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The D‐shaped ion velocity distribution generated by the time‐of‐flight (ToF) effect can be considered as an important characteristic of reconnection exhausts in the magnetopause and magnetotail reconnection processes. In this study, we reported the D‐shaped velocity distribution of O+ ions produced by the ToF effect in the magnetotail reconnection exhaust based on the observations from the Cluster spacecraft. The observed cutoff velocity of O+ ions is smaller than that of H+ ions at the same time, which is different from the previous theoretical prediction. We suggested that the difference between the two cutoff velocities is probably due to O+ ions having a larger reconnection diffusion region than H+ ions. We also demonstrated a remote‐sensing method to estimate the spatial scale ratio between the O+ and H+ diffusion regions and the distance from the observation site to the center of the magnetotail reconnection region. Plain Language Summary: The reconnection processes at the Earth's magnetopause and magnetotail can convert the magnetic energy into the kinetic energy of the plasma, thus producing high‐speed reconnection exhausts. The exhausting ions usually show a D‐shaped velocity distribution caused by the time‐of‐flight effect. Previous works usually focused only on exhausting protons from the reconnection site, while O+ ions could also be abundant during geomagnetic active times. In this study, we analyzed a reconnection event to investigate the behaviors of O+ and H+ ions in the magnetotail reconnection exhaust region using the observations from the Cluster spacecraft. It is suggested that different behaviors of H+ and O+ ions are probably due to O+ ions having a larger reconnection diffusion region than H+ ions. In addition, we proposed a remote‐sensing method to estimate the spatial scale ratio between the O+ and H+ diffusion regions and the distance from the observation site to the center of the magnetotail reconnection region. Key Points: The time‐of‐flight effect of O+ ions in magnetotail reconnection exhaust was observed by the Cluster spacecraftThe observed cutoff velocity of O+ is smaller than that of H+, indicating that O+ may have a larger diffusion region than H+A remote‐sensing method to estimate the spatial scale ratio between O+ and H+ diffusion regions is introduced [ABSTRACT FROM AUTHOR]

Drift‐bounce resonance between ultralow frequency (ULF) waves and ions is essential for ion energization in the magnetosphere. Here, we present the first comprehensive study of drift‐bounce resonance in the dayside outer magnetosphere, where off‐equatorial magnetic field minima would strongly distort ions' bounce and drift motion. A generalized theory is proposed, in which the effects of off‐equatorial minima, time‐evolving fields and ion bounce motion are taken into account. In consequence of these effects, ion pitch angle distributions undergo dramatic changes. In the presence of off‐equatorial minima, the time‐of‐flight effect of ion bounce motion forms latitude‐dependent dispersions besides "paw‐track shaped" structures, while evolving wave fields cause time‐dependent phase shifts in "paw‐tracks." All the predicted signatures have been confirmed by 5 years of Magnetospheric Multiscale spacecraft data and numerical simulations. These results allow us to better understand the interactions between ULF waves and thermal ion species in global magnetospheric dynamics. Plain Language Summary: Ultralow frequency (ULF) waves are widely distributed magnetospheric waves that can efficiently accelerate particles and play important roles in geomagnetic activities. Various patterns of ULF wave‐particle interactions have been investigated in detail near the Earth. However, near the dayside boundary of the magnetosphere, the geomagnetic field changes slightly due to solar wind compression. Consequently, the interaction between ULF waves and particles is modified in this region. This study analyzes how the pattern of ULF wave‐particle interactions varies near the dayside boundary of the magnetosphere. We find the compression of the magnetic field, particle motion and ULF wave evolution change the observational signatures of ULF wave‐particle interactions in the region of interest. The results help us better understand the energy transport from the solar wind to the magnetosphere and the role of ULF waves in magnetospheric dynamics. Key Points: Ultralow frequency wave‐ion drift‐bounce resonance in the dayside outer magnetosphere is investigated with 5 years of Magnetospheric Multiscale observationsA generalized theory is proposed to explain the observed ion pitch angle evolutionThe effects of off‐equatorial minima and time‐evolving fields are taken into account successfully to depict the observations [ABSTRACT FROM AUTHOR]

The ultra‐low frequency wave‐particle drift‐bounce resonance in the inner magnetosphere has been studied in detail, due to its important role in particle energization. However, it remains an open question how drift‐bounce resonance manifests in the dayside outer magnetosphere, where particles' orbits show bifurcations because of off‐equatorial magnetic field minima. In this study, we investigate this question, by analyzing Magnetospheric Multiscale observations of the January 20, 2017 event. A test‐particle simulation is conducted to help us understand the observations. The observed pitch angle‐time spectrograms show "pawtrack‐like" structures. We find there are more than two resonant pitch angles at fixed energy, since off‐equatorial minima change the relationship between the bounce (drift) frequency and pitch angle from unimodal function to trimodal function. These results reveal a new drift‐bounce acceleration mechanism in the dayside outer magnetosphere, which potentially affects the efficiency of particle energization during geomagnetic activities like geomagnetic storms. Plain Language Summary: The sun continuously ejects ionized gases called solar winds toward the Earth, affecting Earth's geomagnetic field. When the solar activity is strong, the geomagnetic field is severely disturbed, influencing the safety of power transmissions and space activities. This phenomenon is connected with particle acceleration in the geomagnetic field. The low frequency oscillation of geomagnetic field lines could efficiently transfer energy to charged particles in space, which plays an important role in particle acceleration. This study analyzes special observations which are different from the conventional pattern. A simulation is conducted to help interpret the observations. We find this low frequency magnetic field oscillation interacts with particles differently near the dayside boundary of the geomagnetic field, as the compression of the geomagnetic field by solar winds affects the motion of charged particles nearby. These results help us better understand the formation of geomagnetic activities. Key Points: The effects of off‐equatorial magnetic field minima on ultra‐low frequency wave‐ion interaction have been studied in this letterOff‐equatorial minima lead to more than two resonant pitch angles in drift‐bounce resonanceThe consequent pitch angle distribution shows "pawtrack‐like" structures [ABSTRACT FROM AUTHOR]

The polar cap arc at 1500 MLT (15MLT‐PCA) has been considered as an auroral signature of the cusp's duskside boundary and been speculated to be caused by lobe reconnection. However, no observational evidence has been provided to support this speculation. Here we report a 15MLT‐PCA event occurred on 29 November 2017 using multi‐instrument observations. During the DMSP observed the 15MLT‐PCA, Cluster, with its footprints at the root of the 15MLT‐PCA, identified two FTEs in the southern hemisphere's lobe region, accompanied by an increase in electron and ion energy from hundreds of eVs to several keVs. AMPERE observed an increase in upward field‐aligned currents associated with the 15MLT‐PCA. SuperDARN observed a single cell convection with an enhancement of sunward plasma flow near the root of 15MLT‐PCA. We suggest that these observations provide the in‐situ observational evidence that the 15MLT‐PCA is generated by a lobe reconnection at the cusp's duskside boundary. Plain Language Summary: Auroras primarily occur within an oval‐shaped region centered on Earth's magnetic poles. Under specific conditions, they can also occur within the polar cap, where auroral structures usually appear as arc‐like or spot‐like. Auroras exhibiting arc‐like structures in this region are collectively termed polar cap auroral arcs (PCA). One such common phenomenon is the 15 magnetic local time‐polar cap arc (15MLT‐PCA), typically observed near the 1500 MLT sector in the summer hemisphere, extending from the poleward boundary of the auroral oval to the pole. Additionally, its occurrence shows a strong dependence on the By component of the interplanetary magnetic field (IMF). It is widely accepted that the generation of north‐south asymmetric auroral structures in the polar cap results from reconnection between the IMF and Earth's magnetic field lines at high latitudes. Based on the observational characterization of 15MLT‐PCA, it is currently believed that this structure is associated with lobe reconnection. However, direct in situ observational evidence for this process is currently lacking. Therefore, in this study, we combined ionospheric and magnetospheric satellites to validate a possible magnetospheric process for 15MLT‐PCA. Key Points: The observations show that the occurrence of 15MLT‐PCA was accompanied by FTEs on the dusk flank of the southern hemisphere's magnetopauseThe 15MLT‐PCA is accompanied by an enhancement of upward FACs and an increase in sunward plasma flow near its rootWe provided evidence that the 15MLT‐PCA results from lobe reconnection occurring at duskside boundary of the cusp [ABSTRACT FROM AUTHOR]

This paper investigates the impact characteristics of the 4 November 2021 magnetic storm across different frequency bands based on the electric field data (EFD) from the China Seismo-Electromagnetic Satellite (CSES), categorized into four frequency bands: ULF (Ultra-Low-Frequency, DC to 16 Hz), ELF (Extremely Low-Frequency, 6 Hz to 2.2 kHz), VLF (Very Low-Frequency, 1.8 to 20 kHz), and HF (High-Frequency, 18 kHz to 3.5 MHz). The study reveals that in the ULF band, magnetic storm-induced electric field disturbances are primarily in the range of 0 to 5 Hz, with a significant disturbance frequency at 3.9 ± 1.0 Hz. Magnetic storms also enhance Schumann waves in the ULF band, with 8 Hz Schumann waves dominating in the southern hemisphere and 13 Hz Schumann waves dominating in the northern hemisphere. In the ELF band, the more pronounced anomalies occur at 300 Hz–900 Hz and above 1.8 kHz, with the 300 Hz–900 Hz band anomalies around 780 Hz being the most significant. In the VLF band, the electric field anomalies are mainly concentrated in the 3–15 kHz range. The ELF and VLF bands exhibit lower absolute and relative disturbance increments compared to the ULF band, with the relative perturbation growth rate in the ULF band being approximately 10% higher than in the ELF and VLF bands. Magnetic storm-induced electric field disturbances predominantly occur in the ULF, ELF, and VLF bands, with the most significant disturbances in the ULF band. The electric field perturbations in these three frequency bands exhibit hemispheric asymmetry, with strong perturbations in the northern hemisphere occurring earlier than in the southern hemisphere, corresponding to different Dst minima. No electric field disturbances were observed in the HF band (above 18 kHz). The conclusions of this paper are highly significant for future anti-jamming designs in spacecraft and communication equipment, as well as for the further study of magnetic storms. [ABSTRACT FROM AUTHOR]

