study utilizes numerical simulations with a climatological tide in the lower boundary to investigate how thermospheric composition disturbance is produced and transported to mid‐latitudes during a geomagnetically quiet period (DOY110‐112, 2019, maximum Kp = 1.7, AE < 300 nT). Through comparing parameters from simulations under real IMF and By = 0, it is found that the IMF By, which was dominant over this period, can significantly enhance the ionospheric potential, Joule heating rate per unit mass, ion drift and neutral winds compared with Bz only condition. As a result, the geomagnetic activity with AE < 300 nT can trigger stronger depletion (30%) of the column density ratio of thermospheric O and N2 (∑O/N2) in the polar region. Then the accelerated meridional wind transports the ∑O/N2 depletion equatorward to mid‐latitudes. The depletion is also transported from local post‐midnight to local early morning. This study shows the crucial role of dominant By during "quiet time". Additionally, it suggests that the response of ∑O/N2 to geomagnetic activity during the "quiet period" is similar to the one during geomagnetic storms. Key Points: IMF with dominant By enhances ion convection pattern, Joule heating rate and neutral windsStrong thermospheric composition disturbance can be generated and propagate into mid‐latitudes with dominant By during "quiet" timeResponse of thermospheric composition to geomagnetic activity with AE < 300 nT is similar to its response to geomagnetic storms [ABSTRACT FROM AUTHOR]

Storm sudden commencements (SSC) often precede geomagnetic storms. Commonly, it takes some hours from the step‐like change that marks the SSC to the start of the magnetic storm activity. In a subset of cases, however, auroral activity starts almost instantaneously after the SSC. To the authors knowledge, the conditions that enable this rapid activation have not been investigated in detail before. Here we consider all the sudden commencements (SC) during the years 2000–2020. Our focus is on the initial response of the auroral currents on the nightside. For that purpose, we make use of the IMAGE Magnetometer Network in Fennoscandia. In about 30% of SC events an initial activation of the westward electrojet is observed. Magnetic deflections of the northward component, surpassing frequently 1,000 nT, are observed only 4 min after the SC. These intense westward currents, flowing typically in narrow channels of 1°–2° latitudinal width, last some 10 min. The electrojets are conjugate to regions in the magnetosphere near geostationary orbits. In several cases geomagnetic substorm onsets are observed about 30 min after the SC. These start typically at fairly high latitude, around 71° magnetic latitude. This is an indication for rather quiet conditions preceding the onset. The magnetic pulse of the SC seems to play an important role in initiating the strong electrojets and the substorms. These initial activities are of relevance for space weather effects because of their strong and rapid variations. This paper provides detailed observations of the initial auroral activity following some SCs. Key Points: First detailed study of intense electrojet activity at auroral latitudes on the nightside following immediately a sudden commencement (SC)Precondition for intense auroral activity is a southward interplanetary magnetic field Bz and a sufficiently large magnetic pulse caused by the SCIn a subset of events also an isolated substorm is initiated at relatively high magnetic latitudes shortly after the SC [ABSTRACT FROM AUTHOR]

Previous spacecraft studies showed that stormtime poloidal ultralow‐frequency (ULF) waves in the ring current region have an antisymmetric (second harmonic) mode structure about the magnetic equator. This paper reports Van Allen Probes observations of symmetric ULF waves in the postnoon sector during a moderate geomagnetic storm. The mode structure is determined from the presence of purely compressional magnetic field oscillations at the equator accompanied by strong transverse electric field perturbations. Antisymmetric waves were also detected but only very late in the recovery phase. The symmetric waves were detected outside the plasmasphere at L = 3.0–5.5 and had peak power at 4–10 mHz, lower than the frequency of the local fundamental toroidal standing Alfvén wave. During the wave events, the flux of protons was enhanced at energies below ∼5 keV, which appears to be a prerequisite for the waves. The protons may provide free energies to waves through drift resonance instability or drift compressional instability, which occur in the presence of radial gradients of plasma parameters. Plain Language Summary: During geomagnetic storms, the intensity of the flux of ions is elevated in the magnetosphere including the region inward of geostationary orbits. The ions are capable of exciting a variety of plasma waves including ultralow‐frequency (ULF) waves in the frequency range 1–20 mHz. Stormtime ULF waves with a strong perturbation of the magnetic field magnitude are commonly detected at geostationary orbit and are known to commonly have a standing wave structure along the background magnetic field that is antisymmetric about the magnetic equator. Observations made by the Van Allen Probes provide new evidence that symmetric waves are also excited inward of geostationary orbits. The waves are observed in association with enhancement of the flux of protons at energies below 5 keV. The waves may be excited by the spatial gradient of parameters associated with the ions. Key Points: Symmetric compressional ULF (4–10 mHz) waves were excited at L = 3.0–5.5 during a geomagnetic stormThe waves were associated with enhancement of the flux of protons at energy <5 keVDrift compressional instability is a possible wave generation mechanism [ABSTRACT FROM AUTHOR]

