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Plasma sheet convection is a key element of storm‐time plasma dynamics in the magnetosphere. While decades of observations have advanced our understanding of convection in general, specifically storm‐time convection remains poorly understood. Using data from ISAS/NASA's Geotail and NASA's MMS, this study characterizes plasma sheet magnetic flux transport across the magnetotail during numerous storms (both recovery and main phases) and contrasts these observations with those from quiet times. Our findings confirm the well‐documented enhancement of the convection electric field during geomagnetic storms. Beyond that, our results reveal a significant dawn‐dusk asymmetry. At dawn, the elevated convection is realized via relatively faster flows while at dusk, through a stronger northward magnetic field. These findings suggest a complex feedback loop between plasma sheet convection and ring current buildup, whereby the latter asymmetrically inflates the magnetotail on the dusk side, shifting the reconnection site and subsequently enhanced earthward flows toward dawn. Plain Language Summary: Strong solar activity creates major disruptions of the Earth's magnetic field known as geomagnetic storms. These major disturbances of near‐Earth space can impact essential technologies like GPS systems and power grids. Our research used data from two space missions, ISAS/NASA's Geotail and NASA's MMS, to study how particles and magnetic fields move in space around Earth during such storms. We compared storm periods to calm times and evaluated the differences. During geomagnetic storms, we found that charged particles transport more magnetic flux due to stronger electric field. We further noted a significant difference between the dawn (morning) and dusk (evening) sectors on the nightside of the near‐Earth space environment. During storm times, plasma moves faster on the dawn side, while the magnetic field is stronger on the dusk side. This discovery reveals that plasma movement and energy buildup during storms are more complex than we thought. The energy buildup on the dusk side pushes plasma movement more toward the dawn side. Understanding these details helps us better predict the impacts of geomagnetic storms on Earth's space environment, which can improve our ability to protect our technology and infrastructure. Key Points: Convection electric field is elevated across the plasma sheet during storms, but convection properties are different between dawn and duskDawn magnetic flux transport during storm time is statistically associated with relatively faster flowsDusk magnetic flux transport during storm times is statistically associated with more dipolar magnetic fields [ABSTRACT FROM AUTHOR]

We present the first direct evidence of an in situ excitation of drift‐compressional waves driven by drift resonance with ring current protons in the magnetosphere. Compressional Pc4–5 waves with frequencies of 4–12 mHz were observed by the Arase satellite near the magnetic equator at L ∼ 6 in the evening sector on 19 November 2018. Estimated azimuthal wave numbers (m) ranged from −100 to −130. The observed frequency was consistent with that calculated using the drift‐compressional mode theory, whereas the plasma anisotropy was too small to excite the drift‐mirror mode. We discovered that the energy source of the wave was a drift resonance instability, which was generated by the negative radial gradient in a proton phase space density at 20–25 keV. This proton distribution is attributed to a temporal variation of the electric field, which formed the observed multiple‐nose structures of ring current protons. Plain Language Summary: During magnetic storms and substorms, energetic ions are sporadically injected into the geospace, which distorts the stable population and velocity distributions of ions in space. At these moments, various plasma instabilities lead to ultra‐low frequency (ULF) wave excitations. The lowest‐frequency waves in the ULF range have a wavelength comparable to the size of the Earth and are typically analyzed using magnetohydrodynamic principles. This approach considers the plasma environment using macroscale parameters such as pressure and density. In this paper, we report a spacecraft observation of a broadband compressional ULF wave that cannot be interpreted using magnetohydrodynamics. Such waves have rarely been reported and analyzed; however, their interaction with energetic ions is important to understand magnetospheric energy dynamics. The plasma conditions were described using the kinetic theory, which involves particle velocity distributions. We observed that a drift resonance occurred between the energetic protons and waves, while the gradient instability condition was satisfied for a part of time. Therefore, we concluded that the wave was in a drift‐compressional mode excited through drift resonance and gradient instability. The interpretation of compressional waves via satellite observations of energetic ions has been receiving increasing attention to understand their excitation mechanism. Key Points: Pc4–5 compressional ultra‐low frequency waves with an azimuthal wave number of −130 were observed in the nose structure on dusksideTheoretically predicted values of drift‐compressional mode frequency match the observed wave frequencyBoth radial ion temperature gradient and drift resonance of 20–25 keV protons serve as energy sources of the wave [ABSTRACT FROM AUTHOR]