In recent years, it has been found that magnetic dip caused by diamagnetic motion of injected plasma can provide an appropriate environment for excitation of electromagnetic ion cyclotron (EMIC) waves. These findings have been widely reported in the Earth's magnetic environment. However, it has rarely been reported in Jupiter's magnetic environment. This paper reports the characteristics of EMIC waves observed by Juno in the magnetic dip of Jupiter. Multiple‐band EMIC waves are observed in frequency range from 10−3 Hz to several Hz. The theoretical analysis shows that in this event both He+ band and O+ band EMIC waves can be constrained in the magnetic dip, which is consistent with the wave emissions observed inside the magnetic dip. Our result provides the first evidence that EMIC wave can be ducted inside a magnetic dip in Jupiter's magnetosphere. Plain Language Summary: The relationship between magnetic structures and electromagnetic waves in the Earth's magnetosphere has been widely reported. Recent studies have theoretically displayed the effects of the magnetic dip on the EMIC wave propagation in Earth's magnetic field. In this paper, we use the event observed by Juno in the magnetic environment of Jupiter to analyze the effect of the magnetic dip on the propagation of EMIC waves. As a result, the observed EMIC waves can be ducted inside the magnetic dip of Jupiter's magnetosphere. Since these ducted waves would influence the dynamics of energetic protons through longtime scattering effects, the magnetic dip is expected to play an important role in the dynamics of Jupiter's magnetosphere. Key Points: Electromagnetic ion cyclotron (EMIC) waves are observed during the magnetic dip by JunoTheoretical analysis about the effects of magnetic dips on the EMIC wave propagation in Jupiter's magnetosphere is performedBoth He+ and O+ band EMIC waves can be ducted inside the magnetic dip in Jupiter's magnetosphere [ABSTRACT FROM AUTHOR]

In recent years, it has become increasingly clear that space weather disturbances can be triggered by transient upstream mesoscale structures (TUMS), independently of the occurrence of large-scale solar wind (SW) structures, such as interplanetary coronal mass ejections and stream interaction regions. Different types of magnetospheric pulsations, transient perturbations of the geomagnetic field and auroral structures are often observed during times when SW monitors indicate quiet conditions, and have been found to be associated to TUMS. In this mini-review we describe the space weather phenomena that have been associated with four of the largest-scale and the most energetic TUMS, namely, hot flow anomalies, foreshock bubbles, travelling foreshocks and foreshock compressional boundaries. The space weather phenomena associated with TUMS tend to be more localized and less intense compared to geomagnetic storms. However, the quiet time space weather may occur more often since, especially during solar minima, quiet SW periods prevail over the perturbed times. [ABSTRACT FROM AUTHOR]

The impact of interplanetary (IP) shocks on the Earth's magnetosphere can greatly disturb the geomagnetic field and electric currents in the magnetosphere-ionosphere system. At high latitudes, the current systems most affected by the shocks are the auroral electrojet currents. These currents then generate ground geomagnetically induced currents that couple with and are highly detrimental to ground artificial conductors including power transmission lines, oil/gas pipelines, railways, and submarine cables. Recent research has shown that the shock impact angle, the angle the shock normal vector performs with the Sun-Earth line, plays a major role in controlling the subsequent geomagnetic activity. More specifically, due to more symmetric magnetospheric compressions, nearly frontal shocks are usually more geoeffective than highly inclined shocks. In this study, we utilize a subset (332 events) of a shock list with more than 600 events to investigate, for the first time, shock impact angle effects on the subsequent GICs right after shock impact (compression effects) and several minutes after shock impact (substorm-like effects). We use GIC recordings from the Finnish natural gas pipeline performed near the Mäntsälä compression station in southern Finland. We find that GIC peaks ( > 5 A) occurring after shock impacts are mostly caused by nearly frontal shocks and occur in the post-noon/dusk magnetic local time sector. These GIC peaks are presumably triggered by partial ring current intensifications in the dusk sector. On the other hand, more intense GIC peaks ( > 20 A) generally occur several minutes after shock impacts and are located around the magnetic midnight terminator. These GIC peaks are most likely caused by intense energetic particle injections from the magnetotail which frequently occur during substorms. The results of this work are relevant to studies aiming at predicting GICs following solar wind driving under different levels of asymmetric solar wind forcing. [ABSTRACT FROM AUTHOR]

The magnetopause is the boundary between the interplanetary magnetic field and the terrestrial magnetic field. It is influenced by different solar-wind conditions, which lead to a change in the shape and location of the magnetopause. The interaction between the solar wind and the magnetosphere can be studied from in situ spacecraft observations. Many studies focus on the equatorial plane as this is where recent spacecraft constellations such as THEMIS or MMS operate. However, to fully capture the interaction, it is important to study the high-latitude regions as well. Since the Cluster spacecraft operate in a highly elliptical polar orbit, the spacecraft often pass through the magnetopause at high latitudes. This allows us to collect a dataset of high-latitude magnetopause crossings and to study magnetopause motion in this region, as well as deviations from established magnetopause models. We use multi-spacecraft analysis tools to investigate the direction of the magnetopause motion in the high latitudes and to compare the occurrence of crossings at different locations with the result in the equatorial plane. We find that the high-latitude magnetopause motion is generally consistent with previously reported values and seems to be more often associated with a closed magnetopause boundary. We show that, on average, the magnetopause moves faster inwards than outwards. Furthermore, the occurrence of magnetopause positions beyond those predicted by the model at high latitudes is found to be caused by the solar-wind parameters that are similar to those in the equatorial plane. Finally, we highlight the importance of the dipole tilt angle at high latitudes. Our results may be useful for the interpretation of plasma measurements from the upcoming SMILE mission as this spacecraft will also fly frequently through the high-latitude magnetopause. [ABSTRACT FROM AUTHOR]

If the interplanetary magnetic field (IMF) has a northward component, a portion of the solar wind ions flowing in the high-latitude magnetosheath drastically alters the flow direction owing to lobe reconnection and follows the Earth's magnetic field lines toward the low-altitude cusp. Partly because of this folded path, the relationship between the speed of ion flow in the magnetosheath and the energy of the precipitating ions in the low-altitude cusp remains unclarified. Herein, we examined particle data from 11 years of observations by Defense Meteorological Satellite Program satellites to obtain 1990 cases through an automated event identification of the cusp. This corresponds to the largest number of cusp ion events reported to date for a stable northward IMF. The results revealed a positive correlation between the energy of the precipitating ions and the estimated flow speed in the magnetosheath, which answers the question that has remained unexplained for decades: why is the brightness of the cusp proton aurora more strongly correlated with the solar wind dynamic pressure than with the solar wind number density? We suggest additional energizations of the exhausted ions from lobe reconnection in the outflow region because of the turbulent electric field formed by the opposite flow of two ion populations—one in the tailward-directed magnetosheath flow and the other in the earthward-directed outflow jet. [ABSTRACT FROM AUTHOR]

We present the first high resolution global MHD with coupled inner magnetosphere simulation results of an observed theta aurora event. We use the Space Weather Modeling Framework in the Geospace configuration, which produces accurate field aligned current closure in the ionosphere that is integral to theta aurora formation. At the location of the observed theta aurora, the simulation produces a narrow channel of Joule heating along both open and closed field lines, and between a pair of oppositely directed field‐aligned current sheets in the ionosphere. We demonstrate that this Joule heating pattern that we identify as theta aurora maps to a reconnection region at the magnetotail flanks as well as in the distant magnetotail. The theta aurora maps to a cross‐tail current disruption and field‐aligned current source region in a highly twisted magnetotail. Plain Language Summary: The light of the aurora observed in the Earth's atmosphere is a signature of magnetic processes in outer space. The Sun produces hot magnetized plasma that blows through the solar system like a wind. The Earth's magnetic field shields us from the harmful impacts of the Sun's magnetized plasma wind. The interaction between the Earth's magnetic field and the Sun's plasma wind causes the ring of auroral light we see in the polar regions. Sometimes the ring of auroral light grows a bar of auroral light across its center (over the Earth's magnetic poles), transforming the ring into a structure like the Greek letter theta. We call this the theta aurora. It is unknown what physical processes in the Earth's magnetic field create the theta aurora. In this study we show the first realistic simulation that uses solar wind driver conditions of an observed theta aurora event, and demonstrate that we can successfully produce the theta aurora structure in the simulation. We find that the theta aurora's center bar of auroral light comes from multiple regions in the vast Earth's magnetotail. This new result helps us better understand the dynamics of the Earth's space environment and better protect ourselves against the Sun's hazardous plasma wind effects. Key Points: We show first global MHD simulation event study that successfully produces a Joule heating signature that we identify as theta auroraSimulation associates transpolar arc signatures with reconnection at the magnetospheric flank and in the distant magnetotailThe transpolar arc maps to a cross‐tail current disruption and field‐aligned current source region in a highly twisted magnetotail [ABSTRACT FROM AUTHOR]

This study presents observations of magnetopause reconnection and erosion at geosynchronous orbit, utilizing in situ satellite measurements and remote sensing ground‐based instruments. During the main phase of a geomagnetic storm, Geostationary Operational Environmental Satellites (GOES) 15 was on the dawnside of the dayside magnetopause (10.6 MLT) and observed significant magnetopause erosion, while GOES 13, observing duskside (14.6 MLT), remained within the magnetosphere. Combined observations from the THEMIS satellites and Super Dual Auroral Radar Network radars verified that magnetopause erosion was primarily caused by reconnection. While various factors may contribute to asymmetric erosion, the observations suggest that the weak reconnection rate on the duskside can play a role in the formation of asymmetric magnetopause shape. This discrepancy in reconnection rate is associated with the presence of cold dense plasma on the duskside of the magnetosphere, which limits the reconnection rate by mass loading, resulting in more efficient magnetopause erosion on the dawnside. Plain Language Summary: The boundary of Earth's magnetosphere, knows as the magnetopause, moves back and forth in response to the upstream solar wind conditions, mainly solar wind dynamic pressure and the north‐south component of the interplanetary magnetic field (IMF). It is well known that the magnetopause moves closer to the Earth during periods of southward IMF compared to northward IMF. This phenomenon occurs because reconnection between the IMF and Earth's magnetic field removes magnetic flux at the boundary layer, resulting in magnetopause erosion. Occasionally, the magnetopause approaches geosynchronous orbit when the magnetosphere is highly compressed and/or eroded, mainly during geomagnetic storms. Statistical studies have shown the magnetopause erosion often shows a dawn‐dusk asymmetry at geosynchronous orbit. This study presents concurrent observations of magnetopause erosion and related dayside reconnection at geosynchronous orbit by utilizing data from THEMIS, Geostationary Operational Environmental Satellites, and Super Dual Auroral Radar Network radar. The observations suggest that the presence of cold dense plasma in the duskside magnetosphere weakens reconnection rates and, consequently, it contributed to the dawn‐dusk asymmetry of magnetopause erosion. Key Points: We present observations of magnetopause erosion at geosynchronous orbit caused by reconnectionMagnetopause erosion showed dawn‐dusk asymmetryAsymmetric erosion can plausibly be attributed to a reduced reconnection rate at the duskside magnetopause due to cold dense plasma [ABSTRACT FROM AUTHOR]

This document is an editorial for a special issue of the journal Remote Sensing, which focuses on using satellite data and new methodologies to understand the preparatory phase of medium-large earthquakes. The issue includes 15 papers from authors in various countries, covering topics such as seismo-electromagnetic processes, lithospheric structure, atmospheric anomalies, ionospheric disturbances, and interactions between the lithosphere, atmosphere, and ionosphere. The editorial emphasizes the need for further research to explain the different patterns observed in earthquakes and the potential role of tectonic settings and water in these phenomena. Additionally, there is an acknowledgment section from a research paper published in the journal, expressing gratitude to the academic editors who helped evaluate the papers in the special issue. [Extracted from the article]

