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The theta aurora is reproduced by global simulation. First, we construct a solution for the stationary northward interplanetary magnetic field (IMF) forming the separatrices, the separators, the nulls, and the stemlines. From the drawing of last‐closed field lines, the overall structure under this condition is summarized as the northern lobe is generated by a separatrix emanating from the southern null. In this paper, all variations are antisymmetric in the southern hemisphere. Afterward, the IMF By is switched to reproduce the theta aurora. The ionospheric theta aurora is reproduced as closed magnetic field regions. The polar cap is divided to old and new parts, by the theta bar. In the magnetosphere, two dayside nulls occur corresponding to the new IMF and two nulls corresponding to the old IMF retreat tailward. The four nulls form a structure connected by four separators, constructing the magnetospheric topology corresponding to the theta aurora. In this topology, old and new nulls in the southern hemisphere generate old and new lobes in the northern hemisphere. Each lobe is projected onto northern old and new polar caps. The origin of the theta bar is the stagnating closed magnetic field region that occurs between old and new lobes. Separator reconnection occurs between the old lobe in the southern and the new lobe in the northern hemispheres, reducing southern old polar caps. This is the cause of the movement of the theta bar. The theta aurora is the phenomenon that demonstrates the existence of the null‐separator structure. Key Points: The theta aurora is an image of the magnetospheric null‐separator structure projected to the virtual mirror in the ionosphereNorthern new and old lobes are respectively generated from new dayside and old retreating nulls in the southern hemisphereThe theta bar is the projection of stagnating closed magnetic field regions accumulating at the interface between old and new lobes [ABSTRACT FROM AUTHOR]

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

Short large‐amplitude magnetic structures (SLAMS) frequently appear near the bow shock. Inside steepening SLAMS, whistler‐mode waves are coherently generated at their leading edges. These waves are crucial for electron dynamics, energy conversion and magnetic reconnection near the shock. Nevertheless, the characteristics and generation of the whistler‐mode waves inside SLAMS are still unclear. In this study, we conducted a statistical analysis of whistler‐mode waves within SLAMS near the Earth's bow shock to investigate their properties and generation mechanisms. We found that these waves are mainly excited by electron temperature anisotropy with 99% of them falling below the nonlinear resonant threshold. The combination of a low background magnetic field and intense steepening at SLAMS' leading edge makes it the main source region of whistler‐mode waves and may induce the nonlinear resonance of whistler‐mode waves. Plain Language Summary: On the quasi‐parallel side of the bow shock, ultra‐low frequency waves and nonlinear structures such as shocklets and SLAMS are commonly observed, which are crucial for the particle dynamics and energy conversion near the shock. Whistler‐mode waves are coherently generated at the leading edge or Earthward side of these nonlinear structures and may subsequently evolve into the shocklets. These waves are crucial for electron dynamics, energy dissipation and energy cascade near the shock. In addition, recent studies have discovered that the propagation of whistler‐mode waves can trigger magnetic reconnection processes inside SLAMS. Nevertheless, the characteristics and generation of the whistler‐mode waves inside SLAMS are still unclear. A comprehensive understanding of whistler‐mode waves inside SLAMS is still missing. In this paper, based on statistical results, we found that most of the whistler‐mode waves are locally generated and trapped inside the SLAMS. The nonlinear resonance condition of whistler‐mode waves is only satisfied at the leading edge of SLAMS with significant steepening. Key Points: Whistler‐mode waves are locally excited and trapped inside the SLAMSMost of the whistler‐mode are excited by electron temperature anisotropy resulting from SLAMS steepeningThe significant steepening seems essential for the nonlinear resonance of whistler‐mode waves at the leading edge of SLAMS [ABSTRACT FROM AUTHOR]

Based on Global Coupled Ionosphere‐Thermosphere‐Electrodynamics Model, the solution of the 3‐dimensional current in the ionospheric region, the equivalent sheet current and filed‐aligned current are examined. The simulation study enables a comprehensive analysis of the effect of the geomagnetic field configuration, especially the non‐dipole component and tilt angle, on the ionospheric electrodynamics phenomena. Different geomagnetic field configurations are specified in the present work, including realistic geomagnetic field (RGF), tilted dipole geomagnetic field (TDGF) and zero‐declination dipole geomagnetic field (ZDGF). Our simulation focuses on the seasonal variation of Sq current, primarily governed by annual and semi‐annual variation. The modulation of the tilt angle of the geomagnetic field is globally distributed, whereas the modulation of the geomagnetic anomaly is localized. At mid‐latitudes, the annual mean and semi‐annual amplitudes of the Sq current are negatively correlated with the magnetic field strength, especially shown in geomagnetic anomaly area, while there is the opposite effect in the geomagnetic conjugate regions of the opposite hemispheres. The annual variation of Sq current system is more affected by the offset of the geomagnetic latitudes and geographical latitudes. The seasonal variation of the total Sq current is also modulated by the geomagnetic field. The annual mean, the annual and semi‐annual components of the total Sq current are negatively correlated with the magnetic field strength, while the annual variation is also controlled by the tilt angle of the geomagnetic field. The solar radiation affects the semi‐annual variation of the current more strongly than the annual variation. Plain Language Summary: Ionospheric currents and magnetic fields exert significant influences on energy and material transport within the ionosphere, playing a pivotal role in Earth's electromagnetic environment, which is the primary focus of ionospheric research. To investigate the impact of the geomagnetic field on the ionospheric current system, we employ a theoretical model (GCITEM) for simulation work. In this simulation, we consider both the realistic geomagnetic field configuration and two hypothetical idealized configurations of the geomagnetic field: one excluding non‐dipole terms and the other further aligning the geomagnetic axis with Earth's spin axis. The analysis of the longitude and the seasonal variation of the simulated currents under realistic magnetic field conditions has been validated through previous observation results. In comparison to simulation results obtained under hypothetical configurations, we discern and analyze the modulation and mechanism of non‐dipole terms and the tilt angle of geomagnetic field on the variability of the ionospheric current system, which are discussed in detail within this paper. This study facilitates the analysis and prediction of long‐term changes in the geomagnetic field's effects on ionospheric current generation and the impact on space's electromagnetic environment. Key Points: The geomagnetic field intensity exerts a significant influence on the Sq currents and exhibits a negative correlation with Sq currentThe annual mean and annual variation amplitude of Sq current are positively correlated with the geomagnetic anomaly at conjugate pointsThe tilt angle of the geomagnetic field affects the annual variation of Sq current globally, but hardly effects the semiannual variation [ABSTRACT FROM AUTHOR]

This paper presents observations of polar cap arc substructure down to scale sizes of meters and temporal resolution of milliseconds. Two case studies containing polar cap arcs occurring over Svalbard are investigated. The first occurred on 4 February 2016 and is consistent with formation on closed field lines; the second occurred on 15 December 2015 and is consistent with formation on open field lines. These events were identified using global‐scale images from the Special Sensor Ultra‐violet Spectrographic Imager (SSUSI) instruments on board Defense Meteorological Satellite Program (DMSP) spacecraft. Intervals when the arcs passed through the small‐scale field of view of the Auroral Structure and Kinetics (ASK) instrument, located on Svalbard, were then found using all sky images from a camera also located on Svalbard. These observations give unprecedented insight into small‐scale polar cap arc structure. The energy and flux of the precipitating particles above these arcs are estimated using the ASK observations in conjunction with the Southampton Ionospheric model. These estimates are then compared to in situ DMSP particle measurements, as well as data from ground‐based instrumentation, to infer further information about their formation mechanisms. This paper finds that polar cap arcs formed on different magnetic field topologies exhibit different behavior at small‐scale sizes, consistent with their respective formation mechanisms. Key Points: We compare observations (at multiple scales) of two polar cap arcs that are consistent with different formation mechanismsSmall‐scale observations of polar cap arcs down to scale sizes of meters and temporal resolution of milliseconds are presentedWe find the small‐scale structures associated with the arcs in each event are very different, consistent with the formation mechanisms [ABSTRACT FROM AUTHOR]

This paper presents the magnetospheric multiscale (MMS) observations of three flux ropes (FRs) sequentially in a reconnection exhaust at Earth's magnetopause. Three active X‐lines, which are responsible for the formation of these FRs, were also detected by MMS, providing strong evidence that the FRs were generated by multiple X‐line reconnection. We find that the core field inside the FRs considerably affects the Hall current in the adjacent X‐lines. The intense core field of the FRs deflected the electron outflow jet, forming the electron‐scale Hall magnetic field on the magnetosheath side. Additionally, an active secondary reconnection was observed at the center of one FR. This secondary reconnection split the FR into two smaller FRs and produced sharp density gradients and intense fluctuations of electric field and current within the FR. These observations advance the understanding of the formation and evolution of FRs during multiple X‐line reconnection at magnetopause. Plain Language Summary: Flux transfer event (FTE) is generally explained as the flux rope (FR) generated by multiple X‐line reconnection at the dayside magnetopause. Although the theory and simulations have shown that the multiple active X‐lines and FRs can be produced simultaneously by the tearing instability in the magnetopause current sheet, there is no in‐situ evidence to directly support this scenario yet. Using high‐resolution measurements from the magnetospheric multiscale mission, we present a case study that all active X‐lines expected at the edges of three successive FRs were observed. These observations confirm that reconnection could simultaneously occur at multiple sites to produce the FRs at Earth's magnetopause. Interestingly, the electron‐scale Hall magnetic field was observed at the magnetosheath side in this multiple X‐line reconnection. This is due to the electron outflow jet deflected into the magnetosheath side by the strong core field of FR. Additionally, an ongoing secondary reconnection was observed at the center of one FR. Intense waves and current perturbations were observed in a smaller FR which was produced by this secondary reconnection. Key Points: Ongoing multiple X‐line reconnection generates a series of flux ropes at the magnetopauseElectron‐scale Hall magnetic fields are formed due to the high deflection of electron jets caused by the core field of the flux ropeSecondary reconnection within the flux rope split the flux rope into smaller flux ropes and produces intense electromagnetic fluctuations [ABSTRACT FROM AUTHOR]