Abstract We present the first direct evidence of an in situ excitation of drift‐compressional waves driven by drift resonance with ring current protons in the magnetosphere. Compressional Pc4–5 waves with frequencies of 4–12 mHz were observed by the Arase satellite near the magnetic equator at L ∼ 6 in the evening sector on 19 November 2018. Estimated azimuthal wave numbers (m) ranged from −100 to −130. The observed frequency was consistent with that calculated using the drift‐compressional mode theory, whereas the plasma anisotropy was too small to excite the drift‐mirror mode. We discovered that the energy source of the wave was a drift resonance instability, which was generated by the negative radial gradient in a proton phase space density at 20–25 keV. This proton distribution is attributed to a temporal variation of the electric field, which formed the observed multiple‐nose structures of ring current protons.

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

The most important parameter driving the solar wind‐magnetosphere interaction is the southward (Bz) component of the interplanetary magnetic field (IMF). While the dawn‐dusk (By) component of the IMF is also known to play an important role, its effects are usually assumed to be independent of its sign. Here we demonstrate for the first time a seasonally varying, explicit IMF By‐dependence of the ring current and Dst index. Using satellite observations and a global magnetohydrodynamic model coupled with a ring current model, we show that for a fixed level of solar wind driving the flux of energetic magnetospheric protons and the growth‐rate of the ring current are greater for By < 0 (By > 0) than for By > 0 (By < 0) in Northern Hemisphere summer (winter). While the physical mechanism of this explicit By‐effect is not yet fully understood, our results suggest that IMF By modulates magnetospheric convection and plasma transport in the inner magnetosphere. Plain Language Summary: The magnetic field carried by solar wind is the most important driver of space weather. Aurora and other phenomena in the near‐Earth space are known to be stronger when the magnetic field on solar wind points to south than north. Usually the space weather response is expected to be equally strong for eastward and westward magnetic fields in solar wind. In this paper show that this is not the case. Using satellite measurements and computer modeling we show that electric currents in the near‐Earth space are stronger for westward than eastward magnetic field in solar wind in northern hemisphere summer, while the opposite is true in northern hemisphere winter. More studies are needed for understanding the physical mechanism of this phenomenon. Key Points: We show for the first time that there is an explicit By‐dependence in the ring current/proton precipitation and in the inner magnetosphereDuring Northern Hemisphere summer (winter) the ring current fluxes/proton precipitation and the rate of change of the Dst index are stronger for By < 0 (By > 0)The By‐dependence of the ring current and energetic proton fluxes is reproduced by a global coupled MHD‐ring current model [ABSTRACT FROM AUTHOR]