Flux transfer events (FTEs) are transient magnetic flux ropes at Earth's dayside magnetopause formed due to magnetic reconnection. As they move across the magnetopause surface, they can generate disturbances in the ultralow frequency (ULF) range, which then propagate into the magnetosphere. This study provides evidence of ULF waves in the Pc2 wave frequency range (>0.1 Hz) caused by FTEs during dayside reconnection using a global 3D hybrid‐Vlasov simulation (Vlasiator). These waves resulted from FTE formation and propagation at the magnetopause are particularly associated with large, rapidly moving FTEs. The wave power is stronger in the morning than afternoon, showing local time asymmetry. In the pre and postnoon equatorial regions, significant poloidal and toroidal components are present alongside the compressional component. The noon sector, with fewer FTEs, has lower wave power and limited magnetospheric propagation. Plain Language Summary: The Earth's magnetosphere is a dynamic region shaped by the interplay between the solar wind and Earth's magnetic field. This interaction occurs at the boundary of the magnetosphere (magnetopause) through a process known as magnetic reconnection, giving rise to Flux Transfer Events (FTEs), which are magnetic structures that carry flux and energy into the magnetosphere. These FTEs form either in sudden bursts, patchy patterns or in a continuous, and relatively stable way making the magnetopause surface dynamic. As the FTEs move along the boundary of the magnetosphere, they create compressed regions and lead to wave generation that can extend into the magnetosphere. The study uses an advanced 3D hybrid‐Vlasov simulation model to analyze waves originated from FTE formation and propagation at the magnetopause. We find that rapidly moving and large FTEs have a significant impact on the magnetopause, leading to the generation of ULF waves with frequency above 0.1 Hz. This shows first direct evidence supporting previous theoretical speculations regarding the ability of FTEs to generate waves near the magnetopause. Key Points: Dayside Pc2 waves (>0.1 Hz) have been detected in a 3D hybrid‐Vlasov simulationThese waves exhibit lower intensity within the magnetosphere at noon, compared to the prenoon and postnoon sectorsPc2 waves observed in the simulation are associated with largest and fast moving flux transfer events initiated by subsolar reconnection [ABSTRACT FROM AUTHOR]

We examined rapid variations in the electron zebra stripe patterns, specifically at L = 1.5, over a three‐month duration, using twin Van Allen Probes within Earth's inner magnetosphere. During geomagnetically quiet intervals, these stripes exhibit a peak‐to‐valley ratio (Δj) ∼1.25 in detrended electron fluxes. However, during geomagnetic storms, they became highly prominent, with Δj > 2.5. The correlation between Δj and net field‐aligned currents (FACs) is observed to be high (0.70). Global magnetohydrodynamic (MHD) simulation results indicate that the westward electric field at midnight at low latitudes in the deep inner magnetosphere correlates well with net FACs. An increase in net FACs could amplify the dawn‐to‐dusk electric field in the deep inner magnetosphere, thereby causing the inward transport of electrons. Given that FACs are linked to the interaction between solar wind and the magnetosphere, our findings emphasize the importance of solar wind‐magnetosphere coupling in the deeper regions of the inner magnetosphere. Plain Language Summary: The intensity of hundreds of keV electron fluxes displays a distinctive pattern in the energy versus L‐value spectrogram, characterized by periodic valleys and peaks, commonly referred to as zebra stripes. These patterns have been observed in the magnetospheres of multiple planets, including Earth, Jupiter, and Saturn. Our study reveals that during geomagnetically quiet intervals, Earth's inner magnetospheric zebra stripes exhibit well‐defined banded features. However, these bands become highly pronounced during geomagnetic storms. The peak‐to‐valley ratio (Δj) of detrended electron fluxes shows a correlation with net field‐aligned currents (FACs), and these FACs, in turn, align with the westward component of the electric field at midnight. Consequently, FACs play a significant role in controlling electron flux dynamics deep within the inner magnetosphere. This research illuminates the solar wind‐magnetosphere‐ionosphere couplings. Key Points: Electron zebra stripes are a persistent feature in Earth's inner magnetosphere, although they become intensified during geomagnetic stormsPeak‐valley ratio (Δj) in detrended electron flux within the zebra stripes enhances by ≥1 factor at L = 1.5 during three geomagnetic stormsΔj is well correlated with the net field‐aligned current (FAC) in polar region, suggesting the dominant role of convection driven by FAC [ABSTRACT FROM AUTHOR]

Geomagnetic field measurements indicate that at present we may be on the brink of the Earth's magnetic field reversal, potentially resulting in all the accompanying negative consequences for the mankind. Mathematical modelling is necessary in order to find precursors for reversals and excursions of the magnetic field. With this purpose in mind, following the Podvigina scenario for the emergence of the reversals, we have studied convective flows not far (in the parameter space) from their onset and the onset of magnetic field generation, and found a flow demonstrating reversals of polarity of some harmonics comprising the magnetic field. We discuss a simulated regime featuring patterns of behaviour that apparently indicate future reversals of certain harmonics of the magnetic field. It remains to be seen whether reversal precursors similar to the observed ones exist and might be applicable for the much more complex geomagnetic dynamo. [ABSTRACT FROM AUTHOR]

Field Line Resonances (FLRs) are a critical component in Earth's magnetospheric dynamics, associated with the transfer of energy between Ultra Low Frequency waves and local plasma populations. In this study we investigate how the polarisation of FLRs are impacted by cold plasma density distributions during geomagnetic storms. We present an analysis of Van Allen Probe A observations, where the spacecraft traversed a storm time plasmaspheric plume. We show that the polarisation of the FLR is significantly altered at the sharp azimuthal density gradient of the plume boundary, where the polarisation is intermediate with significant poloidal and toroidal components. These signatures are consistent with magnetohydrodynamic modeling results, providing the first observational evidence of a 3D FLR associated with a plume in Earth's magnetosphere. These results demonstrate the importance of cold plasma in controlling wave dynamics in the magnetosphere, and have important implications for wave‐particle interactions at a range of energies. Plain Language Summary: Earth's space environment is home to electrons and ions across a wide range of energies, trapped in the region by our global geomagnetic field. Energy can be transferred to and from the trapped particles through oscillations in the magnetic field, and these processes are responsible for the extreme energization of trapped electrons to hazardous levels for local spacecraft. In this paper we explore a type of magnetic field oscillation termed Field Line Resonances (FLRs): standing waves on a field line analogous to the oscillatory motion of guitar strings. We use spacecraft observations to show that the direction of the field line oscillations changes significantly depending on the density of the background plasma. The results confirm previous modeling work, and are the first observational evidence of 3D FLRs at a plume. The findings have important consequences for how FLRs transfer energy between the electrons and ions. Key Points: We present the first observational evidence of a 3D Field Line Resonance at the sharp density gradient of a plume edgeThe observed polarisation change confirms magnetohydrodynamic modeling results and predictions made by Elsden and Wright (2022)The presence of 3D Field Line Resonances during storm times has impacts for how Ultra Low Frequency waves couple and interact with local plasma [ABSTRACT FROM AUTHOR]

We investigate the differences between the effects of geomagnetic storms due to Interplanetary Coronal Mass Ejections (ICME) and due to Stream Interaction Regions or Corotating Interaction Regions (SIR/CIR) on the ionospheric F2-layer during the maximum of solar cycle 24. We have created a unique list of the ICME- and SIR/CIR-driven geomagnetic storm events for the time interval between November 2012 and October 2014. Finally, 42 clear ICME and 34 clear SIR/CIR events were selected for this analysis. The individual geomagnetic storm periods were grouped by seasons, time of day, and local time of Dstmin and were analyzed using three different methods: linear correlation analysis using 4-h averages of foF2 parameters and the geomagnetic indices (1st), daily variation of deltafoF2 (2nd), and 3D plotting: geomagnetic indices vs. time vs. deltafoF2 (3rd). The main phase day of the ICME- and SIR/CIR-induced geomagnetic storms was our main focus. We used manually evaluated ionospheric foF2 parameters measured at the Sopron ionosonde station and the geomagnetic indices (Kp, Dst, and AE) for this analysis. We have found that in most cases, the variation of the Dst index is the best indicator of the impact caused in the F2 layer. We conclude as well that the representation of the data by the third method gives a better description of the ICME and SIR/CIR-triggered storm behavior. In addition, our investigation shows that the SIR/CIR-related perturbations can be predicted with greater accuracy with the second method. [ABSTRACT FROM AUTHOR]

By using the magnetic field data from Van Allen Probes, we analyzed the distribution characteristics of the electromagnetic environment in the inner magnetosphere on different Dst* index and magnetic local time. Our results show that for the response of different current systems, the dawn‐dusk and noon‐midnight asymmetry distribution of the residual magnetic field δB increases with Dst* index. When Dst* < −60 nT, a "banana"‐shaped geomagnetic field negative disturbance peak region appears in the sector from midnight to dusk. Then, we obtained the azimuthal current density and found the asymmetric internal eastward and external westward ring current. Through the vector analysis of three‐dimensional current density, the current density vector distribution in the magnetic equatorial plane is completely displayed for the first time, which directly proves the existence of banana current near r = 3.0–4.0 RE during strong geomagnetic storms. Plain Language Summary: By using the magnetic field data from Van Allen Probes, we find that the electromagnetic environment of the Earth's inner magnetosphere exhibits different distributions on different geomagnetic disturbance strengths. During the occurrence of strong magnetic disturbances, the residual magnetic field δB will exhibit distinct dawn‐dusk and noon‐midnight asymmetric distributions. And a "banana"‐shaped peak region of negative geomagnetic field perturbation will appear in the midnight to dusk sector, while both the eastward and westward ring currents exhibit asymmetry. The presence of the banana current near r = 3.0–4.0 RE during strong geomagnetic storms is then directly demonstrated by vector analysis of the 3D current density. Key Points: The negative perturbation response of space currents to the geomagnetic field in the inner magnetosphere is analyzedBoth the eastward ring current and westward ring current is asymmetricThe vector distribution diagram of banana current density is obtained [ABSTRACT FROM AUTHOR]