The properties of chorus waves have been extensively studied, which are important to understand chorus generation and wave‐particle interactions between chorus and energetic electrons. Observations reveal that there exists a distinct day‐night asymmetry for the properties of chorus waves, such as the sweep rate and duration, which is predicted to be relevant to the asymmetric configuration of Earth's background magnetic field. In this paper, using the one‐dimensional (1‐D) particle‐in‐cell (PIC) simulation model with a compressed/stretched dipole magnetic field, we study the dependences of the properties of rising‐tone chorus waves on the background magnetic field. It is demonstrated that the saturated amplitude and the chorus sweep rate increase while the chorus duration decreases with an increase of the compression factor ξ, which represents the compression/stretch degree of the field line. Moreover, the threshold of exciting chorus waves in the compressed dipole field is generally lower than that in the stretched dipole field. These results are useful to understand the chorus excitation and the day‐night asymmetry of chorus properties. Key Points: A one‐dimensional particle‐in‐cell model is used to study how the configuration of the magnetic field controls the properties of chorus wavesThe amplitude and frequency sweep rate increase with increasing compression factor ξ, while the duration varies oppositelyThe threshold of exciting chorus waves is lowered by a large compression factor or a large field curvature [ABSTRACT FROM AUTHOR]

A broad statistical study addresses for the first time an evolution of ultra‐low frequency waves and/or fluctuations in the terrestrial foreshock around the Moon generated through the interaction between the back‐streaming particles reflected from the bow shock and the incoming solar wind. It is supposed that the waves propagate sunward but they are convected by the solar wind flow back toward the bow shock and their amplitudes grow. However, our study shows that waves/fluctuations could be growing as well as decaying toward the bow shock under the quasi‐radial interplanetary magnetic field. We demonstrate that the growth rate is positive and larger for compressive variations of the magnetic field strength and density than for components of the magnetic field. We show that even if a possible influence of the Moon and its wake is excluded, the growth rate is decreased by nonlinear effects. Plain Language Summary: The paper presents an extensive statistical study of fluctuations in the distant foreshock. The study is based on observations of the ARTEMIS spacecraft pair orbiting around the Moon. The study is motivated by already published values of wave growth rate in this region that are in some sense contradictory. The analysis is based on ratios of standard deviations of the ion density and magnetic field magnitude and its components measured by two spacecraft. The growth rate was computed during intervals when both spacecraft were inside the foreshock in positions ensuring that the wave propagation from the upstream to downstream spacecraft cannot be influenced by the Moon or its wake. Although the determined growth rates can have different signs in individual events, we demonstrate that the growth rate is positive and larger for compressive variations of the magnetic field strength and density than for components of the magnetic field in a statistical sense. We also show that the growth rate is decreased by nonlinear effects. Key Points: Ultra‐low frequency waves/fluctuations in the distant foreshock around the Moon are statistically analyzedThe fluctuations could be growing as well as decaying toward the bow shock under the quasi‐radial interplanetary magnetic fieldThe wave growth rate is positive and larger for compressive variations of the magnetic field strength and plasma density [ABSTRACT FROM AUTHOR]

Previous statistical studies of plasmaspheric hiss investigated the averaged shape of the magnetic field power spectra at various points in the magnetosphere. However, this approach does not consider the fact that very diverse spectral shapes exist at a given L‐shell and magnetic local time. Averaging the data together means that important features of the spectral shapes are lost. In this paper, we use an unsupervised machine learning technique to categorize plasmaspheric hiss. In contrast to the previous studies, this technique allows us to identify power spectra that have "similar" shapes and study their spatial distribution without averaging together vastly different spectral shapes. We show that strong negative correlations exist between the hiss frequency and bandwidth. Key Points: Unsupervised machine learning is used to categorize hiss power spectra of the electric field from Van Allen ProbesFrom the pre‐noon sector toward the afternoon sector the hiss frequency decreases while the bandwidth increasesWe discuss possible source mechanisms that are consistent with the spatial distribution of the hiss waves [ABSTRACT FROM AUTHOR]

Jupiter's magnetic field is tilted by ∼10° with respect to the planet's spin axis, and as a result the Jovian plasma sheet passes over the Galilean satellites at the jovigraphic equator twice per planetary rotation period. The plasma and magnetic field conditions near Ganymede's magnetosphere therefore change dramatically every ∼5 hr, creating a unique magnetosphere‐magnetosphere interaction, and on longer time scales as evidenced by orbit‐to‐orbit variations. In this paper, we summarize the typical magnetic field conditions and their variability near Ganymede's orbit as observed by the Galileo and Juno spacecraft. We fit Juno data from orbit 34, which included the spacecraft's close Ganymede flyby in June 2021, to a current sheet model and show that the magnetospheric conditions during orbit 34 were very close to the historical average. Our results allow us to infer the upstream conditions at the time of the Juno Ganymede flyby. Plain Language Summary: Ganymede is the only moon in the solar system with an intrinsic magnetic field. This field forms a bubble in space around the moon, called a magnetosphere, that is itself contained within Jupiter's magnetosphere. The magnetic field and plasma conditions within Ganymede's magnetosphere can be used to infer information about the satellite's atmosphere, ionosphere, and interior. It is therefore important to understand the interaction between Ganymede's magnetosphere and the Jovian environment in the same way that we study the effects of space weather on the Earth. Here we analyze Galileo magnetic field measurements from Jupiter's magnetosphere in the region near Ganymede's orbit to establish the typical magnetic field magnitude and direction. We discuss the average conditions as well as the nature of the variability that occurs due to dynamic processes occurring in Jupiter's magnetosphere. This information provides useful context for analyzing data from Juno's recent flyby of Ganymede, which we show occurred during typical magnetospheric conditions. Key Points: The magnetic field magnitude and direction upstream of Ganymede vary strongly with longitudeTemporal variations in the magnetosphere also influence Ganymede's upstream field conditionsJuno's Ganymede flyby occurred during typical magnetospheric conditions [ABSTRACT FROM AUTHOR]

The cause of the solar wind is sought on a large‐scale (J × B) force resulting from the solar unipolar induction current, rather than coronal heating and small‐scale processes in the corona. In this paper, we consider the unipolar induction system considered by Alfven (1981, https://doi.org/10.1007/978-94-009-8374-8) the polar current (1.5 × 109 A) flowing out from the polar region of the sun spreads down longitudinally, which produces azimuthal magnetic fields. We considered two models, A and B. In Model A, the current spreads down fairly uniform in five divided regions of the heliosphere (rather than flowing only along the outer surface of the heliosphere). The resulting (J × B) force is mainly directed outward, causing an outward flow of the heliospheric plasmas. However, in this case, the speed at a distance of 1 AU is only 1 km/s, although the acceleration in each divided region is much greater than the solar gravity. In Model B, one third of the dynamo current is assumed to be concentrated in the area between 9.5 R☉ and 10.5 R☉ (where R☉denotes the solar radius) and B = 10−6 T (103 nT) at 10 R☉. The acceleration is estimated to be 1.4 × 102 m/s2 and the resulting speed at 1 AU is 200 km/s. Therefore, there is a possibility that the solar unipolar induction provides partially a new possibility for the cause of a slow, bulk flow of the solar wind. Plain Language Summary: Based on Alfven's solar unipolar induction system, it is shown that the resulting (J × B) force could generate solar wind speed of 200 km/s at 1 AU under several reasonable assumptions. Key Points: The cause of the solar wind is sought on the basis of Alfven's solar unipolar induction current system, rather than an initial heatingThe resulting (J × B) force is applied to the corona between 9.5 and 10.5 solar radii, accelerating the solar wind to 200 km/sThe result is applicable for slow and uniform solar wind with polar angle of 0–70° during the solar cycle minimum phase [ABSTRACT FROM AUTHOR]

Jupiter has a strong magnetic field, and a huge magnetosphere is formed through the solar wind-Jupiter interaction. The generated magnetosphere-ionosphere system is reproduced based on the 9-component Magnetohydrodynamics (MHD) and the current conservation in the ionosphere. Assuming Io plasma emission rate 1.4 t/sec, this paper reproduces self-consistently global magnetic configuration, generations of the field-aligned current (FAC) and aurora, formation of the Io plasma disk at 8-20 RJ, plasma corotation, instability in the plasma disk, transition from the Io plasma disk to the plasma sheet at 20-150 RJ, and the plasmoid ejection. The rotating Io plasma in the disk forms instabilities that promotes radial diffusion. H+ is supplied from the ionosphere along high-latitude magnetic field lines and mixed with heavy ions around 15-20 RJ. Beyond 20 RJ, mixed plasma diffuses further outward by the centrifugal force that can exceed magnetic tension. In the ionosphere, the main oval occurs at 13.7°-15.5° colatitude. The Io disk is inner side of magnetic field lines traced from the low-latitude edge of the main oval. Along magnetic field lines, the main oval is mapped from the outer edge of the Io disk to the entire plasma sheet accompanying rotation delay. Due to the corotation limit, convection is accompanied by plasmoid ejection. Back reaction of plasmoid ejection affects even transport process in the Io disk. The downward FAC occurs in the polar cap showing variability. The region of externally driven Dungey convection seems quite narrow. [ABSTRACT FROM AUTHOR]