In this letter, the statistical characteristics of the structures of giant undulations (GUs) during geomagnetic storms are studied for the first time, based on aurora images during 2005–2019. The GUs mainly occur within ∼4 h following the beginning of the storm main phase and finish in ±4 h around the storm peak. The most interesting feature is that the anti‐sunward edge of GUs is quasi‐stable (18–19 h MLT) during their lifetime. The latitudinal amplitudes of GUs (AGU) peak at 200–300 km while the azimuthal wavelengths (WGU) occupy several discrete values between 100 and 900 km. Both AGU and WGU are quasi‐proportional to the strength of the storms, and AGU is also proportional to WGU. There are some differences in the characteristics of AGU and WGU between pre‐ and post dusk sectors. These results would provide both statistical insight for mechanisms of dynamic auroral structures and data sources for validating magnetosphere‐plasmasphere‐ionosphere coupling models. Plain Language Summary: The inner part of the Earth's magnetosphere, known as the plasmasphere, is full of cold (∼1 eV) and dense plasmas that corotate with the planet. The outer boundary of the plasmasphere is called the plasmapause, which separates two types of plasmas' contents characterized by different temperatures and densities. A recent study found that the storm‐time interaction between the cold plasmasphere and the hot plasma outside the plasmapause (plasma sheet/ring current) excites plasmapause surface wave, which generates giant undulations on the equatorward boundary of the diffuse aurora, that is, the sawtooth aurora, and drives outward‐propagating ultra‐low frequency waves. However, the factors that trigger and control the generation and evolution of the sawtooth aurora remain unclear. As the first step, understanding the statistical characteristics of the sawtooth aurora during magnetic storms is important to uncover the reconfiguration and energy redistribution processed in the magnetosphere. In this letter, we use a large amount of auroral images to statistically investigate the occurrence and general characteristics of the sawtooth aurora during magnetic storms. Our results would provide both physical insights for mechanisms of dynamic auroral structures and data sources for validating magnetosphere‐plasmasphere‐ionosphere coupling models. Key Points: Giant undulations (GUs) usually occur within ∼4 h following the beginning of the storm main phase and end in ±4 h around the storm peakBoth the latitudinal amplitudes and the azimuthal wavelengths of GUs are quasi‐proportional to the strength of the stormsThe anti‐sunward edge of GUs is quasi‐stable during their lifetime, indicating storm injections might excite plasmapause surface waves at a quasi‐fixed location [ABSTRACT FROM AUTHOR]

Abstract Molecular ions in the magnetosphere can be a tracer of fast ion outflows from the deep ionosphere. Statistical properties of molecular ions (O2+/NO+/N2+) in the ring current are investigated based on ion composition measurements (

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

We calculate auroral energy flux and Joule heating in the high‐latitude ionosphere for 27 geomagnetically active days using two‐dimensional maps of field‐aligned currents determined by the Active Magnetosphere and Planetary Response Experiment. The energy input to the ionosphere due to Joule heating increases more rapidly with geomagnetic activity than that due to precipitating particles. The energy flux varies more smoothly with time than Joule heating, which is impulsive in nature on time scales from minutes to tens of minutes. These impulsive events correlate well with recoveries in the Sym‐H index, with the maximum correlation when compared to Sym‐H recoveries 70 min later. Because of prior studies that have associated transient recoveries of Sym‐H with substorm expansions, the delay found here suggests that dissipation of energy in the ionosphere occurs during the substorm growth phase prior to the release of magnetic energy caused by diversion of tail currents. Plain Language Summary: Storms on the sun release radiation and energetic particles that travel through space and impact Earth. Energy from this impact is temporarily stored in the magnetic bubble formed by Earth's magnetosphere, dissipating gradually over time. At high‐latitudes, energy is lost from the magnetosphere by the precipitation of energetic particles into the atmosphere, and by the frictional heating associated with electric currents in Earth's ionosphere. We model the high‐latitude energy input to Earth's atmosphere from these two sources using 27 days of data from the 66 satellites of the Iridium constellation. We find that the energy input from particle impact is smoothly varying with time, in contrast to the frictional energy loss from the electric currents, which is impulsive in nature, characterized by enhancements that last several to several tens of minutes. These short‐lived energy loss events occur about an hour before changes in a magnetic index that measures the energy content of Earth's magnetosphere. The frictional heating occurs in conjunction with magnetic energy storage in the magnetosphere. The subsequent release of this magnetic energy causes the change in the magnetic index. Key Points: We calculate hemispherically integrated precipitating particle energy fluxes and Joule heating from global maps of field‐aligned currentsEnergy input from Joule heating and precipitating particles correlates well with increases in the Sym‐H index occurring about 1 h laterJoule heating in the ionosphere during substorm growth phase is approximately equal to magnetic energy released during substorm expansion [ABSTRACT FROM AUTHOR]

Abstract We show that broadband low‐frequency Alfvénic field variations observed in the inner‐magnetosphere from the Van Allen Probes during geomagnetically disturbed intervals are composed of multiscale current sheets, current filaments, flow shears, and vortices. These observations pertain to spacecraft frame frequencies (fsc) over the range 0.1