Dipolarization fronts (DFs), ion‐scale magnetic transients characterized by dramatic enhancement of northward magnetic field, have been documented as crucial energy transfer regions in the magnetosphere. DF‐driven energy transfer has hitherto been studied mainly in the laminar regime. Energy transfer driven by turbulent processes, however, remains unclear. Here we perform a comprehensive investigation of turbulent energy transfer (TET) developed at DFs, via using high‐cadence data from Magnetospheric Multiscale mission. We find that: (a) TET is equally governed by energy loads and generators, different from laminar energy transfer which is typically dominated by energy loads; (b) ion and electron currents play comparable roles in driving TET; (c) TET is positively correlated with local magnetic field strength and ion speed; (d) TET shows asymmetric global distributions along the dawn‐dusk direction. These features implicate that TET is primarily related to electromagnetic turbulence at electron‐ion hybrid scales. These new results, uncovering unique characteristics of DF‐driven TET, can deeply advance our understanding of energy budgets in the magnetosphere. Plain Language Summary: The sun constantly emits outward‐propagating plasma flows called solar wind. When arriving close to Earth, the solar wind interacts with Earth's internal, dipolar magnetic field, leading to formation of long, stretched magnetic field lines in the Earth's nightside, dubbed as magnetotail. The magnetotail hosts a variety of plasma structures which contribute to energy transport therein. Dipolarization fronts (DFs), characterized by sharp enhancement of northward component of magnetic field at ion scale, have been suggested as the leading boundaries of plasma jets (bursty bulk flows) in the magnetotail. They usually host the interaction between the jets and the ambient plasma. Hence, DFs have been suggested as the crucial regions in which intense energy transfer occurs. So far, it has been well documented that the DF‐driven energy transfer is closely related to large‐scale electric fields and ion currents, which are essentially laminar (i.e., direct‐current). Contributions from turbulent (i.e., alternating ‐current) fields and currents remain not well understood. In this study, we use high‐cadence data from NASA's Magnetospheric Multiscale mission to perform a comprehensive investigation of turbulent energy transfer (TET) developed at DFs, uncovering some important features. These results help better understand energy transport in the terrestrial magnetotail. Key Points: Turbulent energy transfer (TET) is equally governed by energy loads and generators, with comparable contributions from electron and ion currentsTET is positively correlated with local magnetic field strength and ion speedTET may be related to electromagnetic turbulence at electron‐ion hybrid scales [ABSTRACT FROM AUTHOR]

Deep penetration of outer radiation belt electrons to low L (<3.5) has long been recognized as an energy‐dependent phenomenon but with limited understanding. The Van Allen Probes measurements have clearly shown energy‐dependent electron penetration during geomagnetically active times, with lower energy electrons penetrating to lower L. This study aims to improve our ability to model this phenomenon by quantitatively considering radial transport due to large‐scale azimuthal electric fields (E‐fields) as an energy‐dependent convection term added to a radial diffusion Fokker‐Planck equation. We use a modified Volland‐Stern model to represent the enhanced convection field at lower L to match the observations of storm time values of E‐field. We model 10–400 MeV/G electron phase space density with an energy‐dependent radial diffusion coefficient and this convection term and show that the model reproduces the observed deep penetrations well, suggesting that time‐variant azimuthal E‐fields contribute preferentially to the deep penetration of lower‐energy electrons. Plain Language Summary: Electrons trapped by the Earth's magnetic field gather in two regions known as the Van Allen radiation belts. It is well reported that electrons can be transported radially inward from the outer radiation belt during geomagnetically active times. More specifically, low energy (100 s of keV) electrons can be moved radially deeper than higher energy (∼1 MeV) electrons. Previous studies suggested that enhanced convection electric fields could contribute to the earthward transport of low energy (<200 keV) electrons. However, the mechanism which leads to different efficiencies of electron transport at different energies has not been quantified. This study expands the traditional radial diffusion model with an empirically determined convection term and shows that the net convection velocity increases for lower energy electrons. For the first time, we quantitatively modeled the energy‐dependent penetration of radiation belt electrons in a wide energy range (10 s of keV to 2 MeV) in the presence of enhanced large‐scale electric fields, during two geomagnetic storm events observed by the Van Allen Probes mission. Key Points: Convective radial transport of storm‐time enhanced large‐scale E‐fields is an efficient inward transport mechanism of 10–100 s keV electronsThe energy‐dependent electron penetration can be explained by the relation between the timescales of electron drift and large‐scale E‐fieldsA radial diffusion‐convection model is developed to reproduce the storm‐time penetration of lower energy electrons to lower L [ABSTRACT FROM AUTHOR]

Electron heating at collisionless shocks in space is a combination of adiabatic heating due to large‐scale electric and magnetic fields and non‐adiabatic scattering by high‐frequency fluctuations. The scales at which heating happens hints to what physical processes are taking place. In this letter, we study electron heating scales with data from the Magnetospheric Multiscale (MMS) spacecraft at Earth's quasi‐perpendicular bow shock. We utilize the tight tetrahedron formation and high‐resolution plasma measurements of MMS to directly measure the electron temperature gradient. From this, we reconstruct the electron temperature profile inside the shock ramp and find that the electron temperature increase takes place on ion or sub‐ion scales. Further, we use Liouville mapping to investigate the electron distributions through the ramp to estimate the deHoffmann‐Teller potential and electric field. We find that electron heating is highly non‐adiabatic at the high‐Mach number shocks studied here. Plain Language Summary: Shock waves appear whenever a supersonic medium, such as a plasma, encounters an obstacle. The plasma, which consists of charged ions and free electrons, is heated by the shock wave through interactions with the electromagnetic fields. In this work, we investigate how electrons are heated at plasma shocks. A key parameter to electron heating is the thickness of the layer where the heating takes place. Here, we use observations from the four Magnetospheric Multiscale spacecraft that regularly cross the standing bow shock that forms when the supersonic plasma, known as the solar wind, encounters Earth's magnetic field. We find that the thickness of the shock is larger than previously reported and is on the scales where ion physics dominate. We also find that the electron heating deviates significantly from simple adiabatic heating. Key Points: Using multipoint data from Magnetospheric Multiscale, we find that electron heating takes place on ion scales in the quasi‐perpendicular shock rampWe show that the time series of the temperature does not represent the spatial profile due to varying shock ramp speedElectron distributions in the ramp and downstream of the shock show that electrons are heated non‐adiabatically [ABSTRACT FROM AUTHOR]

Dipolarization fronts (DFs) have been documented as important structures contributing to energy conversion and flux transport in Earth's magnetotail. However, energy partition and balance at DFs still remain elusive. Using high‐cadence data from NASA's Magnetospheric Multiscale mission, we preform a comprehensive investigation of energy budgets at the DFs. We find that material derivatives of particle energy densities in the DF frame are basically close to zero, indicating that particles experience negligible heating and/or acceleration and that the energy released at the DFs is predominantly manifested in the form of energy flux. The energy flux is found to be dominated by ion and electron enthalpy flux, ion heat flux, and Poynting flux, with electron enthalpy flux being locally elevated. In addition, the energy flux tends to increase during the DFs' earthward propagation, without exhibiting clear asymmetry in the dawn‐dusk direction. These results help further understand energy budgets at the DFs. Plain Language Summary: Dipolarization fronts (DFs), earthward‐propagating magnetic structures characterized by sharp enhancement of northward component (Bz) of the geomagnetic field, have been suggested as favorable regions for particle acceleration/heating, wave generation, and energy transport, etc. In particular, DFs host significant local energy conversion processes which might be responsible for the global energy transport in the Earth's magnetotail. Therefore they have attracted considerable interest in the recent two decades. In the present study, we aim to investigate one key issue regarding energy conversion at the DFs, that is, energy partition and balance, by taking advantage of unprecedentedly high‐cadence data from the NASA's Magnetospheric Multiscale mission. New results obtained from the present investigation can help better understand energy budgets at the fronts and its role in global energy transport. Key Points: Energy released at dipolarization fronts (DFs) is predominately manifested in the form of energy flux, without significant particle heating and/or accelerationEnergy flux is dominated by particle enthalpy flux, ion heat flux and Poynting flux, with electron enthalpy flux locally elevatedEnergy flux tends to increase during DFs' earthward propagation, without clear asymmetry in the dawn‐dusk direction [ABSTRACT FROM AUTHOR]

An interplanetary shock can abruptly compress the magnetosphere, excite magnetospheric waves and field‐aligned currents, and cause a ground magnetic response known as a sudden commencement (SC). However, the transient (<∼1 min) response of the ionosphere‐thermosphere system during an SC has been little studied due to limited temporal resolution in previous investigations. Here, we report observations of a global reversal of ionospheric vertical plasma motion during an SC on 24 October 2011 using ∼6 s resolution Super Dual Auroral Radar Network ground scatter data. The dayside ionosphere suddenly moved downward during the magnetospheric compression due to the SC, lasting for only ∼1 min before moving upward. By contrast, the post‐midnight ionosphere briefly moved upward then moved downward during the SC. Simulations with a coupled geospace model suggest that the reversed E⃗×B⃗ $\vec{E}\times \vec{B}$ vertical drift is caused by a global reversal of ionospheric zonal electric field induced by magnetospheric compression during the SC. Plain Language Summary: It is well‐known that a shock wave can suddenly compress objects they directly interact with. In this study, we report a special case in the geospace environment in which an interplanetary shock produced a concussion‐like response in the ionosphere that was tens of thousands of kilometers away from the location where the shock first impacted. The ionized part of the atmosphere, or the ionosphere, was remotely connected to the magnetosphere‐the region of geospace dominated by the Earth's magnetic field‐via electric currents. When the magnetosphere was abruptly compressed after the shock arrival, a pair of electric currents flowing along the geomagnetic field lines was generated in the dayside mid‐latitudes. The newly generated currents flipped the dayside ionospheric electric field from eastward to westward, leading to a downward motion of dayside ionospheric charged particles. Within 1 minute, the vertical motion and zonal electric field flipped again to the direction before the compression due to the generation of another pair of electric currents with an opposite sense to the first pair. This study depicts a global picture of the transient ionospheric response using multi‐point high‐resolution measurements and simulations with a state‐of‐the‐art fully coupled geospace model. Key Points: Dayside ionospheric plasma undergoes a transient motion from downward to upward during a sudden commencement (SC)Both observations and simulations show that the reversed vertical drift is a global response of the ionosphere to the SCThe transient response is caused by a reversal of induced zonal electric field during the SC [ABSTRACT FROM AUTHOR]

On 19 September 2021, La Palma Cumbre Vieja Volcano started an eruption classified as Volcanic Explosive Index (VEI) 3. In this study, at least the six months prior to such an event have been investigated to search for possible lithosphere–atmosphere–ionosphere bottom-up interactions. The lithosphere has been analysed in terms of seismicity getting advantages from the high-density local seismic network. Possible atmospheric alterations related to the volcano emissions or release of gases due to the uplift of the magmatic chamber have been searched in SO2, aerosol, dimethyl sulphide, and CO. The magnetic field on Earth's surface has been studied by ground geomagnetic observatories. The status of the ionosphere has been investigated with two satellite missions: China Seismo Electromagnetic Satellite (CSES) and European Space Agency Swarm constellation, with Total Electron Content (TEC) retrieved from global maps. We identified a temporal migration of the seismicity from November 2020 at a depth of 40 km that seems associable to magma migration, firstly to a deep chamber at about 15 km depth and in the last 10 days in a shallow magma chamber at less than 5 km depth. The atmospheric composition, ground geomagnetic field, and ionosphere showed anomalies from more than three months before the eruption, suggesting a possible influence from the bottom geo-layers to the upper ones. CSES-01 detected an increase of electron density, confirmed by TEC data, and alterations of vertical magnetic field on ground Guimar observatory that are temporal compatible with some volcanic low seismic activity (very likely due to the magma uplift), suggesting an eventual electromagnetic disturbance from the lithosphere to the ionosphere. A final increase of carbon monoxide 1.5 months before the eruption with unusually high values of TEC suggests the last uplifting of the magma before the eruption, confirmed by a very high shallow seismicity that preceded the eruption by ten days. This work underlines the importance of integrating several observation platforms from ground and overall space to understand geophysics better, and, in particular, the natural hazard affecting our planet. [ABSTRACT FROM AUTHOR]