The turbulent foreshock region upstream of the quasi‐parallel bow shock is dominated by waves and reflected particles that interact with each other and create a large number of different foreshock transients. The structures with the enhanced magnetic field, Short Large Amplitude Magnetic Structures, and density spikes named plasmoids are frequently observed. They are one of the suggested sources of transient flux enhancements or jets in the magnetosheath. Using measurements of the Magnetospheric Multiscale Spacecraft (MMS) and OMNI solar wind database between the 2015 and 2018 years, we have found that there is a category of events exhibiting both magnetic field and density enhancements simultaneously and we introduce the term "mixed structures" for them. Consequently, we divided our set of observations into three groups of events and present their comparative statistical analysis in the subsolar foreshock. Based on our results and previous research, we discuss the properties, possible origin and occurrence rates of these events under different upstream conditions and their possible relation to the jet and plasmoid formation in the magnetosheath. Plain Language Summary: The solar wind is a plasma stream of charged particles that expands outward from the solar corona. The solar wind on its path first encounters the bow shock, whose structure and properties depend strongly on the angle between the interplanetary magnetic field and the shock normal, θBn. Most of the foreshock phenomena, which are the main subject of this study, are observed for θBn < 45°, that is, upstream of the quasi‐parallel bow shock. The transients like ULF waves, the Short Large Amplitude Magnetic Structures, plasmoids, magnetic holes, etc. are created by interaction of the incoming solar wind and ions reflected from the bow shock. In this paper, we use Magnetospheric Multiscale (MMS) Mission data inside the foreshock and solar wind OMNI database for a statistical study of the properties of these compressive foreshock structures. We show differences between the morphology of the magnetic field, the temperature anisotropy and the occurrence rate of the structures and discuss possible implications for magnetosheath jets. Key Points: Both paramagnetic and diamagnetic plasmoids exist in the foreshockThe occurrence rate of the foreshock compressive structures increases on highly decelerated solar wind in the foreshockVelocity inside the structures is deflected from the surrounding foreshock, but deflection inside the plasmoids is very low [ABSTRACT FROM AUTHOR]

We present a case study of plasma and magnetic field observations in the Martian magnetotail using data from the Mars Atmosphere and Volatile EvolutioN (MAVEN) mission during an orbit when the spacecraft was in the optical shadow, past the dusk terminator and downstream of the strongest crustal magnetic fields. In this region, we observed multiple magnetic field rotations (a signature of currents) closely associated with energized (up to 100 eV) electron populations. Several transitions between closed and draped magnetic topologies also occur in this region, which are likely to be caused by magnetic reconnection between the interplanetary magnetic field (IMF) and crustal magnetic fields. We also observe two regions of energized, counter‐streaming electrons, which are rare in the magnetotail, but twice as likely to occur downstream of strong crustal magnetic fields when they are near the evening terminator. Together, the multiple magnetic field rotations, topological changes, and counter streaming electrons suggest the presence of an electric potential structure similar to those observed above the auroral arc regions at Earth. Plain Language Summary: Mars does not have a global dipole magnetic field, but a fraction of the planet's crust carries strong remanent magnetization, which affects the plasma environment. When the magnetized solar wind encounters Mars, its magnetic field drapes around the conducting ionosphere and crustal magnetic fields and forms a magnetotail downstream of the planet. Reconnection between the solar wind magnetic field and crustal magnetic fields occurs often, resulting in a complex and changing configuration of open, closed, and draped magnetic field lines. Reconnection and reconfiguration of the magnetic field releases stored energy and drives currents, which energize the plasma. The energized plasma can precipitate onto the atmosphere and cause aurora. In this paper, we present a case study of plasma and magnetic field observations when the MAVEN spacecraft was downstream of strong crustal magnetic fields located near the evening terminator. Multiple magnetic field rotations, topological changes, and energized plasma indicate that reconnection gives rise to a complex magnetic environment downstream of the crustal fields. We found counter‐streaming electrons in the magnetotail, an observation which is thought to be a signature of the potential structure that energizes the auroral electrons, and are commonly observed at high altitudes above auroral regions at Earth. Therefore, we consider the observations in this study to be possible precursors to discrete auroral activity in Mars' crustal magnetic field regions. Key Points: The Martian magnetotail exhibits complex morphology and topology downstream of strong crustal magnetic field sourcesMultiple magnetic field rotations on spatial scales that are small compared to the magnetotail dimensions suggest multiple current regionsRare counter‐streaming electrons, are twice as likely to occur downstream of crustal fields when they are located near the dusk terminator [ABSTRACT FROM AUTHOR]

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

Particle precipitation is a loss mechanism from the radiation belts whereby particles trapped by the Earth's magnetic field are scattered into the loss cone due to wave-particle interactions. Energetic electron precipitation creates ozone destroying chemicals which can affect the temperatures of the polar regions, therefore it is crucial to accurately quantify this impact on the Earth's atmosphere. We use bounce-averaged pitch angle diffusion coefficients for whistler mode chorus waves, plasmaspheric hiss and atmospheric collisions to calculate magnetic local time (MLT) dependent electron precipitation inside the field of view of the Polar Orbiting Environmental Satellites (POES) T0 detector, between 26–30 March 2013. These diffusion coefficients are used in the BAS Radiation Belt Model (BAS-RBM) and this paper is a first step toward testing the loss in this model via comparison with real world data. We find the best agreement between the calculated and measured T0 precipitation at L* > 5 on the dawnside for the >30 keV electron channel, consistent with precipitation driven by lower band chorus. Additional diffusion is required to explain the flux at higher energies and on the dusk side. The POES T0 detector underestimates electron precipitation as its field of view does not measure the entire loss cone. We demonstrate the potential for utilizing diffusion coefficients to reconstruct precipitating flux over the entire loss cone. Our results show that the total precipitation can exceed that measured by the POES >30 keV electron channel by a factor that typically varies from 1 to 10 for L* = 6, 6.5, and 7. [ABSTRACT FROM AUTHOR]

Geomagnetic disturbances caused by solar storms have the potential to create large‐scale geomagnetically induced currents in long conductors at the Earth's surface. These may disrupt the operation of electric power grids and cause blackouts. Ocean‐continent boundary regions are of particular concern because of the sharp contrast between the higher ocean electrical conductivity compared to the lower continental conductivity. This contrast may generate high‐amplitude geoelectric fields and cause power grids in coastal regions to be more vulnerable to space weather hazards. Previously, analytical calculations were used to estimate geomagnetically induced currents at ocean‐continent boundaries. However, for the analytical equations to be solvable, the physics and geometries of the problem were simplified. As a result of these simplifications and due to a lack of published measurements examining this issue in coastal regions, it is difficult to know for sure whether there are unique hazards to electric power grids at ocean‐continent boundaries. In this paper, a grid‐based, time domain modeling approach is used to solve the complete Maxwell's equations, which permits accommodation of (1) the complete physics of the propagating electromagnetic fields from disturbed ionospheric currents through the air and into the lithosphere and even into the ocean via the skin effect and (2) more realistic coastal geometries. Using this more robust approach, in the variety of scenarios studied in this paper, only a local enhancement of the electric fields was observed, which are expected to only pose potential hazards to power grids only a local scale near ocean‐continent boundaries. Key Points: Electromagnetic fields across ocean‐continent boundaries are solved via FDTD while accounting for a finite‐depth ocean and topographyAnalytical approaches are found to provide a close approximation to the more realistic FDTD results for the range of studied scenariosSurface electric field enhancements are highly localized along the coast, even for low frequencies and shallow oceans [ABSTRACT FROM AUTHOR]

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

Auroral streamers have been found to be a particularly important cause of magnetic perturbations within the auroral oval during substorms. We present a detailed investigation on the association of equatorward extending auroral streamers with ground magnetic perturbations at locations initially equatorward of the auroral oval using Time History of Events and Macroscale Interactions imagers and ground magnetometers. Based on the analysis of four events, we show that negative perturbations in the horizontal (H) component at initially subauroral latitudes were not seen prior to streamer onset, and the peak negative H‐bay at subauroral latitudes coincided with the equatorward extending auroral streamers. Although stations close to the poleward boundary of auroral oval showed modest H‐component perturbations in response to the substorm onset and poleward auroral intensifications, the abrupt and large responses within the oval were associated with the onset of streamers. Results confirm that ground magnetic perturbations that are traditionally perceived as signatures of magnetic substorm onset are not always an auroral substorm onset but can also be related to auroral streamers. These results are supported by the remarkable correlation of positive bay onset and sudden Pi2 enhancement with the onset of auroral streamers, irrespective of the classical substorm onset. In addition, the magnitude of positive bay and amplitude of Pi2 pulsations, particularly at subauroral latitudes, are enhanced during the events of equatorward extending streamers. In general, all streamer events were accompanied by dipolarization and electron flux enhancement at geosynchronous orbit that continue to strengthen during the emergence of bright streamers. Plain Language Summary: Large, rapid perturbations in the ground magnetic field at high latitudes have been considered a major space weather hazard as they can harm technological systems. Such magnetic perturbations are common at auroral latitudes during geomagnetic storms and substorms. However, it is a matter of research to comprehend under what conditions such perturbations can extend equatorward of the auroral oval. The ground magnetic perturbations at auroral latitudes are shown to be linked with auroral streamers. Auroral streamers are roughly north‐south oriented narrow bands of auroral luminosity. They originate near the poleward edge of the auroral oval and propagate equatorward. Auroral streamers are related to flow bursts in the plasma sheet. The equatorward motion of auroral streamer is related to the earthward motion of flow bursts in the plasma sheet. This paper shows that the abrupt magnetic perturbations at latitudes equatorward of the auroral oval (i.e., nominal subauroral latitudes) are linked with the equatorward extending auroral streamers. The observations suggest that as the streamers extended to lower latitudes, the auroral oval expands equatorward to reach previously subauroral latitudes. We found that auroral streamers are, in general, followed by abrupt decreases in the H (horizontal)‐component at auroral and subauroral latitudes. Key Points: The onset of abrupt negative perturbations in H‐component at auroral and part of subauroral latitudes coincided with the onset of auroral streamerThe peak negative bay in H‐component at subauroral latitudes coincided with the equatorward extending auroral streamersBright equatorward extended auroral streamers were associated with the peak enhancement in electron flux at geosynchronous orbit [ABSTRACT FROM AUTHOR]