Sporadic E (Es) layers could be composed of metallic ions and formed, modified, or transported by the action of convective electric fields in the high latitude ionosphere. In this paper, by utilizing simultaneous observations from Digisonde and Super Dual Auroral Radar Network (SuperDARN) HF radars at Zhongshan Station (ZHS, 69.4°S, 76.4°E), Antarctica, a thin Es layer, which initially formed in the lower F region and descended into the lower E region, with wavelike structures, was recorded by Digisonde on 14 November 2019. The Es layer‐related concurrent ionospheric irregularities were also detected by the SuperDARN ZHS HF radar. By using a global‐scale 2‐D convection map, combined with images from the Special Sensor Ultraviolet Spectrographic Imager instruments onboard Defense Meteorological Satellite Program (DMSP) spacecraft, it is proposed that the flow shears associated with the duskside convective circulation are responsible for the evolution of the Es layer. Moreover, using the HF radar elevation angle data to measure the scatter height, it is strongly suggested that the Es layer was elongated with convection circulation. The electrodynamic processes responsible for the formation and evolution of the Es layer are discussed. Key Points: An Es layer formed in the lower F region and descended to the E region, observed simultaneously by Digisonde and Super Dual Auroral Radar Network (SuperDARN) HF radarsThe formation and evolution of the Es layer related to the afternoon convection reversalSuperDARN HF radar measurements suggest the Es layer is elongated with convection circulation [ABSTRACT FROM AUTHOR]

Using ground‐based magnetic field measurements and numerical simulations from the Thermosphere‐Ionosphere Electrodynamic General Circulation Model (TIEGCM), a first paper (Zhang, Yamazaki, et al., 2021, doi: https://doi.org/10.1029/2021JA029681; under review) introduced the potential roles of disturbance dynamo electric field due to subauroral polarization streams (SAPS) on the equatorial electrojet (EEJ) during a moderate geomagnetic storm on 1 June 2013. Our second study investigated the temporal responses of equatorial electrojet to SAPS. At noon, the residual EEJ (ΔEEJ) owing to SAPS flows westward, that is, counter equatorial electrojet (CEJ). The temporal variation of CEJ excited by the dynamo electric field was basically consistent with that by SAPS, and the effects of zonal wind were larger than those of meridional wind. The relative time delay of CEJ and SAPS was related to the propagation time of disturbance wind from mid‐latitudes to low‐latitudes. It took 2–3 hr for SAPS‐related disturbance wind to propagate to the equatorial region and change the polarity of EEJ. The influence of meridional winds on the temporal variations of ΔEEJ is related to the generation of eastward currents at mid‐latitudes, which can accumulate the positive charges at dusk terminator and then generate a westward electric field at lower latitudes. Plain Language Summary: The equatorial electrojet (EEJ) represents a ribbon of intense electric current flowing in the ionospheric E region (approximately 110 km) on the dayside along the dip equator. Its behaviors are controlled by the daytime ionospheric electric field and ionospheric conductivity. However, the temporal variations of EEJ in response to the subauroral polarization streams (SAPS), an interesting and important physical phenomenon at subauroral latitudes, are seldom explored and poorly understood. The understanding of EEJ behaviors and their associated physical drivers during SAPS periods can contribute to the modeling and forecasting of the equatorial space environment, and the understanding of the coupling between ionosphere‐thermosphere systems at high‐latitudes and dip equator. Key Points: ubauroral polarization streams‐induced counter equatorial electrojet (CEJ) shows a 2–3 hr delay with respect to SAPSThe time delay of CEJ attributes to the traveling atmospheric disturbances in zonal windsThe effects of meridional winds on CEJ depend greatly on the geomagnetic declination [ABSTRACT FROM AUTHOR]