Earth's radiation belt relativistic electron dropouts frequently occur during the main phase of geomagnetic storms, which have been partially attributed to the ring current ion enhancements causing geomagnetic field reconfigurations. In contrast to early studies on the global radiation belt response to the ring current buildup on a timescale of several hours, we here describe a rapid, localized, significant decay of relativistic electrons driven by freshly injected energetic protons during the 27 May 2017 geomagnetic storm. Near the proton injection front, the magnetic field lines were stretched outward, with an inferred migration of their equatorial crossings by ∼0.5 RE during several minutes. Because of the betatron and Fermi decelerations, the relativistic electron fluxes decreased by 2–3 orders of magnitude with pitch‐angle distributions evolving from pancake/flat‐top type to cigar type. This rapid localized response pattern of relativistic electrons could be general over strong particle injections during both geomagnetic storms and substorms. Plain Language Summary: Relativistic electrons of the Van Allen belts pose a significant radiation hazard for both satellites and astronauts in the geospace. During geomagnetic storms, the radiation belt relativistic electron fluxes characteristically exhibit a sharp decrease at the main phase preceding a substantial increase at the recovery phase. How these relativistic electrons are depleted at the storm main phase remains not fully understood. One class of depletion processes is betatron and Fermi decelerations that act when electrons are transported outward along with the stretching of geomagnetic field lines driven by the ring current ion enhancement. On the basis of multi‐satellite observations and modeling for the 27 May 2017 geomagnetic storm, we here describe a rapid localized deceleration of relativistic electrons driven by a proton injection. The relativistic electron fluxes decreased by 2–3 orders of magnitude within several minutes on the stretched magnetic field lines near the proton injection front, different from the commonly known global radiation belt response to the ring current buildup on a timescale of several hours. This rapid localized response pattern of relativistic electrons could be general over strong particle injections that are able to stretch geomagnetic field lines during both storms and substorms. Key Points: Storm proton injection caused a significant localized stretching of magnetic field lines on a timescale of several minutesRelativistic electrons were locally decelerated through betatron and Fermi processes on the stretching magnetic field linesRelativistic electron fluxes on the same magnetic field lines decreased by 2–3 orders of magnitude (depending on pitch‐angle) [ABSTRACT FROM AUTHOR]

Based on 1‐year equatorial electrojet (EEJ) measurements from Swarm, and ion velocity from the Defense Meteorological Satellite Program, this study first examines the simultaneous effects of the subauroral polarization streams (SAPS) on EEJ during quiet time. The changes in EEJ (ΔEEJ) vary significantly with local time. ΔEEJ flows westward at dawn and is gradually enhanced to eastward at dusk. The eastward ΔEEJ at post‐noon might be driven indirectly by the upward electric field associated with SAPS electric field at SAPS channel, whereas ΔEEJ at pre‐noon might be directly attributed to the penetrating SAPS‐related polarization electric field. Under stronger geomagnetic forcing, a stronger intensity of westward ΔEEJ at dawn and noon is produced, whereas a stronger eastward ΔEEJ at dusk is generated because of a faster SAPS velocity. Plain Language Summary: A series of interesting phenomenon have been observed in Earth's ionosphere‐thermosphere (I‐T) coupled system, for instance, equatorial electrojet (EEJ), subauroral polarization streams (SAPS). EEJ is the strong currents at geomagnetic equator, and SAPS is westward ion flows at subauroral latitudes. These two locates at different latitudes. However, the whole I‐T is a non‐linear system, indicating a potential physical connection between those two. This is the first observational report on the local time variation of the electrodynamic coupling between these two physical processes at different latitude bands. Key Points: Equatorial electrojet (EEJ) owing to both low SME‐subauroral polarization stream (LSME‐SAPS) and high SME‐SAPS (HSME‐SAPS) is westward at dawn and eastward at dusk∆EEJ owing to HSME‐SAPS are stronger westward than that owing to LSME‐SAPS at pre‐noonDusk eastward ∆EEJ is attributed to the upward electric field mapping from SAPS electric field [ABSTRACT FROM AUTHOR]

Multi‐MeV electron drift‐periodic flux oscillations observed in Earth's radiation belts indicate radial transport and energization/de‐energization of these radiation belt core populations. Using multi‐year Van Allen Probes observations, a statistical analysis is conducted to understand the characteristics of this phenomenon. The results show that most of these flux oscillations result from resonant interactions with broadband ultralow frequency (ULF) waves and are indicators of ongoing radial diffusion. The occurrence frequency of flux oscillations is higher during high solar wind speed/dynamic pressure and geomagnetically active times; however, a large number of them were still observed under mild to moderate solar wind/geomagnetic conditions. The occurrence frequency is also highest (up to ∼30%) at low L‐shells (L∗∼3−4 ${L}^{\ast }\sim 3-4$) under various geomagnetic activity, suggesting the general presence of broadband ULF waves and radial diffusion at low L‐shells even during geomagnetically quiet times and showing the critical role of the electron phase space density radial gradient in forming drift‐periodic flux oscillations. Plain Language Summary: Earth's radiation belts contain 100s of keV–10 MeV electrons which pose great threats to the avionics and humans in space. Understanding their dynamics is critical due to both scientific interests and practical needs. Periodic flux oscillations of multi‐MeV electrons at their drift frequencies have been previously observed in Earth's radiation belts. These flux oscillations are believed to be indicators of the radial transport of these electrons and contain critical information of their energization processes. Using Van Allen Probes observations, a statistical analysis is conducted to further understand this phenomenon and associated mechanisms. It is shown that the majority of observed flux oscillations result from resonant interactions between multi‐MeV electrons and broadband ultralow frequency (ULF) waves, which lead to radial diffusion, one of the most important mechanisms causing radiation belt electron acceleration/loss. These flux oscillations preferentially occur under active solar wind conditions and geomagnetic activity. They also occur more often at lower altitudes, which is due to the steeper radial gradient of the electron distribution closer to the Earth. This indicates the general presence of broadband ULF waves and radial diffusion at low altitudes and suggests the critical role of the radial gradient of the electron distribution in forming these flux oscillations. Key Points: Most multi‐MeV electron drift‐periodic flux oscillations in the outer belt result from resonant interaction with broadband ultralow frequency wavesThese flux oscillations preferentially occur during high solar wind speed/dynamic pressure and geomagnetically active timesThe occurrence rate of these flux oscillations is highest at L*∼ 3–4 due to higher phase space density radial gradients at lower L [ABSTRACT FROM AUTHOR]

During a paleomagnetic reversal, Earth's geomagnetic field is about 10% of today's magnitude and its topology is far more complex than dipolar. Revealing the solar wind energy transmission during the reversal is of importance for understanding the historic space environment. Using a global magnetohydrodynamic model implemented with a data‐reconstructed time‐dependent paleomagnetic field, this study simulates the solar wind‐magnetosphere interactions during the last Matuyama‐Brunhes reversal that occurred about 780,000 years ago. As the magnetosphere shrinks in the midst of the reversal, the stand‐off distance of the subsolar magnetopause approaches as close as 3 Re. Moreover, multiple magnetic reconnection sites emerge due to the irregular geomagnetic configuration, opening up many channels on the magnetopause surface for the solar wind to directly access. The power transmission efficiency through the magnetopause increases to 18%. Plain Language Summary: Earth's dynamo at the core produces the intrinsic geomagnetic field that helps prevent solar wind from directly impacting the atmosphere. We present simulation results on the evolution of the space environment during the Matuyama‐Brunhes event that occurred about 780,000 years ago and show that the protection was weakened in two ways: The reduction of the size of the magnetosphere and the increase of power input efficiency. The stand‐off distance of the subsolar magnetopause during the polarity reversal approaches as close as 3 Re. Due to the irregular magnetic field configuration, multiple channels for solar wind energy appear on the magnetosphere, increasing the power transmission efficiency through the magnetopause up to 18%. This implies that many satellites may become more vulnerable to solar storms if a new reversal is underway. Key Points: We simulate the solar wind‐magnetosphere interactions during the Matuyama‐Brunhes reversal event with the space weather modeling frameworkDuring the reversal, the magnetosphere shrinks, and the transmission efficiency of solar wind power increases up to 18%Multiple magnetic reconnection sites open up channels of solar wind power transmission globally [ABSTRACT FROM AUTHOR]

Today's geomagnetic field can prevent energetic particles, including solar energetic particles and galactic cosmic rays, from directly hitting the Earth's atmosphere. However, when the geomagnetic field strength is significantly decreased during geomagnetic field excursions or reversals, the geomagnetic field shielding effect becomes less prominent. Geomagnetic cutoff rigidity, as a quantitative estimation of the shielding effect, can be calculated using trajectory tracing or theoretical equations. We use a recent high‐resolution continuous geomagnetic field model (LSMOD.2) to study the geomagnetic cutoff rigidity during the Laschamps excursion. Global grids of the geomagnetic cutoff rigidities are presented, in particular for the excursion midpoint when the geomagnetic field is weak and not dipole‐dominated anymore at Earth's surface. We compare the cutoff rigidity calculation results between a trajectory tracing program and theoretical equations and we find that the influence of the non‐dipole component of the geomagnetic field cannot be ignored during the excursion. Our results indicate that the exposure of Earth's atmosphere to energetic particles of cosmic and solar origin is high and nearly independent of latitude in the middle of the Laschamps excursion. Our results will be useful for future studies associated with cosmic radiation dose rate and cosmogenic isotope production rate during the Laschamps excursion. Plain Language Summary: The geomagnetic field is a natural shield, prohibiting that solar energetic particles and galactic cosmic rays directly hit the Earth's atmosphere. However, the geomagnetic field is not steady, and even polarity reversals or excursions have occurred numerous times in the geological past. During the geomagnetic field transitions, the morphology of the geomagnetic field is highly complex and the geomagnetic field strength is significantly decreased so that energetic particles can much easier access the Earth's atmosphere. The knowledge of the geomagnetic field variation during excursions has been greatly improved by global models of the paleomagnetic field. A recent high‐resolution continuous geomagnetic field model (LSMOD.2) provides a view of the characteristics of the Laschamps excursion all over Earth's surface. Using the LSMOD.2 model, we analyze the geomagnetic field shielding effect during the Laschamps excursion. Our results suggest that the exposure of Earth's atmosphere to energetic particles, the cosmic radiation dose rate, and cosmogenic isotope production rate increased strongly during the excursion. Key Points: The global distribution of geomagnetic cutoff rigidities during the Laschamps excursion is presentedBoth a trajectory tracing program and theoretical equations are utilized to calculate the cutoff rigidity using the LSMOD.2 modelAt the midpoint of the Laschamps excursion, the cutoff rigidities are lower than 4 GV globally and nearly independent of latitude [ABSTRACT FROM AUTHOR]