The paper presents results from the statistical study of very low frequency (VLF) whistler‐mode waves observed inside field‐aligned density inhomogeneities by the NASA Van Allen Probe satellites. These observations were conducted in the night‐side equatorial magnetosphere during April 2013 and in the dusk‐side magnetosphere during August 2015. The total number of analyzed events is 225. The events include wave ducting in the so‐called "narrow" high‐density (HDD) ducts, where the variation of the background magnetic field over the duct width is insignificant compared to the change in the density, and "wide" ducts; where the variation of the magnetic field needs to be taken into account. Of the 225 total events, 176 of them were "narrow" events. The analysis revealed that the most common size of narrow, HDD ducts are 100–200 km. These ducts are characterized by the increase in the amplitude of the density of 161%. The background magnetic field in these ducts is in the range of 100–300 nT. The wide ducts are formed by the broader regions, consisting of large‐scale gradients in the density and the magnetic field. We analyzed 49 wide ducts observed in the dusk magnetosphere during August 2015. Frequency of the waves trapped in these ducts is commonly in the range of 400–500 Hz. Our results demonstrate that ducting of the VLF waves by the HDD narrow and wide ducts is a common phenomenon and it may significantly contribute to the VLF energy propagation and distribution in the magnetosphere. Key Points: We create a catalog of whistler‐mode waves from the observation data of the NASA Van Allen ProbesWe identify common parameters of both narrow and wide ducting eventsWe conclude that whistler‐mode wave ducting events are a common occurrence in the magnetosphere [ABSTRACT FROM AUTHOR]

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

This study presents a number of previously unreported auroral features that form shortly after the IMF Bz ${\mathrm{B}}_{\mathrm{z}}$–component turns sharply northward. The auroral images were captured by the Boreal Aurora Camera Constellation (BACC) All‐sky camera at the Kjell Henriksen Observatory, located in Longyearbyen, Svalbard. These observations are different from those previously reported. There are several differences: (a) The Eastward‐Moving Auroral Forms (EMAFs) do not originate from type 1 or type 2 aurora. The EMAFs originate from a diffuse vertical auroral form in the far west part of the ASC image. Vertical refers to the auroral form oriented in the north‐south direction in the frame of the ASC image. (b) The EMAFs form crescent shapes after they begin to drift eastward. (c) The EMAFs are discrete auroral forms; previously reported ionospheric signatures of high‐latitude reconnection are described as patches of brightness that form within a pre‐existing auroral band. (d) The EMAFs are observed to first move eastward and then slightly poleward during their journey. The movements of previously reported Ionospheric signatures of high‐latitude reconnection are described as expanding patches of brightness within a pre‐existing auroral band. (e) Some of the EMAFs brighten again after fading, during their northeastward drift across the field of view. (f) The EMAFs begin their eastward motion aligned in the north‐south direction. During their northeast drift, they begin to tilt toward the east. Some of the EMAFs keep tilting until they are aligned in the east‐west direction. Plain Language Summary: Magnetic reconnection is a process which combines antiparallel magnetic field lines from different sources. Since magnetic field lines permeate our universe, there are many places where magnetic reconnection can occur. The solar‐terrestrial environment provides a great laboratory to study magnetic reconnection. Different orientations of the interplanetary magnetic field (IMF) allow it to connect to different regions of the Earth's geomagnetic field. Information from a magnetic reconnection event is transmitted to the ionosphere via the terrestrial magnetic field and is manifested in dazzling auroral displays. This paper presents ionospheric signatures of magnetic reconnection at high‐latitudes which are extremely different than past studies: (a) the origin of the observations in this study are different, (b) the ionospheric signatures form crescent shapes after they begin to drift eastward, (c) the ionospheric signatures are discrete auroral forms, (d) The ionospheric signatures are observed to first move eastward and then slightly poleward during their journey, (e) some of the ionospheric signatures brighten again after fading, during their northeastward drift across the sky, (f) during their northeast drift, they begin to tilt toward the east. Some of the EMAFs keep tilting until they are aligned in the east‐west direction. Key Points: Discrete auroral forms for northward IMF are aligned in the north‐south direction, propagating eastwardThese discrete auroral forms rotate clockwise during their eastward drift, some becoming aligned in the east west directionHigh‐latitude reconnection is a possible explanation for these discrete dayside auroral forms since the IMF clock angle is less than 90° [ABSTRACT FROM AUTHOR]

Alfven waves have a very small‐scale radial (across magnetic shells) structure, in an inhomogeneous magnetospheric plasma. This structure is determined by the Alfven wave type. In spacecraft observations, it is difficult to separate local oscillations of the wavefield from oscillations due to the spacecraft crossing the small‐scale transverse structure of such a wave. We propose a new method ("phase portraits") to determine the type of Alfven waves observed onboard. What we call a "phase portrait" of monochromatic Alfven wave are two related plots: (a) radial distribution of the transverse components of the electric or magnetic field of the wave; and (b) radial distribution of the phase shift between these components. In order to construct such a "phase portrait," we propose using the transverse components of the wavefield as registered onboard a spacecraft crossing the region occupied by the wave. It is demonstrated that the phase shift between the oscillations of the transverse components of the electric/magnetic field in a monochromatic Alfven wave is uniquely related to its radial structure. The transverse structure of the spacecraft‐observed Alfven wave can be determined by plotting the radial dependence of the phase shift between the wavefield transverse components. Comparing this plot to the calculated theoretical "phase portraits" we can select the most appropriate (similar) portrait in order to determine the radial structure and type of the wave. In this paper, "phase portraits" are constructed for most types of ULF monochromatic Alfven waves observed in the Earth's magnetosphere. Key Points: Equations describing the spatial structure of MHD oscillation field in a dipole‐like magnetosphere are derivedA method is proposed to determine the radial structure of Alfven waves using phase shifts between transverse components of their field"Phase portraits" are plotted for the main types of monochromatic Alfven waves in the Earth's magnetosphere [ABSTRACT FROM AUTHOR]

Using a resistive MagnetoHydroDynamic (MHD) simulation, we study how the magnitude and shape of diffusion influence magnetic reconnection. Specifically, we investigate how and why the reconnection rate is influenced by variations in the diffusion distribution and magnitude. By running multiple MHD simulations where we vary the localized resistivity, we find that the properties of the diffusion region greatly influence the rate of reconnection. Increasing the magnitude of the imposed resistivity results in a higher reconnection rate, but the rate saturates at approximately 0.2. We show how a redistribution of the current density, leading to a bifurcated current sheet, play a major role in this limitation. In addition, we investigate the impact of different shapes of resistive region. The shape of the diffusion region also plays a major role in how efficient the reconnection energy conversion can operate. The highest reconnection rate, approximately 0.25, is achieved for an optimal opening angle. Our results imply that reconnection has a speed limit that may depend on properties outside the diffusion region. Plain Language Summary: Magnetic reconnection is a fundamental plasma process, which can explosively convert magnetic energy to particle energy. When reconnection operates, it releases almost all of the energy stored in the magnetic field to plasma acceleration and heating. The consequences of reconnection depend on the magnetic energy available and the process' ability to rapidly release the energy. Thus, the effectiveness of reconnection, which can be quantified by the rate at which energy is converted, is a key factor in understanding the consequences and implications of this universal process. This paper investigates how the reconnection rate depends on the resistivity in the system. In our fluid‐based scheme, resistivity determines the plasma's ability to diffuse across the magnetic field ‐ allowing new magnetic topologies to form. We show that, even when inserting very strong resistive spots with varying shapes, there appears to be a maximum rate of reconnection the system can support. In addition, we find that a sub‐optimal choice of resistivity magnitude or shape of the resistive spot leads to lower overall reconnection rates. These results imply that the reconnection rate depends significantly on properties of the diffusion region, even if the size of that region is much smaller than the system. Key Points: The reconnection rate depends significantly on the properties of the diffusion regionEven for very strong resistivities and varying shapes of the resistive region there exists a maximum rate that the system can supportWe show how the properties of the diffusion region are altered for various resistive spot magnitudes and shapes [ABSTRACT FROM AUTHOR]

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

A quantitative model of the magnetosheath (MS) magnetic structure is developed, using a multi‐year set of Geotail, Themis, Cluster, and MMS magnetometer and plasma instrument data. The MS database is created using an identification algorithm, based on observed magnetic field magnitudes and proton densities, normalized by their concurrent interplanetary values, followed by additional filtering with the help of standard bow shock (BS) and magnetopause (MP) models. The model architecture is based on the toroidal/poloidal formalism and a coordinate system that naturally accounts for the tailward flaring of both boundaries. The magnetic field expansions include 960 free coefficients, derived by fitting the model to a grand data set, split into independent training and validation subsets with 1,291,380 and 411,933 1‐min records, respectively. The model faithfully reproduces basic types of the interplanetary magnetic field (IMF) wrapping around the MP. Regular IMF sectors result in strongly dawn‐dusk asymmetric draping, with much larger magnitudes at the quasi‐perpendicular dusk side of BS, and weaker at the quasi‐parallel dawn side, where the MS field lines are bent and dragged tailward. Except in the case of the flow‐aligned IMF orientation, the subsolar field steadily grows toward the MP, and the effect is clearly IMF Bz‐dependent: the field and its gradient are larger (smaller) for northward (southward) IMF Bz, implying a pile‐up of the magnetic flux in the first case and stronger reconnection in the second. Model distributions of the MS field magnitude reveal local depressions, associated with polar cusps near the high‐latitude limits of data coverage. Plain Language Summary: The terrestrial magnetosheath is a relatively wide transition region, separating our planet's magnetosphere from the undisturbed flow of magnetized solar wind. Due to the sudden compression of the incoming plasma flow at the bow shock and high conductivity of the magnetosheath medium, the relatively weak magnetic field of interplanetary origin undergoes abrupt compression and drapes around the magnetosphere boundary, the magnetopause. Huge amounts of archived spacecraft data accumulated in the world data centers during past decades of space flight made it possible to develop quantitative models of the magnetosheath magnetic field, based on direct in situ observations. This paper presents first results of such a modeling study, providing a closed analytical representation of the magnetosheath magnetic field, driven by input from upstream monitors of the interplanetary medium. Key Points: A closed empirical model of the magnetosheath magnetic field is developed, based on satellite data and a flexible mathematical architectureDriven by interplanetary parameters and dipole tilt, the model faithfully reproduces basic features of the IMF draping around the magnetopauseNorthward (southward) IMF results in higher (lower) subsolar fields, indicating magnetic flux pile‐up (stronger reconnection), respectively [ABSTRACT FROM AUTHOR]