The dynamics of substorms are known to be dominated by the North‐South (Bz) component of the Interplanetary Magnetic Field (IMF), which is the most important driver of the dayside reconnection. Even though the dawn‐dusk (By) component is also known to play a role in substorm dynamics, its effects are not yet fully understood. In this paper we study how IMF By modulates the onset latitude, strength and occurrence frequency of substorms as well as the isotropic boundary (IB) latitude of energetic protons. We show that the substorm onset latitude and the IB latitude are about one degree lower for large magnitude By (|By|>3 nT) than for small By. In contrast, the substorm occurrence frequency is larger for small |By|. We suggest that the magnetotail is more stable during large |By|, requiring the magnetotail lobes (and hence the polar cap) to contain more flux to initiate a substorm compared to the situation when By is small. Plain Language Summary: Substorms are global magnetic disturbances in which energy stored in the Earth's magnetic field is suddenly released, leading to intense aurorae and other space weather effects. Substorms are most frequent and strongest when the magnetic field incident to the Earth at the Sun‐Earth line has a strong southward component. In this paper we study how the occurrence and strength of substorms are affected by the east component of this magnetic field. We show that substorms are less frequent but stronger, with associated aurora extending to lower latitudes, when the east component is strongly positive or negative. These results help in developing more accurate space weather predictions in the future. Key Points: For similar solar wind forcing, increased IMF |By| influence both substorm onset latitude and the proton isotropic boundary latitudeSubstorms are less frequent for large IMF |By| for similar solar wind forcingThe results indicate that the magnetotail response depends on the magnitude of the IMF By component [ABSTRACT FROM AUTHOR]

A quasi‐semidiurnal type pattern was observed earlier in the diurnal UT variation of the geomagnetic storms studied using mainly Kyoto Dst (disturbance storm‐time) index. However, the pattern has been argued as apparent due to uneven longitude distribution of the four Dst observatories. Unlike earlier studies, this paper investigates the diurnal UT variation of the storms automatically identified in six available indices including Kyoto Dst, USGS (United States Geological Survey) Dst, SymH (symmetric‐H), RC (ring current), Dcx (corrected extended Dst), and AER (Atmospheric and Environmental Research) in 50, 50, 36, 21, 5, and 7 years, respectively. The indices are derived using 4, 4, 12, 14, and 15 ground observatories (with maximum longitude separations of ∼120°, 120°, 70°, 110°, and 50°) and four DMSP (Defense Meteorology Satellite Program) satellites, respectively. The UT distribution of the storm intensity (minimum value of an index during the storm main phase) in all indices shows a striking quasi‐semidiurnal type variation with maxima around 06–08 UT and 21–23 UT and minima around 03–05 UT and 13–15 UT. Similar quasi‐semidiurnal variation is also observed in the computed values of the main energy input in the ring current. The variation correlates well with the variations of the dipole tilt angles μ and θ involved in the equinoctial hypothesis and Russell‐McPherron (RM) effect, respectively. These observations indicate that the quasi‐semidiurnal variation is real. Plain Language Summary: Large disturbances in the geomagnetic field lasting form several hours to several days are known as geomagnetic storms. The variations of the occurrence and intensity of the storms with solar activity and season have been understood thanks to the works of a large number of scientists. The variation of the storms with the time‐of‐day studied using mainly the low latitude geomagnetic activity index Dst has shown a quasi‐semidiurnal pattern. The pattern, however, has been argued as apparent due to the uneven longitude distribution of the four magnetic observatories used for deriving Dst. The present study investigates the diurnal UT variation of the storms using six available indices. The results show similar striking quasi‐semidiurnal patterns in the UT distribution of the storm intensity in all indices and computed value of the main energy input in the ring current. The quasi‐semidiurnal pattern also correlates well with the angles μ and θ involved in the mechanisms of equinoctial hypothesis and RM effect. These observations indicate that the quasi‐semidiurnal variation is real. Key Points: A striking quasi‐semidiurnal pattern is observed in the UT distribution of the geomagnetic storm intensity in six low latitude indicesSimilar pattern exists in the UT variation of the computed value of the main energy input in the ring currentThe quasi‐semidiurnal pattern correlates well with the angles μ and θ involved in the mechanisms of equinoctial hypothesis and RM effect [ABSTRACT FROM AUTHOR]