When the interplanetary magnetic field (IMF) is northward, magnetic reconnection between the IMF and the geomagnetic field can occur at the high latitude magnetopause, which allows energy transfer from the solar wind into Earth's magnetosphere. In this letter, by using three‐dimensional global hybrid simulations, we find that during the northward IMF, high latitude magnetic reconnection both poleward and equatorward of the cusp can occur almost simultaneously. High latitude magnetopause flux ropes (HLMFRs) with four topologies of magnetic field lines can be formed by multiple X‐line reconnection. At the center of the HLMFRs, there is a decrease in the surrounding magnetic field magnitude (weak core field) and an increase in parallel temperature. When the IMF is purely northward, the HLMFRs are located at the noon‐midnight meridian plane. When the IMF has a positive By component, the HLMFRs in the northern hemisphere shift duskward. Plain Language Summary: When the interplanetary magnetic field (IMF) is southward, magnetic reconnection between the IMF and the Earth's magnetic field can occur at the low latitude magnetopause, and flux ropes are formed. However, when the IMF is northward, the reconnection site is at high latitude magnetopause. In many previous studies, the high latitude magnetopause reconnection with the northward IMF was considered as no flux rope is formed. In this letter, we use computer simulations to investigate the high latitude magnetopause reconnection during the northward IMF in different directions. When the IMF is northward, the high latitude magnetopause flux ropes (HLMFRs) with four types of magnetic field lines are formed. At the center of the HLMFRs, there is a decrease in the surrounding magnetic field magnitude, an increase in parallel temperature. Compared with the flux ropes formed during the southward IMF, the HLMFRs are fewer and smaller. When the IMF is purely northward, the HLMFRs are located at the noon‐midnight meridian plane; when the IMF rotates to the duskward (but still northward), the HLMFRs in the northern hemisphere shift duskward. Key Points: High latitude magnetopause reconnection with different northward IMF clock angles is studied by using 3‐D global hybrid simulationsHigh latitude magnetopause flux ropes (HLMFRs) with four topologies of magnetic field lines are formed by multiple X‐line reconnectionIMF clock angle effects where the HLMFRs occurs and the direction of the HLMFRs' axis [ABSTRACT FROM AUTHOR]

Surface waves process the turbulent disturbances which drive dynamics in many space, astrophysical and laboratory plasma systems, with the outer boundary of Earth's magnetosphere, the magnetopause, providing an accessible environment to study them. Like waves on water, magnetopause surface waves are thought to travel in the direction of the driving solar wind, hence a paradigm in global magnetospheric dynamics of tailward propagation has been well-established. Here we show through multi-spacecraft observations, global simulations, and analytic theory that the lowest-frequency impulsively-excited magnetopause surface waves, with standing structure along the terrestrial magnetic field, propagate against the flow outside the boundary. Across a wide local time range (09–15h) the waves' Poynting flux exactly balances the flow's advective effect, leading to no net energy flux and thus stationary structure across the field also. Further down the equatorial flanks, however, advection dominates hence the waves travel downtail, seeding fluctuations at the resonant frequency which subsequently grow in amplitude via the Kelvin-Helmholtz instability and couple to magnetospheric body waves. This global response, contrary to the accepted paradigm, has implications on radiation belt, ionospheric, and auroral dynamics and potential applications to other dynamical systems. The magnetopause surface waves (SW) that drive global plasma dynamics are thought, like waves on water, to travel with the driving solar wind. Here, the authors show that impulsively-excited SW, with standing structure along the geomagnetic field, are stationary by propagating against this flow. [ABSTRACT FROM AUTHOR]

The magnetopause marks the outer edge of the Earth's magnetosphere and a distinct boundary between solar wind and magnetospheric plasma populations. In this study, we use global magnetohydrodynamic simulations to examine the response of the terrestrial magnetopause to fast‐forward interplanetary shocks of various strengths and compare to theoretical predictions. The theory and simulations indicate the magnetopause response can be characterized by three distinct phases; an initial acceleration as inertial forces are overcome, a rapid compressive phase comprising the majority of the distance traveled, and large‐scale damped oscillations with amplitudes of the order of an Earth radius. The two approaches agree in predicting subsolar magnetopause oscillations with frequencies 2–13 mHz but the simulations notably predict larger amplitudes and weaker damping rates. This phenomenon is of high relevance to space weather forecasting and provides a possible explanation for magnetopause oscillations observed following the large interplanetary shocks of August 1972 and March 1991. Plain Language Summary: The Earth's magnetic field carves out an approximately spherical region of space called a magnetosphere which charged particles streaming from the Sun are unable to directly penetrate. In this study we use computer simulations and theory to examine how the edge of the magnetosphere, the magnetopause, moves following the impact of interplanetary shock waves originating at the Sun. These shocks act to shift the magnetopause to a more tightly compressed position and we find that the magnetopause notably features large‐scale oscillations of decreasing magnitude before settling into a new stable location. When very large shock waves strike, the magnetopause can be compressed inside geosynchronous orbits and will therefore expose telecommunication satellites to increased radiation levels in the solar wind. These results provide an explanation for periodic magnetopause motion observed when large shock waves struck the Earth in August 1972 and in March 1991. Key Points: The response of the terrestrial magnetopause to fast‐forward interplanetary shocks is studied using theory and global magnetohydrodynamic simulationsTheory and simulations predict large‐scale damped oscillations with amplitudes up to an Earth radius and frequencies 2–13 mHzThis phenomenon may explain magnetopause oscillations observed following the large interplanetary shocks of August 1972 and March 1991 [ABSTRACT FROM AUTHOR]

GPS total electron content (TEC) measurements were used to investigate high‐m ultralow frequency (ULF) waves during the recovery phase of a geomagnetic storm. ULF wave signals in TEC data show high coherence and significant common power in the wavelet coherence and cross wavelet transform analyses with magnetic field radial component data from GOES‐15. They did not cause significant ionospheric scintillation or ground magnetic signatures due to ionospheric screening effects. An automatic identification procedure is developed to identify ULF wave signature in TEC data from 10 GPS receivers on January 25, 2016. The waves were mainly distributed on the dayside and post dusk sector from ∼64° to ∼71° magnetic latitude. This is the first time that the large‐scale 2D spatial structure and temporal evolution of high‐m ULF waves are revealed, which demonstrates TEC measurements as an effective high‐m ULF wave remote sensing tool. Plain Language Summary: Ultralow frequency (ULF) waves with periods from 1 s to more than 10 min can interact with particles in the magnetosphere and affect the ionospheric electric field as well as the geomagnetic field. Determining how ULF waves propagate from the magnetosphere to the ionosphere, and ground, and how the wave power is distributed spatially and evolves over time is important for understanding the role of ULF waves in magnetosphere‐ionosphere coupling. ULF waves with large azimuthal wave numbers (high‐m) are usually screened from the ground by the ionosphere and thus not detected by magnetometers. Due to the limitations in spatial coverage of satellites and ground radar observations, it has been difficult to examine the large‐scale spatial structure and temporal evolution of high‐m ULF waves during a single event. In this study, we used data from multiple instruments to analyze the characteristics of the high‐m ULF waves that occurred during January 24–26, 2016. The ULF wave signatures in GPS total electron content (TEC) showed similar time evolution and frequency variations to GOES satellite measurements. By using abundant GPS TEC data and automatic identification procedure, the large‐scale 2D spatial structure and time evolution of high‐m ULF waves are disclosed for the first time. Key Points: 2D spatial structures and time evolution of high‐m ultralow frequency (ULF) waves are revealed for the first time using total electron content (TEC) dataULF waves in TEC data show high coherence, similar temporal distribution and frequency evolution with wave signatures in GOES Br dataThe high‐m waves caused neither ionospheric scintillations nor ground magnetic signatures [ABSTRACT FROM AUTHOR]

The ionosonde at the Sodankylä Geophysical Observatory (SOD; 67 ∘ N, 27 ∘ E; Finland) routinely performs vertical sounding once per minute which enables the study of fast ionospheric variations at a frequency of the long-period geomagnetic pulsations Pc5–6/Pi3 (1–5 mHz). Using the ionosonde data from April 2014–December 2015 and colocated geomagnetic measurements, we have investigated a correspondence between the magnetic field pulsations and variations of the critical frequency of radio waves reflected from the ionospheric F2 layer (fo F2). For this study, we have developed a technique for automated retrieval of the critical frequency of the F2 layer from ionograms. As a rule, the Pc5–6/Pi3 frequency band fluctuations in fo F2 were observed at daytime during quiet or moderately disturbed space weather conditions. In most cases (about 80%), the coherence between the fo F2 variations and geomagnetic pulsations was low. However in some cases (specified as "coherent") the coherence was as large as γ2≥0.5. The following conditions are favorable for the occurrence of coherent cases: enhanced auroral activity (6 h maximal auroral electrojet (AE) ≥800 nT), high solar wind speed (V>600 km/s), fluctuating solar wind pressure and northward interplanetary magnetic field. In the cases when the coherence was higher at shorter periods of oscillations, the magnetic pulsations demonstrated features typical for the Alfvén field line resonance. [ABSTRACT FROM AUTHOR]

Recent observations and particle‐tracing models have revealed a counterintuitive difference between the behaviors of magnetospheric protons and oxygen ions when they encounter the magnetopause. The oxygen ions usually meander around the magnetopause without a full escape to the magnetosheath, whereas the protons can more easily stream across the magnetopause despite their smaller gyroradii. Here, we analytically identify the mechanisms that cause this species dependence. As magnetospheric ions drift toward the magnetopause, the grazing angle of their magnetopause encounter is constrained within a narrow range near 0°, with its maximum being proportional to the square root of the ion gyroradius. It is the grazing angle that largely determines the follow‐up ion motion. The ions with larger grazing angles would meander around the magnetopause, whereas the meandering motion for ions with smaller grazing angles could be easily disrupted to enable their escape even if the normal magnetic field is very weak. Plain Language Summary: The Earth's magnetic field is constrained by the interplanetary field within a finite region named the magnetosphere. The boundary of this region, the magnetopause, separates the magnetospheric plasma from the cooler and denser population in the shocked solar wind. Some of the magnetospheric particles, however, can still penetrate the magnetopause and escape into the interplanetary space. One may expect that heavy ions, like singly charged oxygen ions, are more likely to escape than protons due to their larger gyroradii; however, the opposite trend has been identified in spacecraft observations. This counterintuitive feature, although reproduced in a particle‐tracing model, has not yet been fully understood. Here, we analyze the particle trajectories to identify the mechanism underlying the species dependence of the ion escape. When magnetospheric ions encounter the magnetopause, their grazing angles are constrained within a narrow range near 0°, with the maximum angle depending positively on ion gyroradius. We find that the ions with larger grazing angles would meander around the magnetopause without a full escape, whereas the meandering motion for those with smaller angles could be more easily disrupted by a weak normal magnetic field to enable their field‐aligned escape. Key Points: Despite their smaller gyroradii, protons are found more likely to escape from the magnetosphere than oxygen ionsA mechanism leading to the counterintuitive species dependence is analytically identified based on a simple modelThe model also explains the bifurcated gyrophase bunching of the oxygen ions observed in the magnetosheath [ABSTRACT FROM AUTHOR]