We investigate the electron dynamics and generation of whistler‐mode waves inside a short large‐amplitude magnetic structure (SLAMS) based on Magnetospheric Multiscale Satellite observations and test particle simulations. The buildup of energetic reflected ions caused by the SLAMS's intense magnetic fields favors the resonance condition of solar wind ions of ion‐ion non‐resonant mode. This might further steepen the SLAMS, leading to the Betatron acceleration of electrons and generating the whistler‐mode waves locally ahead of the peak of the magnetic field. In regions behind the peaked magnetic field, electrons are decelerated by the convection electric field during gradient drift and the whistler‐mode waves are rapidly decayed. Our study provides new insights into electron acceleration and the generation of whistler‐mode waves near the quasi‐parallel shock. Plain Language Summary: On the quasi‐parallel side of the bow shock, ultra‐low frequency waves and nonlinear structures such as shocklet and short large‐amplitude magnetic structure (SLAMS) are commonly observed, which are powered by energetic reflected ions and form in different stages during steepening. Solar wind ions are heated significantly inside these nonlinear structures. However, what role electrons played inside these structures is full of mystery. Recent numerical simulations reveal that electron dynamics and whistler‐mode waves inside these structures are responsible for triggering bow shock reconnection on the quasi‐parallel side. In this paper, we focus on electron distributions and acceleration inside SLAMS. Electrons are accelerated/decelerated in regions prior to/after the peaked magnetic field, exciting/decaying whistler‐mode waves locally which well‐regulated the electron distribution. Our work reveals the electron dynamics inside the SLAMS and provides new insights into plasma heating and acceleration near the quasi‐parallel shock. Key Points: Whistler‐mode waves are locally generated through the first‐order resonance of the electrons inside the short large‐amplitude magnetic structure (SLAMS)SLAMS steepening primarily impacts electron acceleration and whistler‐mode wave generationElectron deceleration by the v × B electric field during gradient drift leads to whistler‐mode wave decay at the trailing edge of SLAMS [ABSTRACT FROM AUTHOR]

This paper studies extremely low frequency (ELF) whistler‐mode waves' behavior within small‐scale magnetic field irregularities in the Earth's magnetosphere, known as magnetic ducts. Based on the magnetic fields' magnitude inside and outside these ducts, they are categorized as high‐magnetic ducts and low‐magnetic ducts (LBD). Using the whistler‐mode dispersion relation analysis, our primary focus is to show that LBDs are prone to leak electromagnetic energy outside the duct. We further investigate the hypothesis that whistlers can propagate within LBDs without any signal loss when the width of the duct corresponds to an integer multiple of the perpendicular wavelengths of the waves inside it. This condition offers a straightforward and effective method for identifying non‐leaking eigenmodes of LBDs. Our analysis of this non‐leaking condition reveals that every LBD possesses a finite number of non‐leaking eigenmodes directly proportional to the duct's width and the magnitude of the ambient magnetic field within it. The analytical results are then validated using two‐dimensional, time‐dependent simulations of the electron‐Magnetohydrodynamics model. Also, we model the non‐leaking propagation of an ELF whistler‐mode wave observed inside the LBD by the NASA Magnetospheric Multiscale mission satellites. Key Points: Magnetospheric Multiscale mission satellites observe whistler‐mode waves inside the low‐magnetic ducts (LBD)We provide an analytical explanation for the occurrence of leaking ducts and show that the LBD are prone to leakSimulations confirm our analytical prediction of the most trapped extremely low frequency wave in the LBD, provided by least degree of leakage [ABSTRACT FROM AUTHOR]

The location of the polar cap boundary is typically determined by using low‐orbit satellite measurements in which the boundary is identified by its unique signature of a sharp decrease in energy and particle flux poleward of the auroral oval. In principle, this decrease in precipitating particles should appear as a concomitant sharp change in auroral luminosity. Based on a few events, Blanchard et al. (1995, https://doi.org/10.1029/94JA02631) suggested that a dramatic gradient in redline aurora may also be an indicator of the polar cap boundary. In recent years, advances in capabilities and the deployment of ground‐based all‐sky imagers have ushered in a new era of auroral measurements. Auroral imaging has moved well beyond the capabilities of the instrumentation in the previous study in terms of both spatial and temporal resolution. We now have access to decades of optical data from arrays spanning a huge spatial range, enabling a fresh examination of the relationship between redline aurora, particle precipitation, and the polar cap open closed boundary. In this study, we use data from the Defense Meteorological Satellite Program satellites in conjunction with the University of Calgary's Redline Emission Geospace Observatory (630.0 nm) data to assess the viability of automated detection of the 2‐dimensional polar cap boundary. Our results exhibit good agreement between the optical and particle polar cap boundary and suggest that a luminosity in redline emission could serve as a reasonable proxy for the location of the electron poleward boundary, while providing both high temporal and spatial resolution maps of the open‐closed boundary. Plain Language Summary: Named for its shape, the auroral oval is an oval‐shaped region located near the north and south poles where the aurora is formed. Studying the location of the equatorward and poleward limits of the auroral oval is fundamental to evaluate the dynamics and responses of the near‐Earth space environment. In particular, the poleward boundary corresponds to a region in which Earth's magnetic field transitions from closed (connected to Earth on both sides) to open field lines (connected to the solar wind on one side); because of this topological change in magnetic field, the location of this boundary is fundamental to understand mass and energy transport in the whole magnetosphere‐ionosphere system. While satellite data can determine the exact location of this boundary, they only provide point measurements. Auroral images are capable of detecting the luminosity around this boundary; however, many qualitative studies consider the region immediately poleward of this luminosity as the oval's poleward boundary. In this paper, we analyze the location of this boundary in a case study by using satellite data combined with auroral imaging data. Key Points: Polar cap boundary determined with Defense Meteorological Satellite Program particle data allows for examination of the boundary in optical redline dataWe find good agreement between the auroral boundary determined by auroral luminosity and those determined by particle fluxLuminosity method can be applied during active geomagnetic conditions, providing high spatial and temporal resolution polar cap boundary [ABSTRACT FROM AUTHOR]

This study presents results from magnetic field line conjunctions between the medium‐Earth orbiting Demonstration and Science Experiments (DSX) satellite and the low‐Earth orbiting (LEO) very low frequencies (VLF) Propagation Mapper (VPM) satellite. DSX transmitted at VLF toward VPM, which was equipped with a single‐axis dipole electric field antenna, when the two spacecraft passed near the same magnetic field line. VPM did not observe DSX signals in any of the 27 attempted conjunction experiments; the goal of this study, therefore, is to explain why DSX signals were not received. Explanations include (a) the predicted power at LEO from DSX transmissions was too low for VPM to observe; (b) VPM's trajectory missed the "spot" of highest intensity due to the focused ray paths reaching LEO; or (c) rays mirrored before reaching VPM. Different combinations of these explanations are found. We present ray‐tracing analysis for each conjunction event to predict the distribution of power and wave normal angles in the vicinity of VPM at LEO altitudes. We find that, for low‐frequency (below 4 kHz) transmissions, nearly all rays mirror before reaching LEO, resulting in low amplitudes at LEO. For mid‐ and high‐frequency transmissions (∼8 and 28 kHz respectively), the power at LEO is above the noise threshold of the VPM receiver (between 0.5 μV/m and 1 μV/m). We conclude that the antenna efficiency and plasmasphere model are critical in determining the predicted power at LEO, and are also the two most significant sources of uncertainty that could explain the apparent discrepancy between predicted amplitudes and VPM observations. Plain Language Summary: In this study we present results from transmissions between two near‐Earth spacecraft. The Demonstration and Science Experiments (DSX) satellite transmitted signals at very low radio frequencies (VLF) toward the VLF Propagation Mapper (VPM) satellite when the two satellites passed near the same magnetic field line. VLF broadcasts tend to follow magnetic field lines as they are guided by the plasma in the magnetosphere. This study is important for understanding VLF wave propagation in the near‐Earth space environment. We analyze the data from each experiment and conclude that DSX broadcasts were not observed by VPM. The goal of this paper is to describe our analysis to explain possible reasons for the lack of observation. We perform ray‐tracing, or solving for the paths of the VLF broadcasts, to explain why VPM missed the signals. We conclude that in some cases, the broadcasts mirrored, or reversed direction in the near‐Earth space environment before they were able to reach VPM. In other cases, the ray‐tracing analysis predicts we should have observed the signals. However, we find that the DSX antenna performance and the model of the near‐Earth environment we use in these simulations are significant sources of uncertainty that could explain this discrepancy. Key Points: Results from active conjunction experiments between Demonstration and Science Experiments (DSX) and Very Low Frequency Propagation Mapper are presentedRay‐tracing is performed to investigate the lack of DSX signal observationThe effects of the antenna efficiency and plasmasphere model are explored [ABSTRACT FROM AUTHOR]