The coupling response between solar wind structures and the magnetosphere is highly complex, leading to different effects in the outer radiation belt electron fluxes. Most Coronal Mass Ejections cause strong geomagnetic storms with short recovery phases, often 1–2 days. By contrast, High‐Speed Solar Wind Streams lead to moderate and weak storms often with much longer recovery phases, from several to ∼10 days. The magnetosphere receives energy for a long time under the influence of the HSSs, considerably changing its dynamics. This in turn has an effect on the charged particles trapped in the outer radiation belt. Although the high‐energy electron flux enhancements have received considerable attention, the high‐energy electron flux enhancement pattern (L > 4) has not. This paper identifies 37 events with this enhancement pattern in the high‐energy electron flux during the Van Allen Probes era. We find the enhancements coincident with HSS occurrence. The interplanetary magnetic field (IMF) exhibits north/south Bz fluctuations of Alfvénic nature with moderate amplitudes. The high‐energy electron flux enhancements also correspond to long periods of auroral activity indicating a relationship to magnetotail dynamics. However, the AE index only reaches moderate values. Ultra‐Low Frequency waves were present in all of the events and whistler‐mode chorus waves were present in 89.1% of the events, providing a convenient scenario for wave‐particle interactions. The radial gradient of the ULF wave power related to the L, under the influence of the HSSs, is necessary to trigger the physical processes responsible for this type of high‐energy electron flux enhancement pattern. Key Points: Each of 37 high‐energy electron flux enhancement events began at L > 4Ultra Low Frequency wave activity localized in L is decisive to the position of the high‐energy electron flux enhancement pattern (L > 4)Each event occurred during High‐Speed Stream in conjunction with Alfvénic fluctuations, Bz southward, substorms and chorus/ULF waves [ABSTRACT FROM AUTHOR]

The interaction of the solar wind with the Earth's magnetic field produces geomagnetic activity, which is critically dependent on the orientation of the interplanetary magnetic field (IMF). Most solar wind coupling functions quantify this dependence on the IMF orientation with the so‐called IMF clock angle in a way, which is symmetric with respect to the sign of the By component. However, recent studies have suggested that the sign of By is an additional, independent driver of high‐latitude geomagnetic activity, leading to higher (weaker) geomagnetic activity in Northern Hemisphere (NH) winter for By > 0 (By<0). In this paper we quantify the size of this explicit By effect with respect to the solar wind coupling function, both for northern and southern high‐latitude geomagnetic activity. We show that high‐latitude geomagnetic activity is significantly (by about 40%–50%) suppressed for By < 0 in NH winter and for By > 0 in Southern Hemisphere winter. When averaged over all months, high‐latitude geomagnetic activity in NH is about 12% weaker for By < 0 than for By > 0. The By effect affects the westward electrojet strongly, but hardly at all the eastward electrojet. We also show that the suppression of the westward electrojet in NH during By < 0 maximizes when the Earth's dipole axis points toward the night sector, that is, when the auroral region is maximally in darkness. Key Points: IMF By is an explicit driver of high‐latitude geomagnetic activityHigh‐latitude geomagnetic activity is suppressed in local winter for By < 0 in Northern Hemisphere and for By > 0 in Southern HemisphereExplicit By effect maximizes when the Earth's dipole axis points toward night [ABSTRACT FROM AUTHOR]