Localized magnetic field depressions in the inner magnetosphere, known as magnetic dips, are produced by the diamagnetic motion of energetic ions injected via substorm activities. The magnetic dips, if deep enough, can produce a local minimum in the radial profile of the field strength to trap the injected protons. Therefore, the trapped protons would drift at the same speed as the dip propagation, which leads to the simultaneous enhancements of proton fluxes in multiple energy channels at the leading edge of the dip structure. On the trailing side, the reduction of proton fluxes shows dispersive features, which can be attributed to the energy‐dependent drift motion of the injected protons in the absence of the local field minimum. This scenario is examined based on comparisons between multi‐spacecraft observations and test‐particle simulations, and their good agreement validates the scenario to shed new light on the dynamics of the inner magnetosphere‐magnetotail coupled system. Plain Language Summary: During magnetospheric substorms, energetic ions are injected from the magnetotail toward the inner magnetosphere, and the diamagnetic motion of these ions produces longitudinally localized field depressions named magnetic dips. Recent studies with multi‐spacecraft observations have revealed the westward propagation of the magnetic dips, which are believed to originate from the westward drift of the injected ions in the geomagnetic field. One may expect from the wide energy range of the injected ions and the energy‐dependence of their drift motion that these ions would be gradually dispersed in the longitude to cause the expansion and shallowing of the dip structure. These expectations, however, are not consistent with the observations of sustained magnetic dips associated with simultaneous ion flux enhancements at multiple energy channels. To reconcile this inconsistency, we propose the ion trapping process for magnetic dips with sufficient depths, which explains the simultaneous flux enhancements observed on the leading edge of the dip structure. On the trailing side, the ions cannot be trapped due to the weaker field depressions, which explains the observations of dispersive ion flux reduction given a discontinued and/or localized process of substorm injection. This scenario is supported by the good agreement between test‐particle simulations and multi‐spacecraft observations. Key Points: We analyze the behavior of energetic protons associated with magnetic dip structures based on observations and test‐particle simulationsMagnetic dips with strong field depressions can trap energetic protons inside and have them drift at the same speed as the dip propagationObserved dispersionless increases and dispersive reductions of proton fluxes on dip leading and trailing sides are reproduced by simulations [ABSTRACT FROM AUTHOR]

This study reports the temporal and spatial distributions of the extremely/very low frequency (ELF/VLF) wave activities and the energetic electron fluxes in the ionosphere during an intense storm (geomagnetic activity index Dst of approximately −174 nT) that occurred on 26 August 2018, based on the observations by a set of detectors onboard the China Seismo-Electromagnetic Satellite (CSES). A good correlation of the ionospheric ELF/VLF wave activities with energetic electron precipitations during the various storm evolution phases was revealed. The strongest ELF/VLF emissions at a broad frequency band extending up to 20 kHz occurred from the near-end main phase to the early recovery phase of the storm, while the wave activities mainly appeared at the frequency range below 6 kHz during other phases. Variations in the precipitating fluxes were also spotted in correspondence with changing geomagnetic activity, with the max values primarily appearing outside of the plasmapause during active conditions. The energetic electrons at energies below 1.5 MeV got strong enhancements during the whole storm time on both the day and night side. Examinations of the half-orbit data showed that under the quiet condition, the CSES was able to depict the outer/inner radiation belt as well as the slot region well, whereas under disturbed conditions, such regions became less sharply defined. The regions poleward from geomagnetic latitudes over 50° were found to host the most robust electron precipitation regardless of the quiet or active conditions, and in the equatorward regions below 30°, flux enhancements were mainly observed during storm time and only occasionally in quiet time. The nightside ionosphere also showed remarkable temporal variability along with the storm evolution process but with relatively weaker wave activities and similar level of fluxes enhancement compared to the ones in the dayside ionosphere. The ELF/VLF whistler-mode waves recorded by the CSES mainly included structure-less VLF waves, structured VLF quasi-periodic emissions, and structure-less ELF hiss waves. A wave vector analysis showed that during storm time, these ELF/VLF whistler-mode waves obliquely propagated, mostly likely from the radiation belt toward the Earth direction. We suggest that energetic electrons in the high latitude ionosphere are most likely transported from the outer radiation belt as a consequence of their interactions with ELF/VLF waves. [ABSTRACT FROM AUTHOR]

We present the first clear observational connection between interplanetary shock impact angles and magnetospheric ultralow frequency (ULF) wave activity. We perform a comparative study of two solar wind shocks with relatively similar strengths, but one being nearly frontal and, the other, highly inclined. We utilize multipoint observations with magnetometers based in space and on the ground for comparisons. For satellites and ground stations occupying similar local time positions, we find that the ULF waves have larger amplitudes and tend to be more narrowband in the case of the nearly head‐on impact, confirming previous simulation results. Additionally, we provide evidence that, due to their symmetric compression nature, nearly frontal shocks can excite only odd mode waves, while asymmetric compressions caused by inclined shocks can excite both odd and even mode waves. These results suggest that shock impact angles play crucial roles in mediating ULF wave‐particle interactions in the inner magnetosphere. Plain Language Summary: The interaction of solar perturbations with the Earth's magnetic field is of paramount importance in space weather investigations. The subsequent response is almost always characterized by magnetic field disturbances measured in space and on the ground, which in turn can modulate energetic particle populations that damage satellite electronics and cause electric current surges in large‐scale power transmission lines. These disturbances are also manifested as geomagnetic pulsations which in part are responsible for the distribution of energy in the near‐Earth space environment. In this work, we connect, for the first time, observational properties of these pulsations or waves with the impact angle of the perturbation. We find that nearly head‐on impacts are associated with stronger wave response in comparison to inclined impacts. These results show that space weather forecasters should take the perturbation impact angle into account when predicting geomagnetic effects caused by solar perturbations. Key Points: We perform a comparative study of space‐ and ground‐based magnetic field response to two interplanetary shocks with different orientationsWe suggest nearly frontal shocks excite magnetospheric odd mode ULF waves, while highly inclined shocks can excite both odd and even modesWave magnetic field amplitudes caused by similarly strong shocks are higher in the case of the head‐on impact due to symmetric compression [ABSTRACT FROM AUTHOR]

This paper presents observations of dayside auroral signatures resulting from a massive localized compression of the magnetopause, magnetosheath, and bow shock. On 10 December 2016, the Magnetospheric Multiscale Mission (MMS) spacecraft observed a compression of the entire magnetosheath and bow shock, which moved past the spacecraft in 1 min and 45 s. Shortly afterward, the resulting auroral signature was observed at the Kjell Henriksen Observatory located in Longyearbyen, Norway. The characteristics of this unique event have major ramifications for understanding dayside magnetospheric physics. Plain Language Summary: The aurora borealis and australis are the visible signatures of interactions between the Earth's magnetic field and the solar wind. When the solar wind's magnetic field interacts with the Earth's magnetic field, there can be large fluctuations in its positioning and shape. This paper presents observations from satellites of a very large compression of the magnetic field, resulting in unique ionospheric signatures directly over an all‐sky camera at Kjell Henricksen Observatory in Svalbard, Norway. One minute and 57 s after the compression at the magnetosphere, previously unstructured aurora began to intensify and organize, first propagating eastward and then brightening westward. The auroral arcs then began to spiral in a counterclockwise direction forming a spiral structure over the field of view. This defined shape only lasted for a few seconds before dispersing and moving poleward. The characteristics of this unique event have major ramifications for understanding dayside magnetospheric physics. Key Points: Analysis of rapid compression of the magnetopause, magnetosphere, and bow shockObservations of unusual auroral signature over Svalbard, Norway resulting from compression [ABSTRACT FROM AUTHOR]

Magnetic reconnection is a fundamental and important physical process that can release magnetic energy and change magnetic topology. A statistic study of reconnected magnetic field lines has been carried out by Cluster spacecraft 1 during 58 current sheet crossing events. We calculated the values of at different distances from reconnection site to describe the loosing process of magnetic field lines. Our results suggest that current sheets in ion diffusion regions have small values of. As the distance from reconnection site increases, the reconnected magnetic field lines relax, and subsequently have relatively large values of. However, such large values of are found to be closely related to open magnetic field lines that connect the Earth's magnetosphere and the solar wind. [ABSTRACT FROM AUTHOR]

At the Earth's low‐latitude magnetopause, clear signatures of the Kelvin‐Helmholtz (KH) waves have been frequently observed during periods of the northward interplanetary magnetic field (IMF), whereas these signatures have been much less frequently observed during the southward IMF. Here, we performed the first 3‐D fully kinetic simulation of the magnetopause KH instability under the southward IMF condition. The simulation demonstrates that fast magnetic reconnection is induced at multiple locations along the vortex edge in an early nonlinear growth phase of the instability. The reconnection outflow jets significantly disrupt the flow of the nonlinear KH vortex, while the disrupted turbulent flow strongly bends and twists the reconnected field lines. The resulting coupling of the complex field and flow patterns within the magnetopause boundary layer leads to a quick decay of the vortex structure, which may explain the difference in the observation probability of KH waves between northward and southward IMF conditions. Plain Language Summary: Space between planets is filled with ionized gas released from the upper atmosphere of the Sun, called the solar wind. Although the Earth's magnetic field basically acts as a barrier to prevent energetic solar wind from penetrating into the region filled with the Earth's magnetic field, called the magnetosphere, it is known that the solar wind frequently leaks into the magnetosphere. The Kelvin‐Helmholtz (KH) instability, which is a flow‐driven instability and can be unstable by the antisunward flowing solar wind, has been considered as an important candidate process for the solar wind leaks. However, past spacecraft observations have revealed that the observation probability of KH waves is very low when the magnetic field in the solar wind, called the interplanetary magnetic field (IMF), is oriented southward, that is, opposite from the Earth's magnetic field. In this study, based on a plasma kinetic simulation, it is found that when the IMF is southward, magnetic reconnection, which is an explosive plasma process that rearranges the magnetic topology across the boundary, occurs at multiple locations in the KH waves and rapidly destroys the wave structures. This may explain the low observation probability of the KH waves under the southward IMF. Key Points: Three‐dimensional fully kinetic simulation of Kelvin‐Helmholtz instability at the Earth's magnetopause under the southward IMF condition is performedFast reconnection causes a rapid decay of the nonlinear vortex structure in the early nonlinear growth phase of the instabilityThe vortex decay can lead to a lower probability of observing magnetopause Kelvin‐Helmholtz waves/vortices during southward IMF periods [ABSTRACT FROM AUTHOR]