Resonance Maps depict the possible locations and polarizations of resonant Alfvén waves (Field Line Resonances – FLRs) for a given equilibrium and driving frequency. Previously the use of Resonance Maps has been developed for gaining insight into the behavior of Alfvén waves in basic potential magnetic fields that allow the use of an orthogonal field aligned coordinate system. In more general magnetic fields these coordinates do not exist. In this paper we explore the application of Resonance Maps to such equilibria. A number of simulations of resonant Alfvén waves are presented and interpreted using the Maps. We find that Resonance Maps remain useful when some of the constructions used previously are generalised to accommodate the properties of more general magnetic fields. For example, Resonance Maps are able to predict the location and polarisation of Alfvén waves which are driven strongly by fast mode waves using a "tangential alignment condition". Unusual properties, such as Alfvén waves crossing flux surfaces persist in the more general magnetic fields we consider. Key Points: Resonance Maps are useful for interpreting resonantly excited Alfven in complex potential magnetic fieldsEfficient resonant fast‐Alfven wave coupling can occur where the plasma displacement is tangential to the Resonant Zone boundary"Toroidal" and "Poloidal" have limited usage in complex magnetic fields. Directions based on the Resonance Map have more utility [ABSTRACT FROM AUTHOR]

Waves from nine major ground‐based very low frequency (VLF) transmitters are modeled from their sources to 660 km altitude with a full‐wave code, which reliably treats transionospheric attenuation, and then ray‐and‐power traced throughout the plasmasphere. Lightning‐generated whistlers, previously modeled at 660 km altitude, are ray‐and‐power traced throughout the plasmasphere as well. The resulting profiles of electric and magnetic fields, including wave normal angles, are organized by L value. Two versions of a realistic plasmaspheric density model are used, and ducted as well as nonducted propagation are treated. Results are compared to empirical models based on near‐equatorial measurements by Van Allen Probes. A companion paper will evaluate resonant interactions of these waves with radiation belt electrons. Key Points: Waves from VLF transmitters and lightning are modeled from their sources to the plasmasphereResults are compared to empirical models based on near‐equatorial measurements by Van Allen ProbesA companion paper evaluates their resonant interactions with radiation belt electrons [ABSTRACT FROM AUTHOR]

In this paper, we present Magnetospheric Multiscale (MMS) observations of a flux transfer event (FTE) characterized by a clear signature in the magnetic field magnitude, which shows maximum at the center flanked by two depressions, detected during a period of stable southward interplanetary magnetic field. This class of FTEs are called "crater‐FTEs" and have been suggested to be connected with active reconnection X line. The MMS burst mode data allow the identification of intense fluctuations in the components of the electric field and electron velocity parallel to the magnetic field at the borders of the FTE, which are interpreted as signatures of the magnetic separatrices. In particular, the strong and persistent fluctuations of the parallel electron velocity at the borders of this crater‐FTE reported for the first time in this paper, sustain the field‐aligned current part of the Hall current system along the separatrix layer, and confirm that this FTE is connected with an active reconnection X line. Our observations suggest a stratification of particles inside the reconnection layer, where electrons are flowing toward the X line along the separatrix, are flowing away from the X line along the reconnected field lines adjacent to the separatrices, and more internally ions and electrons are flowing away from the X line with comparable velocities, forming the reconnection jets. This stratification of the reconnection layer forming the FTE, together with the reconnection jet at the trailing edge of the FTE, suggests clearly that this FTE is formed by the single X line generation mechanism. Key Points: Intense high frequency fluctuations of parallel electron velocity component are an evident signature of encounters with magnetic separatrixThese observations suggest a stratification of reconnection layer in the crater FTEs, with magnetic separatrices at the external bordersAt the borders of a crater FTE, far from the X line, are observed field aligned current fluctuations, related to the separatrix layer [ABSTRACT FROM AUTHOR]

The ionospheric electron temperature is important for determining the neutral/photochemical escape rate from the Martian atmosphere via the dissociative recombination of O2+. The Langmuir Probe and Waves instrument onboard MAVEN (Mars Atmosphere and Volatile EvolutioN) measures electron temperatures in the ionosphere. The current paper studies electron temperatures in the dayside for two regions where (1) crustal magnetic fields are dominant and (2) draped magnetic fields are dominant. Overall, the electron temperature is lower in the crustal‐field regions, namely, the strong magnetic field region, which is due to a transport of cold electrons along magnetic field lines from the lower to upper atmosphere. The electron temperature is also greater for high solar extreme ultraviolet conditions, which is associated with the local extreme ultraviolet energy deposition. The current models underestimate the electron temperature above 250‐km altitude in the crustal‐field region. Electron heat conduction associated with a photoelectron transport in the crustal‐field regions is altered due to kinetic effects, such the magnetic mirror and/or ambipolar electric field because the electron mean free path exceeds the relevant length scale for electron temperature. The mirror force can affect the electron and heat transport between low altitudes, where the neutral density and related electron cooling rates are the greatest, and high altitudes, while the ambipolar electric field decelerates the electron's upward motion. These effects have not been included in current models of the electron energetics, and consideration of such effects on the electron temperature in the crustal‐field region should be considered for future numerical simulations. Key Points: At high altitudes, the electron temperature is lower for the crustal‐field region than for the IMF‐dominated draped regionsThe low temperatures for the crustal‐field region are due to a heat transport along the field lines between lower and upper atmosphereKinetic effects such the magnetic mirror and the ambipolar electric field may affect the heat transport along crustal magnetic field lines [ABSTRACT FROM AUTHOR]

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

Magnetic reconnection is universal in various plasma environments like the solar atmosphere, planetary magnetosphere, and laboratory to trigger explosive phenomena. Its nature and magnetic structure in two dimensions (2D) have been abundantly analyzed. The three‐dimensional (3D) characteristics of reconnection are different from 2D, and the mysteries of its 3D geometry are under exploration. A critical character in 3D is the presence of magnetic nulls, which is essential when the magnetic field topology is changing. The four identical spacecraft from the Cluster mission enable several methods for analyzing and visualizing the 3D nature of the magnetic structure. In this paper, we focus on reviewing the magnetic topology of the reconnection geometry containing 3D magnetic null points. Applying the fitting‐reconstruction method to Cluster data, magnetic null pairs or multiple magnetic nulls linked by separators or helically wrapped spines are found in the reconnection diffusion region and turbulent outflow region. The reconstructed magnetic structures show that the 2D features are parts of local approximations of the 3D geometries. The 2D regimes of antiparallel and component reconnection can simultaneously occur in 3D separator reconnection. The flux ropes are formed along the spine line or in a region enclosed by wrapped fan surfaces in the presence of spiral null points. Key Points: Cluster and magnetospheric multiscale observations and data‐based reconstruction of 3D reconnection geometry with magnetic nulls are reviewedNull pairs can form configurations with both X‐type and O‐type 2D projectionsThe null points serve as skeletons of the 3D magnetic reconnection region [ABSTRACT FROM AUTHOR]

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

TOI‐700 d is the first Earth‐sized planet in the habitable zone (HZ) discovered by the Transiting Exoplanet Survey Satellite. Here, we assess whether a Venus‐like exoplanet at the TOI‐700 d location could retain an atmosphere for a time comparable to the age of the host star based on multispecies magnetohydrodynamics simulations. We investigate the effects of X‐ray and EUV (XUV) radiation from the host star, the interplanetary magnetic field (IMF) orientation, and the planetary intrinsic magnetic field. In unmagnetized cases, major ion loss is caused by O+ escape through a ring‐shaped region by the mass loading process after the ionization of the extended oxygen corona. As the IMF Parker spiral angle increases, the escape flux in the magnetotail shows stronger enhancement around the meridional current sheet, and the escape rate of molecular ions (O2+ ${\mathrm{O}}_{\mathrm{2}}^{+}$ and CO2+ $\mathrm{C}{\mathrm{O}}_{\mathrm{2}}^{+}$) increases by an order of magnitude due to acceleration in the ionosphere by magnetic tension forces. In magnetized cases, the intrinsic magnetic field suppresses ion pickup loss from the neutral oxygen corona by deflecting the stellar wind and preventing ion pickup while promoting cusp‐origin escape from the lower ionosphere. These results suggest that the unmagnetized exoplanet would have difficulty retaining its atmosphere over a few billion years under extreme conditions where XUV is 30 times stronger than at the current Earth. However, the dipole intrinsic magnetic field of 1,000 nT at the equatorial surface reduces the escape rate and would help the exoplanet to retain its atmosphere even under strong XUV conditions. Plain Language Summary: The recent discovery of Earth‐sized planets in the habitable zones (HZ) of M dwarfs has garnered attention over whether liquid water and life exist on these planets. We investigate the habitability of planets around M dwarfs in terms of atmospheric retention. Planetary atmospheres are necessary not only to retain liquid water on the surface but also to protect life from high‐energy particles and radiation. In this paper, we assess the feasibility of atmospheric retention on a Venus‐like exoplanet at the location of TOI‐700 d, which is the first Earth‐sized planet in the HZ discovered by the Transiting Exoplanet Survey Satellite. Our simulations show that the unmagnetized exoplanet cannot retain an atmosphere under extreme conditions where the planet is exposed to the intense light from the host star that energizes the upper layers of the planetary atmosphere, but the presence of an intrinsic magnetic field might make this possible. The escape rate of atmospheric ions, especially molecular ions, is suppressed by the interplanetary magnetic field nearly parallel to the stellar wind velocity, which may be typical as the planet and the host star get closer. Key Points: Stellar X‐ray and EUV (XUV) must be within 30 times that of Earth to retain a Venus‐like atmosphere for an unmagnetized exoplanetIn unmagnetized cases, the Parker spiral angle affects molecular ion escape due to the tension force of draping magnetic field linesA strong intrinsic magnetic field suppresses ion escape, allowing the atmosphere to be retained even when XUV is 50 times that of Earth [ABSTRACT FROM AUTHOR]