During the magnetic storm starting on September 7, 2017, the MEP‐i instrument onboard the Arase (ERG) satellite observed molecular ions (O2+/NO+/N2+) in the ring current. The molecular ions were observed by Arase in four orbits during this magnetic storm. This indicates that there was a continuous molecular ion supply from the ionosphere. During the storm main phase around the second Dst minimum (∼−100 nT) on September 8, 2017, the European Incoherent Scatter (EISCAT) radar observed the ion upflow (∼50–150 m s−1) in the low‐altitude (250–350 km) ionosphere together with strong ion heating up to >2,000 K. The convective electric field derived from the electron heating observed by EISCAT at an altitude of approximately 110 km was also enhanced by a factor of 2. The observations suggest that the additional ion heating at low altitudes helps to cause the fast upflow and transport molecular ions upward. The flux decreases from 280 to 350 km altitudes due to the dissociative recombination was estimated to be approximately two orders of magnitude. This resulted in significant molecular ion flux remaining at 350 km altitude. These results suggest that the low‐altitude ion upflow caused by the ion frictional heating enables molecular ions to escape to space against rapid loss by the dissociative recombination. Plain Language Summary: Molecular ions (O2+/NO+/N2+) in the ring current are sometimes observed during magnetic storms. These molecular ions come from the deep ionosphere and considered good tracers of escape mechanisms from the Earth's ionosphere to the magnetosphere. However, it has not been revealed how these molecular ions are transported upward, especially by ion upflows in the low‐altitude ionosphere (∼250–350 km). It is difficult to transport sufficient flux of molecular ions due to deceleration by the strong gravitational force and rapid decrease by dissociative recombination even during magnetic storms. In this paper, we report the analysis results of the fast ion upflow (∼100 m s−1) event in the low‐altitude ionosphere observed by the European Incoherent Scatter (EISCAT) radar, while the Arase (ERG) satellite observed molecular ions in the inner magnetosphere during the magnetic storm starting on September 7, 2017. The results suggest that ion frictional heating created the ion upflow and could be a source of outflows at higher altitudes in the ionosphere to supply the molecular ions into the magnetosphere. Key Points: Arase satellite observed molecular ions in the ring current during the magnetic storm starting on September 7, 2017An ion upflow from the deep ionosphere was observed with enhancements of electric field and ion temperature by EISCATThe fast upflow caused by the ion frictional heating enables molecular ions to escape to space against dissociative recombination [ABSTRACT FROM AUTHOR]

Total electron content (TEC) map is one of the important ionospheric parameters. The International Global Navigation Satellite System Service (Ionosphere Working Group) provides the combined vertical TEC maps. However, the postprocessing of the IGS TEC maps may cost quite a long time, and it's not easy for the organization to collect the complete data. It is necessary for researchers to figure out a method to complete the global TEC maps efficiently with regard to the problems of lack of data or not available to the standard IGS TEC. With the rapid development of the deep learning methods, the Deep Convolutional Generative Adversarial Network exhibits the great potential in computer vision. In this paper, we propose a new method called Global and Local GAN (GLGAN) based on the DCGAN and apply it on completing the global TEC maps. Different from the traditional GAN, the GLGAN consists of a generator (or called completion network) and two discriminators. The completion network is powerful enough to Extract features of IGS TEC maps to complete the TEC maps. The design of two discriminators enhances the ability of judging the quality of output images, and improves the accuracy of the completion network. After analyzing the results, we find the GLGAN have a better performance in complicate structures during geomagnetic storm time. The success of the GLGAN in completing the TEC maps suggests that the deep learning methods are able to solve many problems regarding to data and images in ionospheric parameters' reconstruction or forecasting. [ABSTRACT FROM AUTHOR]