We studied the space weather effects on the South Atlantic Anomaly (SAA) magnetic response using Tsyganenko models. For the physical parameters characterizing the SAA, the study considered the minimum magnetic field, the location (longitude and latitude) of the SAA center, and the area of the SAA. Regarding the space weather parameters, we considered the solar wind dynamic pressure, the interplanetary magnetic field components, B yIMF and B zIMF , the Dst index, and the geodipole tilting angle. To study the magnetic field response of the SAA, several different versions of the Tsyganenko models, namely, T96, T01, and TS05, were used to describe the external magnetic field contributions. The main internal magnetic field was calculated by the International Geomagnetic Reference Field (IGRF-12). The magnetic field study of the SAA was realized in long- and short-term (seasonal and diurnal) variations. We found that the Dst index and the geodipole tilting angle were the strongest influencing parameters on the SAA magnetic field response at all altitudes. Moreover, it was revealed that both magnetic poles might be a possible cause of the SAA magnetic field response, resulting from the space weather conditions. Furthermore, the magnetic field behavior of the SAA was affected by hourly variations, where the largest changes occurred at dayside. [ABSTRACT FROM AUTHOR]

The plasmasphere is a highly dynamic toroidal region of cold, dense plasma around Earth. Plasma waves exist both inside and outside this region and can contribute to the loss and acceleration of high energy outer radiation belt electrons. Early observational studies found an apparent correlation on long time scales between the observed inner edge of the outer radiation belt and the modeled innermost plasmapause location. More recent work using high‐resolution Van Allen Probes data has found a more complex relationship. For this study, we determine the standoff distance of the location of maximum electron flux of the outer belt MeV electrons from the plasmapause following rapid enhancement events. We find that the location of the outer radiation belt based on maximum electron flux is consistently outside the plasmapause, with a peak radial standoff distance of ∆L ~ 1. We discuss the implications this result has for acceleration mechanisms. Plain Language Summary: We present a statistical study of the location of particles, particularly electrons, in currents around the Earth whose intensity is increased, or enhanced, rapidly (within 1 day). We use over 5 years of particle intensity and plasma density data from the Van Allen Probes satellites. We find that these quick enhancement events occur when the particles are outside the edge of the plasmasphere, an ever‐changing, often asymmetric donut‐shaped region of plasma around the Earth. Specifically, we find that the locations of particles are correlated with the location of plasma waves outside the plasmasphere, which supports models of electron acceleration from interactions with these waves over short periods of time. Understanding these wave‐particle interactions will improve our understanding of how the plasmasphere under differing conditions can either shield Earth from or worsen the impacts of geomagnetic activity and space weather. Key Points: During periods of rapid electron flux enhancement, the outer radiation belt persists outside the plasmapauseThe belt at enhancement times stands off from the plasmapause, spatially correlated with the region of peak chorus activityThis statistical finding supports a model of chorus enhancement dominating on short time scales [ABSTRACT FROM AUTHOR]

Based on assumptions that substorm field line dipolarization at geosynchronous altitudes is associated with the arrival of high-velocity magnetotail flow bursts referred to as bursty bulk flows, the following sequence of field line dipolarization is proposed: (1) slow magnetoacoustic wave excited through ballooning instability by enhanced inflows in pre-onset intervals towards the equatorial plane; (2) in the equatorial plane, slow magnetoacoustic wave stretching of the flux tube in dawn–dusk directions resulting in spreading plasmas in dawn–dusk directions and reduction in the radial pressure gradient in the flux tube. As a consequence of these processes, the flux tube assumes a new equilibrium geometry in which the curvature radius of new field lines increased in the meridian plane, suggesting an onset of field line dipolarization. The dipolarization processes associated with changing the curvature radius preceded classical dipolarization caused by a reduction of cross-tail currents and pileup of the magnetic fields. Increasing the curvature radius induced a convection surge in the equatorial plane as well as inductive westward electric fields of the order of millivolts per meter (mV m -1). Electric fields transmitted to the ionosphere produce an electromotive force in the E layer for generating a field-aligned current system of Bostrom type. This is also equivalent to the creation of an incomplete Cowling channel in the ionospheric E layer by the convection surge. [ABSTRACT FROM AUTHOR]

While magnetopause reconnection naturally occurs in three‐dimensional environments, our understanding of the process has been mainly confined to the two‐dimensional reconnection plane. How reconnection varies in the third direction, that is, the azimuthal direction, and what causes the variation remain open questions. We study the azimuthal variation of reconnection at an underexplored scale, which is below an Earth radius, using THEMIS spacecraft. Our results show that reconnection jets transition from being present to absent within tenths of a RE, corresponding to tens of ion inertial lengths. The absence of jets cannot be explained by an ion or electron diffusion region, or a magnetic flux rope flanked by two reconnection sites. The sharp transition of reconnection is associated with spatially varying waves. No drivers of the transition are identified locally at the magnetopause or in the upstream, implying that reconnection may naturally have a finite extent at certain stages of its development. Plain Language Summary: Magnetic reconnection extracts energy from the solar wind to power plasma convection, auroral displays, and geomagnetic substorms and storms in Earth's magnetosphere. Although reconnection is often pictured as a two‐dimensional laminar process, where oppositely directed magnetic field lines in a plasma break and reconnect, the actual reconnection is three‐dimensional where it varies along the direction perpendicular to the plane. Since the scale in the third dimension determines how much energy is imparted to the magnetosphere, knowledge of how reconnection varies in this direction and what causes the variation is extremely important. Among existing reports, the utilized spacecraft constellations are often too small or too large, only loosely constraining the range of variability in the third dimension. Here, we use serendipitous constellations with a spatial size that is right at the predicted scale for reconnection to vary. Our results show that reconnection jets transition from being present to absent within tenths of an Earth radius. The sharp transition of reconnection is associated with spatially varying waves. Causes of the transition cannot be identified from the surrounding or upstream electromagnetic field and plasma conditions, implying that reconnection may naturally have a finite extent at certain stages of its development. Key Points: We study the azimuthal variation of magnetopause reconnection at sub‐RE scale using serendipitous THEMIS constellationsReconnection jets transition from present to absent within tens of ion inertial lengths, in association with spatially varying wavesWe do not observe clear drivers of the finiteness of reconnection locally at the magnetopause or in the upstream magnetosheath [ABSTRACT FROM AUTHOR]

This paper presents a multi-parameter global statistical model of extreme horizontal geomagnetic field fluctuations (dBH/dt), which are a useful input to models assessing the risk of geomagnetically induced currents in ground infrastructure. Generalised Pareto (GP) distributions were fitted to 1-min measurements of |dBH/dt| from 125 magnetometers (with an average of 28 years of data per site) and return levels (RL) predicted for return periods (RP) between 5 and 500 years. Analytical functions characterise the profiles of maximum-likelihood GP model parameters and the derived RLs as a function of corrected geomagnetic latitude, λ. A sharp peak in both the GP shape parameter and the RLs is observed at |λ| = 53° in both hemispheres, indicating a sharp equatorward limit of the auroral electrojet region. RLs also increase strongly in the dayside region poleward of the polar cusp (|λ| > 75°) for RPs > 100 years. We describe how the GP model may be further refined by modelling the probability of occurrences of |dBH/dt| exceeding the 99.97th percentile as a function of month, magnetic local time, and the direction of the field fluctuation, dBH, and demonstrate that these patterns of occurrence align closely to known patterns of auroral substorm onsets, ULF Pc5 wave activity, and (storm) sudden commencement impacts. Changes in the occurrence probability profiles with the interplanetary magnetic field (IMF) orientation reveal further details of the nature of the ionospheric currents driving extreme |dBH/dt| fluctuations, such as the changing location of the polar cusp and seasonal variations explained by the Russell-McPherron effect. [ABSTRACT FROM AUTHOR]

Earth's present dipolar magnetic field extends into the interplanetary space and interacts with the solar wind, forming a magnetosphere filled up with charged particles mostly originating from the Earth's atmosphere. In the elongated tail of the magnetosphere, the particles were observed to move either Earthward or tailward at different locations, even outside the Moon's orbit. We hypothesize that the lunar soil, on both the nearside and farside, should have been impacted by these particles during the geological history, and the impact was controlled by the size and morphology of the magnetosphere. We predict that the farside soil could also have the features similar to those in the nearside soil, e.g., 15N‐enrichment. Furthermore, we may infer the evolution of the magnetosphere and atmosphere by examining the implanted particles in the lunar soil from both sides. This hypothesis could provide an alternative way to study the evolution of Earth's dynamo and atmosphere. Key Points: The configuration of ancient geomagnetic field could be inferred from lunar soilBoth nearside and farside lunar soil might have recorded Earth's volatilesThe farside soil was impacted by ~100 km/s atmospheric ions for 3.5 billion years [ABSTRACT FROM AUTHOR]

Short-term upper atmosphere variations due to magnetospheric forcing are very complex, and neither well understood nor capably modeled due to limited observations. In this paper, mass density variations from 10 years of GRACE observations (2003–2013) are isolated via the parameterization of annual, local solar time (LST), and solar cycle fluctuations using a principal component analysis (PCA) technique. The resulting residual disturbances are investigated in terms of magnetospheric drivers. The magnitude of high-frequency (δ < 10 d) disturbances reveals unexpected dependencies on the solar cycle, seasonal, and an asymmetric behavior with smaller amplitudes in June in the south polar region (SPR). This seasonal modulation might be related to the Russell–McPherron (RM) effect. Meanwhile, we find a similar pattern, although less pronounced, in the northern and equatorial regions. A possible cause of this latitudinal asymmetry might be the irregular shape of the Earth's magnetic field (with the north dip pole close to Earth's rotation axis, and the south dip pole far from that axis). After accounting for the solar cycle and seasonal dependencies by regression analysis to the magnitude of the high-frequency perturbations, the parameterization in terms of the disturbance geomagnetic storm-time index Dst shows the best correlation, whereas the geomagnetic variation Am index and merging electric field Em are the best predictors in terms of time delay. We test several mass density models, including JB2008, NRLMSISE-00, and TIEGCM, and find that they are unable to completely reproduce the seasonal and solar cycle trends found in this study, and show a clear overestimation of about 100 % during low solar activity periods. [ABSTRACT FROM AUTHOR]

Within the last two solar cycles (from 2001 to 2018), the location of the outer radiation belt (ORB) was determined using NOAA/Polar-orbiting Operational Environmental Satellite (POES) observations of energetic electrons with energies above 30 keV. It was found that the ORB was shifted a little (∼1 ∘) in the European and North American sectors, while in the Siberian sector the ORB was displaced equatorward by more than 3 ∘. The displacements corresponded qualitatively to the change in the geomagnetic field predicted by the IGRF-12 model. However, in the Siberian sector, the model has a tendency to underestimate the equatorward shift of the ORB. The shift became prominent after 2012, which might have been related to a geomagnetic "jerk" that occurred in 2012–2013. The displacement of the ORB to lower latitudes in the Siberian sector can contribute to an increase in the occurrence rate of midlatitude auroras observed in the Eastern Hemisphere. [ABSTRACT FROM AUTHOR]