In 27 October 2012 an Alfvén wave of the Pc4 range was observed with Van Allen Probe A. The parallel Poynting flux of the wave was directed toward the Northern ionosphere. Assuming that this may be caused by the asymmetry of the ionospheric conductivity between the Northern and Southern hemispheres, its effect on the standing structure of the Alfvén wave was investigated. For this purpose, the analytical model with straight magnetic field lines was used. As a result, the method for estimation of the Northern and Southern ionospheric conductivity was developed. It allows to reconstruct the parallel structure of Alfvén waves under various conditions of the ionospheric conductivity. With the method developed, the ionospheric conductivity for the 27 October 2012 event was evaluated and compared with ionosphere model IRI‐2016 results. Plain Language Summary: The most commonly observed ultra low frequency waves in the magnetosphere are Alfvén waves standing along a magnetic field line between magnetically conjugated points of the ionosphere located in opposite hemispheres. In this paper, we considered how the north‐south asymmetry of the ionospheric conductivity influences standing Alfvén wave's parallel structure. The model predicts that the structure of the Alfvén waves may differ depending on the conductivity values of the Northern and Southern ionospheres. Based on the model we developed a method of a qualitative estimation of the ionospheric height‐integrated Pedersen conductivity. The method is used parameters of Alfvén waves observed with spacecraft in the magnetosphere. As a result, for the 27 October 2012 event the parallel structure of the observed Alfvén wave was reconstructed, and the ionospheric conductivity was estimated. We found a significant difference in the conductivity values of the Northern and Southern ionospheres. The mean is that the magnetically conjugated footprints of the spacecraft trajectory, where the event was observed, were located on the opposite sides of the terminator line (a line dividing day and night sides of a planet). Key Points: A method for estimating the ionospheric conductivity based on spacecraft observations of Alfvén waves in the magnetosphere is proposedThe parallel structure of an Alfvén wave under various conditions of the ionospheric conductivity is reconstructedThe height‐integrated Pedersen conductivity of the ionosphere were estimated for the 27 October 2012 event [ABSTRACT FROM AUTHOR]

Within the inner magnetosphere, the occurrence of plasma waves and/or magnetic field structures plays an important role in the dynamics of the particle populations, accelerating some particles to higher energies while scattering others. The 2013 Cluster Inner Magnetosphere Campaign was designed to investigate this process based on observations made at small separation scales. In addition, it provides an ideal opportunity for the study of small‐scale structures. This paper presents observations of previously unreported small‐scale magnetic field structures and investigates their characteristics and spatial properties. These structures, seen in the STAFF‐SC data set, are characterized by a rotation in the field direction and are observed to have spatial sizes of a few tens of kilometers, with a central core region around 10 km and are probably filamentary current structures. Using a variance analysis, it was determined that the maximum variance direction was close to being perpendicular to the external magnetic field. Based on the size of the central core region the current density is of the order of 10–40 nAm−2. While they may affect the profile of these structures, data calibration effects were ruled out as their origin. Key Points: Cluster STAFF‐SC magnetometer observations of previously unreported nonlinear structures in the inner magnetosphere are presentedThe small separation of two of the Cluster satellites enables their spatial and temporal properties to be investigatedWe argue that these structures can be generated by thin current filaments whose size is a few tens of kilometers [ABSTRACT FROM AUTHOR]

A geomagnetic storm is a phenomenon of weakening of Earth's magnetic field from a few hours to several days, which directly or indirectly affects navigation, transportation, communication, power grids, satellite electronics, etc. In general, the duration of the recovery phase is larger. Thus, its cumulative effect is greater than the main phase. Therefore, it is important to understand the physical processes involved during the recovery phase. In this paper, we have studied the recovery phase of 31 extreme geomagnetic storms that occurred from 1990 to 2020. Each storm demonstrates two distinct features of the recovery phase that is, initial fast and later slow recovery phase. During the fast recovery phase, the rate of recovery either linearly or non‐linearly depends on SYM−H, which has been characterized by an exponential or hyperbolic decay function in various reported studies. In our study, we have noted that the hyperbolic function explains the complete recovery phase of only 11 extreme events. Furthermore, both decay functions fail to explain the late recovery phase of storms. Interestingly, we have found that the rate of recovery during the slow phase shows steady variation (independent to SYM−H), that is, d(SYM−H)dt=constant. Our analysis suggests that the magnitude of the recovery rate during the late phase is proportional to the magnitude of the storm. Key Points: The exponential or hyperbolic decay function can only explain fast recovery phase of a stormsWe observe a constant recovery rate during the late (slow) recovery phase of extreme storms that is, d(SYM−H)dt=constantWe also find that the recovery rate during the late phase is directly proportional to the amplitude of the storm [ABSTRACT FROM AUTHOR]

The plasma environment of Mars is highly influenced by its crustal magnetic fields. In this work, we investigate whether the crustal magnetic fields can be displaced from their original positions due to advection by ionospheric plasma flow. The ranges of interest are altitudes between 200and1000 km and solar zenith angles between 45°and135°. We examine magnetic field data using measurements from the Magnetometer (MAG) onboard the Mars Atmosphere and Volatile EvolutioN (MAVEN) spacecraft. We conduct statistical analyses (∼5 years of data) in order to investigate if horizontal shifts are observed between the data and the crustal magnetic field model by Langlais et al. (2019), https://doi.org/10.1029/2018je005854. We also show two case studies of individual measurements within a specific region of Mars. The results show statistical and observational evidence that the crustal magnetic fields are often being advected towards the night‐side of the planet. The displacement seems to increase exponentially as a function of altitude, reaching a horizontal shift of 100 km at 650 km altitude. Moreover, we use measurements from the SupraThermal And Thermal Ion Composition (STATIC) instrument in order to compare the dynamic pressure of ionospheric plasma flow to the magnetic pressure of the crustal magnetic fields. There are two possible causes for the displacement: a forcing by external magnetic pressure or an advection of the field to the electron fluid, which follows the ion fluid. In this paper, we argue that by the observed relation between ion fluid and displacement, the latter explanation is more probable. Plain Language Summary: Crustal magnetic fields keep information of a past time, when Mars still had its own internal magnetic field generator. These structures do not tend to change in the long‐term, so several models have been built throughout the last decades with the aim of reproducing the observed Martian fields. The crustal fields can influence the dynamics of the atmosphere. The region of the atmosphere, which is dominated by atmospheric plasma (ions and electrons) is the ionosphere. At Mars, the ionosphere lies at ∼130 km and above. In theory, the flow of the plasma is able to move the crustal magnetic fields in a process called advection. In this work, we indicate that this process happens often at Mars. Our results may be important for improving crustal magnetic field models, and also for estimating how much electricity the ionospheric plasma can conduct. Key Points: The crustal magnetic fields on Mars are being displaced relative to their expected locationThe displacement increases with solar zenith angle and altitudeIonospheric plasma flow is most likely responsible for this process [ABSTRACT FROM AUTHOR]

The available magnetic field data from the terrestrial magnetosphere, solar wind and planetary magnetospheres exceeds over 106 hours. Identifying plasma waves in these large data sets is a time consuming and tedious process. In this Paper, we propose a solution to this problem. We demonstrate how self‐organizing maps (SOMs) can be used for rapid data reduction and identification of plasma waves in large data sets. We use 72,000 fluxgate and 110,000 search coil magnetic field power spectra from the Magnetospheric Multiscale Mission (MMS1) and show how the SOM sorts the power spectra into groups based on their shape. Organizing the data in this way makes it very straightforward to identify power spectra with similar properties and therefore this technique greatly reduces the need for manual inspection of the data. We suggest that SOMs offer a time effective and robust technique, which can significantly accelerate the processing of magnetic field data and discovery of new wave forms. Key Points: We develop a robust method to classify large data sets of power spectra of magnetic fieldThe technique significantly reduces the need for time consuming manual inspection of the data and allows the discovery of new wave formsThe classification technique can be used to categorize plasma waves based on frequency, amplitude and bandwidth [ABSTRACT FROM AUTHOR]

Magnetosonic waves play an important role on the overall dynamics of relativistic radiation belt electrons. Numerical codes modeling the evolution of the radiation belts often account for wave‐particle interaction with magnetosonic waves. The diffusion coefficients incorporated in these codes are generally estimated based on the results of statistical surveys of the occurrence and amplitude of these waves. These statistical models assume that the spectrum of the magnetosonic waves can be considered as continuous in frequency space. This assumption can only be valid if the discrete nature of the waves satisfy the Chirikov overlap criterion. Otherwise, the assumption of a continuous frequency spectrum could produce erroneous results in wave models and hence estimates of the electron diffusion coefficients used in numerical models of the inner magnetosphere. Recently, it was demonstrated, through a case study conducted on a single short (10 s) period snapshot within a longer wave event, that the discrete nature of the equatorial magnetosonic waves do satisfy the Chirikov overlap criterion and so the assumption of a continuous frequency spectrum is valid for the calculation of diffusion coefficients. This paper expands this study to a broader range of time with many magnetosonic wave events to determine whether the discrete nature of the waves always satisfy the Chirikov overlap criterion. The results show that most, but not all, discrete magnetosonic emissions satisfy the Chirikov overlap criterion. Therefore, the use of the continuous spectrum, employed in quasi‐linear theory, may not always be justified. Key Points: Equatorial magnetosonic waves have a discrete spectrum at harmonics of the proton gyrofrequencyThe Chirikov resonance criterion is applied to investigate the validity of applying quasi‐linear theory with the assumption of a continuous Gaussian spectrumCluster observations of equatorial magnetosonic waves [ABSTRACT FROM AUTHOR]