With the interplanetary magnetic field (IMF) that turns from northward to southward, the global simulation successively reproduces the growth phase, the onset, and the expansion phase of the substorm. The calculated ionospheric convection for the growth phase reproduces the development of the Harang reversal (HR), the upward field-aligned current (FAC), and the upcoming onset point as observed. Magnetic field lines traced from the center of the nightside upward FAC are open, while the magnetic field line traced from the onset point is closed. These open magnetic field lines map to flow shear just outside the O/C boundary. Seen from the magnetic configuration, the growth phase proceeds as the replacement process of nulls. Two new nulls appear on the dayside under the southward IMF, while two old nulls under the northward IMF retreat tailward forming two bifurcation regions on the dawn and dusk flanks. Flow shear around the O/C boundary forms by magnetospheric convection that returns to the dayside via bifurcation regions. The expansion phase proceeds through a topological change by the nearearth neutral line (NENL). The NENL occurs inside the thinned structure of the northward IMF remnant, on the low-latitude side of the flow shear, and projects down to the low-latitude edge of the upward FAC. Associated with the NENL, the convection return path changes to the center of the plasma sheet and reveals in the ionosphere as the release of the HR. By the shrinkage of projecting magnetic field line, the O/C boundary migrates poleward in the ionosphere. [ABSTRACT FROM AUTHOR]

Collisionless shocks are one of the most effective particle accelerators in the known universe. Even low Mach number shocks could have a significant role in particle heating and acceleration. Theory suggests that kinematic collisionless relaxation, the process whereby a downstream nongyroptopic ion population becomes thermalized through collisionless gyrophase mixing, is the dominant energy redistribution mechanism in quasi‐perpendicular, low Mach number, and low β shocks. However, there have only been a limited number of observations of these shocks using in situ measurements at Venus, Earth and in interplanetary space. This paper presents the results of the first detailed study using in situ measurements, of the effect of fundamental parameters on the formation of these shocks. All low Mach number shocks occurring during the magnetic cloud phase of an interplanetary coronal mass ejection are identified in Venus Express magnetic field data over the duration of the mission. From the 92 shock crossings identified, 38 show clear evidence of kinematic relaxation. It is shown that kinematic relaxation is dominant at Venus when the angle between the local shock normal and upstream magnetic field is greater 50° and the Alfvén Mach number is less than 1.4. These shocks are also observed across a range of solar‐zenith‐angles indicating that it is likely that any location on the Venus bow shock could form such a structure. Venus Express plasma measurements are used to verify the parameters estimated from the magnetic field and indicate the importance of heavy ions, including potential pickup O+. Key Points: First detailed study using in situ measurements of the effect of fundamental parameters on kinematic relaxation at low Mach number shocksOnly quasi‐perpendicular very low Mach number shocks show evidence of the downstream oscillations created by kinematic relaxationA low Mach number bow shock with kinematic relaxation as the dominant energy re‐distribution mechanism can form at most locations at Venus [ABSTRACT FROM AUTHOR]

The paper summarizes the properties of precipitation of magnetospheric electrons with energy above several hundred keV recorded by observing X‐ray bremsstrahlung in the polar stratosphere above the Murmansk region, Russia, in 1961–2019. Precipitation occurrence rate demonstrates a clear dependence on the solar activity with a maximum at the decay phase of the 11‐year solar cycle, similarly to the variability in occurrences of the high‐speed solar wind streams (HSSWS). The energetic electron precipitation (EEP) event series is often initiated by a moderate geomagnetic storm caused by a HSSWS and continues during geomagnetic storm recovery. EEP demonstrates the seasonal rate variation with the maxima in occurrence rate around the spring and the autumn solstices and correlates with fluences of relativistic electrons in the outer radiation belt. For 59 years, 589 events of precipitation were observed. Analysis of the long‐term time series revealed a growing trend in the rate of precipitation occurrence, especially in the 1990s to 2000s that is not properly explained yet. Plain Language Summary: Since the beginning of the 1960s, the group from Lebedev Physical Institute watches the precipitation of energetic electrons from the outer radiation belt to the atmosphere. The precipitation reflects the condition in the interplanetary space and in the magnetosphere, that is, it is governed by solar activity. In the 1990s, solar activity started weakening: the maximum annual mean of sunspot number decreased from 233 in Solar Cycle 21 to 116 in Cycle 24. But the occurrence rate of precipitation increased, which has not found an explanation yet. Key Points: Balloon observations allow recording precipitation of magnetospheric electrons via bremsstrahlung in the atmosphereSince 1961 till 2019, 589 electron precipitation events were observed in the Murmansk regionAn unexpected increase of precipitation occurrence rate was found in the 1990s to 2000s [ABSTRACT FROM AUTHOR]

In this paper, we have evaluated the role of interplanetary magnetic field (IMF) By on the asymmetry of the ring current during the main phase of geomagnetic storms. The mean H variations have been calculated using 31 ground magnetic stations over magnetic latitudes of 09–45° following the methodology of Li et al. (2011, https://doi.org/10.1029/2011JA016886). Further, the magnetic local time (MLT) variations in the H component at these stations w.r.t. the mean H were investigated for three cases of geomagnetic storms with varying southward IMF Bz and IMF By conditions. Significant ring current asymmetries were observed during the main phase of geomagnetic storms. The primary role of IMF Bz on the asymmetry of the ring current is observed from these cases. More importantly, the investigation brings out for the first time, the additional role of IMF By in influencing the MLT distribution of ring current observed at ground magnetic stations. Under southward IMF Bz conditions, it is shown based on SuperDARN and AMPERE data that IMF By can alter the MLT distribution of ring current under suitable conditions. The timescales of IMF By also play very important role in determining the asymmetry in the ring current. Under steady convection state, IMF By can rotate the convection cells based on its polarity, which in turn can change the MLT distribution of ring current observed by low‐latitude ground stations. This investigation, thus, brings out the important role of IMF By on the asymmetric MLT distribution of ring current under southward IMF Bz. Key Points: IMF By plays important role in asymmetry of the ring current observed at ground stations in addition to IMF BzSuperDARN convection cells and AMPERE‐derived FACs show the association of IMF By with MLT distribution of the ring currentUnder suitable conditions, IMF By can alter the MLT distribution of the ring current [ABSTRACT FROM AUTHOR]

The characteristics of isolated and storm‐time injections are important in the understanding of storm‐substorm relationship. The injection region, flux ratio and energy‐flux spectra have been investigated to find the differences and similarities for isolated and storm‐time injections. A scanning procedure automatically identifies a total of 94 isolated injections and 229 storm‐time injections with the LANL data in 2000 and 2001. A significant difference has been found in longitudinal distribution for isolated and storm‐time injections which have wider injection regions than the former. Based on the trends of characteristic energies and number density from spectra analysis, two types of isolated injections have been found. Type I injections are characterized by characteristic energies increasing with the increase of number density, while Type II has an opposite trend. Further statistical surveys suggest that the flux ratios before and after injections have opposite trends in Type I and II injections. Storm‐time injection is very similar to the Type II injection in characteristic energies‐number density trend and features of flux ratio variations. Our results provide injection regions, flux ratios and spectral features, which may improve our understanding of the isolated and storm‐time injections and their relationship. Plain Language Summary: The injection of energetic particles can occur during magnetic storms and substorms. The differences and similarities in isolated and storm‐time injections have been studied through several factors (the injection regions, flux ratios and spectral characteristics). The investigation may show some characters of isolated and storm‐time injections and their relationship. A clear distinction between isolated and storm‐time injections has been found that isolated injections are rarely observed in the 7–13MLT region, while storm‐time injections cover all magnetic local times. The storm‐time injection events have wider injection regions than isolated injection ones. By spectral analysis, two types of isolated injections have been found from trend of relations between characteristic energies and number density. Statistical surveys also show the opposite trends in variation of the flux ratios for two types of isolated injections. The spectral characteristics and flux ratios of storm‐time injections are very similar to the one of isolated injections. It is possible to find the relations between ion injections and Dst index including those factors which affect the ring current for further study. Key Points: An automatic scanning procedure has been developed to identify dispersionless and dispersive ion injectionsThe differences in injection regions and its longitudinal extension between storm‐time and isolated injections are foundThrough spectral analysis, we find two types of isolated injections, one of which is consistent with characters of storm‐time injection [ABSTRACT FROM AUTHOR]