The horizontal currents in the high‐latitude ionosphere are the primary driver of the magnetic field perturbations that are observed at the surface of the Earth. These currents and their ground effects are an important aspect of the magnetosphere‐ionosphere coupling process. This paper discusses the method of inversion that uses spherical harmonic potential function, in which magnetic field measurements on the ground can be used to derive maps of the "ionospheric equivalent currents," a mathematical representation of the horizontal currents flowing on a thin shell. It is shown that the use of both internal telluric and external current sources is required when fitting the spherical harmonic series; otherwise, the ionospheric currents will be overestimated. Furthermore, the inversion needs to compensate for magnetic effects of the magnetospheric ring current; otherwise, this current is projected onto the ionosphere. The amplification of the surface horizontal magnetic field and the suppression of the vertical magnetic field are demonstrated. The equivalent currents may be useful for estimating the ionospheric conductivity values. Additionally, these currents can be compared with the results from simulation models as a means of validation. Plain Language Summary: Currents in the high‐latitude ionosphere produce changes in the magnetic field at the surface of the Earth. This paper discusses a technique that uses measurements of these changes in the magnetic field to solve the problem of deriving maps of the currents flowing in the ionosphere. While the first description of this method dates back to the 1940s, this obscure practice can now be more useful with the more recent availability of globally distributed magnetic field measurements. The details of this particular "inversion" technique are described. It is shown that for greatest accuracy, the mirror image currents that occur beneath the Earth's surface need to be considered, as well as the currents that are actually located in the magnetosphere, far above the ionosphere. This result is useful in the study of the interaction between the solar wind and the Earth's magnetosphere, and the resulting currents. Key Points: Calculations of ionospheric equivalent current functions from ground‐level magnetic field values are reviewedThe vertical magnetic field is needed to derive the telluric currents, and effects of the ring current should be consideredThe telluric currents enhance the horizontal field and suppress the vertical component [ABSTRACT FROM AUTHOR]

This paper presents a comprehensive study of geomagnetic storm time currents using magnetic field recorded by multispacecraft polar‐orbiting mission, Swarm. During geomagnetic storm period, the magnetic field variations obtained after removing the internal geomagnetic field and quiet time contributions can be considered as a proxy for storm time currents and are found to follow the temporal profile of Dst index very closely. These variations at the equatorial crossings recorded by multiple‐spacecraft are used to estimate the Dst values and are found to have a good match with the ground‐based Dst index. The average deviation between these two is around 4–13%. We have estimated the asymmetry by taking the difference between the magnetic field variations at two local time sectors separated by 12 hr. The estimated asymmetry shows a good match with the AsyH‐index, especially when satellite traverses in the dawn‐dusk sector. In general, the magnetic field variations are stronger in the night‐to‐dusk sector than day‐to‐dawn sector, which could be due to the larger pressure‐gradients near night‐to‐dusk caused by ion movements. The important advantage of Swarm mission is that it provides an opportunity to investigate the longitudinal gradients in the storm time magnetic fields. It is observed that in general, the gradients are stronger during the main phase of the storm, centered near the equator with a latitudinal width of ~20–30° in both the hemispheres, and are supportive to the scenario of particle‐injection from the magnetotail. The stronger gradients are observed at higher latitudes (~40°) during the episodes of substorms and might be associated with the ionospheric/field‐aligned currents. Plain Language Summary: The space weather events such as geomagnetic storms can have a huge impact on the terrestrial magnetic and plasma environment, producing large currents, which in turn, impact spaceborne communication, navigation systems, aviation and satellite operations, and electric power grids, etc. The paper presents a new approach to estimate geomagnetic indices equivalent to Dst and AsyH during geomagnetic storms, making use of the increased observations from space‐based instruments in recent times, and demonstrates their utility in quantifying the geomagnetic environment. The important advantage of multispacecraft Swarm mission is that it provides an opportunity to investigate the spatial gradients in the storm time magnetic fields. The paper reports that strong azimuthal gradients in the magnetic field, near the equatorial sector, are associated with particle injection from the magnetotail. The paper demonstrates the utility of space‐based measurements in understanding the geomagnetic environment. Key Points: Geomagnetic storm time magnetic field variations are stronger in the dusk‐night than the dawn‐day sectorLongitudinal gradients in the storm time magnetic field are stronger near the equator and support the idea of particle injection from tailDuring episodes of substorm, stronger longitudinal gradients are observed at higher latitudes (~40°), which might be associated with FACs [ABSTRACT FROM AUTHOR]

The proton temperature anisotropy in the solar wind, when plotted as a two‐dimensional histogram in (β∥,T⊥/T∥) phase space, shows that the bulk of the data are broadly distributed in quasi‐isotropic state, while the fringe of data distribution is apparently regulated by various marginal stability conditions. The physical origin of magnetic field fluctuations in the stable regime is an open question. Among plausible mechanisms is the spontaneous thermal emission process. Discrete particle effects are central to the theory of spontaneous emission and reabsorption of electromagnetic fluctuations in plasmas. The present paper formulates the quasi‐linear theory of temperature anisotropy‐driven instabilities in which discrete particle effects are taken into account. The hybrid simulation is also carried out with varying number of superparticles per cell in order to unveil the underlying relationship between the theoretical and simulated discrete particle effects. The present paper demonstrates that the inherent numerical noise in the particle simulation is not always undesirable but may actually be exploited in order to control the degree of particle discreteness. Such a finding may have a significant implication for simulating the solar wind plasmas. Key Points: The standard quasi‐linear kinetic theory is reformulated in order to incorporate the effects of spontaneous thermal emissionThis formalism and hybrid simulations are used to study proton cyclotron and parallel firehose instabilitiesA possible relationship between inherent numerical noise of simulations and the particle discreteness is explored [ABSTRACT FROM AUTHOR]

The Magnetospheric Imaging Instrument/Charge Energy Mass Spectrometer onboard the Cassini spacecraft measured suprathermal H+ and water group (W+) ions during the 13‐year mission from 2004 to 2017. This paper generates statistical maps of pulse height analyzed fluxes for 3.24–3.34 keV H+ and 8.79–9.05 keV W+ from the start of day 2 in 2005 through the end of day 366 in 2016. The fluxes of both H+ and W+ are more intense on the nightside than on the dayside. The dayside magnetodisk thickness of H+ is greater than on the nightside, while the day and night thicknesses are comparable for W+. On both dayside and nightside, fluxes concentrate near the equator and also along the magnetic field lines near and just outside the Rhea L shell (L ≈ 8.75RS, 1RS = 60,268 km). The flux peaks map to the ionosphere well equatorward of the aurora. This intensity peak is near the boundary between where interchange events occur and where tail collapse (reconnection) events occur. The intensity variations can be represented by lognormal distributions in range r for each latitude λ, and the lognormal coefficients can then be fitted in latitude, thus allowing construction of simple models of the H+ and W+ spatial distributions. The models can then be used to estimate the fluxes of low energy suprathermal ions from ~5 to ~21RS on the dayside and ~5 to ~30RS on the nightside in both r and λ, or alternately in ρ and z. Plain Language Summary: The spatial distribution of ions in a magnetosphere is of fundamental interest to space scientists. This paper describes the distribution of protons (H+) and water group ions (mostly, O+) in the magnetosphere of Saturn as derived from 12 years of observations by the MIMI/CHEMS instrument on the Cassini spacecraft. Definite patterns in these flux maps appear. For instance, the fluxes of these ions follow magnetic field lines that cross the equator just outside the orbit of the moon Rhea. A model is constructed that has wide applicability to other investigations of Saturn's magnetosphere. Key Points: Energetic H+ and W+ fluxes form field‐aligned patterns when meridionally mapped onto the day and night sides of Saturn's magnetosphereFor a given latitude, the fluxes have lognormal distributions in range, and these distributions peak just outside the L shell of Rhea (L ≈ 9)A simple latitude‐dependent model can be devised to describe this global morphology [ABSTRACT FROM AUTHOR]

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

Plasma composition observations provide a useful mechanism for investigating plasma sources and subsequent evolution within planetary magnetospheres. While He++ is the second most abundant ion species in the solar wind, there are no known sources of He++ ions within the magnetosphere of Saturn, allowing He++ to serve as a tracer of solar wind ions within the Kronian magnetosphere. Meanwhile, water group ions (W+, consisting of O+, OH+, H2O+, and H3O+) and H2+, known to originate within the magnetosphere of Saturn, serve as a tracer of internally ionized plasma. In this paper, we investigate the relative abundances and properties of energetic (32–220 keV) ion species originating from sources within the magnetosphere and from the solar wind. Solar wind‐originating ions are observed to have significant relative abundance (up to ~0.05) in the midnight, dawn, and noon local time quadrants at high radial distances (~40, ~45, and ~20 RS, respectively). Several possible entry processes, such as reconnection and Kelvin‐Helmholtz instabilities, are outlined in this paper as well as a discussion of subsequent transport. Plain Language Summary: While the sources of plasma within the magnetosphere of Saturn have long focused on internal generation, this study seeks to estimate the amount of plasma entering into the magnetosphere from the solar wind. Using a mission averaged vantage point of the global relative abundances of 32 to 220 keV ions, the internal to external plasma is investigated in order to estimate the relative amount of solar wind‐originating ions within the magnetosphere of Saturn, as well as the regions of their importance. Solar wind‐originating ions are predominantly seen within the midnight and dawn quadrants of Saturn, while the dayside and inner magnetosphere is dominated by internally generated plasma. The energetic plasma is also seen to peak in density near a radial distance of 10 RS as a consequence of loss processes closer to the planet and radial diffusion further from Saturn. Key Points: Energetic (32‐220 keV) H+, H2+, He+, He++, and W+ densities are used to investigate internal versus external plasma sourcesEnergetic solar wind He++ observed in the magnetotail suggest access through Kelvin‐Helmholtz Instabilities and reconnection processesBoth internally and externally generated plasma species observe a peak in density at a radial distance of ~10 RS [ABSTRACT FROM AUTHOR]