The Rice Convection Model‐Equilibrium (RCM‐E) and SuperThermal Electron Transport (STET) are combined to investigate electron heat flux formation in the region of the diffuse aurora for the geomagnetic storms of 17 March 2013 and 17 March 2015. The primary electron precipitation into the atmosphere resulting from wave particle scattering in the magnetosphere are simulated by the magnetically and electrically RCM‐E during these two geomagnetic storms. The primary precipitating electron fluxes are modified by the STET model by taking into account atmospheric backscatter processes. The modified electron energy fluxes and their mean energies are coupled to the STET code to calculate electron thermal fluxes associated with diffuse aurora on a global scale. We use the simulated heat flux to estimate electron temperatures at the upper ionospheric altitudes and compare them with corresponding observations from the Defense Meteorological Satellite Program satellite. Key Points: Electron heat flux formation in diffuse aurora associated with electron precipitation eventsSimulated electron thermal fluxes during the St. Patrick's 2013 and 2015 stormsThermal electron heat flux validation using Defense Meteorological Satellite Program observations [ABSTRACT FROM AUTHOR]

Recent advances in reconstructing Earth's magnetic field and associated currents by utilizing data mining of in situ magnetometer observations in the magnetosphere based on geomagnetic indices and solar wind parameters have proven remarkably accurate at reproducing observed ion diffusion regions. We investigate the effect of placing regions of localized resistivity in global simulations of the magnetosphere at specific locations inspired by the data mining results for the substorm occurring on 6 July 2017. When explicit resistivity is included, the simulation forms an x‐line at the same time and location as the Magnetospheric Multiscale Mission observation of an ion diffusion region at 15:35 UT on that day. Without this explicit resistivity, reconnection forms later in the substorm and far too close to Earth (≳−15RE), a common problem with global simulations of Earth's magnetosphere. A consequence of reconnection taking place farther down the tail due to localized resistivity is that the reconnection outflows transport magnetic flux Earthward and thus prevent the current sheet from thinning enough for reconnection to take place nearer Earth. As these flows rebound tailward from the inner magnetosphere, they can temporarily and locally (in the dawn‐dusk direction) stretch the magnetic field allowing for small scale x‐lines to form in the near Earth region. Due to the narrow cross‐tail extent of these x‐lines (≲5RE) and their short lifespan (≲5 min), they would be difficult to observe with in situ measurements. Future work will explore time‐dependent resistivity using 5 min cadence data mining reconstructions. Plain Language Summary: Recently, data mining of spacecraft magnetometers have created highly accurate reconstructions of Earth's magnetic field. These reconstructions capture the location and timing of so‐called x‐lines on the night side of Earth. Magnetic field lines reconnect at x‐lines and convert magnetic energy to plasma energy via heating and acceleration of flows. Nightside reconnection is a crucial part of geomagnetic storms and substorms that pose a danger to spacecraft, astronauts, and the power grid on Earth. By identifying x‐lines in reconstructions we can influence simulations of specific substorms to increase their accuracy and make them more physically realistic. We demonstrate this ability by matching simulation results with observations and find that reconnection further from Earth suppresses extended x‐line formation near Earth. Additionally, the flows from reconnection can bounce off the strong magnetic field close to Earth (a well‐known effect) and locally stretch the magnetic field creating small scale x‐lines near Earth. Key Points: Localized Resistivity in global magnetohydrodynamic simulations can "encourage" magnetic reconnection in specific locations to match data mining resultsActive x‐lines at XGSM ≲ −20RE can suppress the formation of extended x‐lines at XGSM ≳ −15REReconnection outflows rebound from the near‐Earth region causing transient and narrow (in YGSM) secondary reconnection [ABSTRACT FROM AUTHOR]