Abstract Ultra‐low frequency (ULF) waves can effectively accelerate ring current ions through drift‐bounce resonance. Conventional theories have been developed to understand this resonance since the dawn of the space age, which usually assume ULF waves are quasi‐steady in time. However, ULF waves would experience growth and damping stages in the real magnetosphere, which requires an adjustment of the conventional theories. Here, we attempt to reveal how ions respond to evolving ULF waves. By analyzing an event observed by Van Allen Probe A and reproducing the observations via simulation, we find the phase shift of ion flux oscillation across resonant pitch angles varies with time, when wave growth and damping cannot be neglected. As a result, the inclination angle of stripes in pitch angle spectrogram varies with time, causing “fishbone‐like” pitch angle structures. One should take into account such ion behaviors when considering the role of drift‐bounce resonance in the real magnetosphere.

Suprathermal electrons are a major heat source of ionospheric plasma. How the suprathermal electrons evolve during their bounces inside the plasmasphere is a fundamental question for the magnetosphere‐ionosphere coupling. On the basis of Van Allen Probes observations and quasi‐linear simulations, we present here the first quantitative study on the evolution of suprathermal electrons under the competition between Landau heating by whistler mode hiss waves and Coulomb collisional cooling by background plasma inside a plasmaspheric plume. We show that the Landau heating can prevail over the collisional cooling for >50 eV electrons and cause the field‐aligned suprathermal electron fluxes to increase by up to 1 order of magnitude within 1.5 hr. Our results imply that the plasmaspheric plume hiss waves could mediate energy from the ring current electrons to the ionospheric plasma. Plain Language Summary: The ionospheric plasma temperature variation can affect the Earth's atmospheric escape and the spacecraft orbit decay. A major heat source of the ionospheric plasma is the suprathermal (several eV to hundreds of eV) electrons, which can bounce along the magnetic field lines inside the plasmasphere. How these suprathermal electrons evolve during their bounces is a fundamental question for the magnetosphere‐ionosphere coupling. In the past, the plasmaspheric suprathermal electrons were usually considered to gain energy from the ring current ions through Coulomb collisions and wave‐particle interactions. Here we show that strong whistler mode hiss waves can grow from the instability of the ring current electrons in the plasmaspheric drainage plume, and their Landau heating can prevail over the collisional cooling by background plasma for >50 eV electrons. The enhanced field‐aligned suprathermal electrons could eventually heat the ionospheric plasma. These results have significant implications for understanding the energy transfer process from the magnetosphere to the ionosphere. Key Points: The collisional cooling by background plasma counteracts largely the Landau heating by hiss waves for <50 eV electronsThe Landau heating can increase the field‐aligned fluxes of >50 eV electrons by 1 order of magnitude within 1.5 hrPlasmaspheric plume hiss could mediate energy from the ring current electrons through the suprathermal electrons to the ionospheric plasma [ABSTRACT FROM AUTHOR]

The Naval Research Laboratory (NRL) Sami3 is Also a Model of the Ionosphere (SAMI3) ionosphere/plasmasphere code is used to examine the effect of ring current heating during a storm. With a ring current heating function added to SAMI3, a cold thermal (<1 eV) oxygen ion outflow is produced, with O+ density and location similar to observations of the so‐called "oxygen torus." The ring current heating function is based on a Comprehensive Inner Magnetosphere‐Ionosphere (CIMI) model simulation of the 7 October 2015 storm. We find that the ring current can heat plasmasphere electrons, subsequently heating plasmasphere H+, and ionosphere O+. The resulting O+ outflows resemble observed O+ enhancements in the inner magnetosphere. Plain Language Summary: The near‐Earth environment includes relatively low‐energy "cold" ions with temperatures less than 105 K (8.6 eV) and much higher energy ions. We specifically consider thecold plasmasphere, the magnetically contained extension of the ionosphere into space, and the ring current, a high‐energy (1–100 keV) population of O+ and H+ ions generated during a geomagnetic storm. We numerically model the interaction between these populations in search of an explanation for the cold oxygen ions that are observed near the edge of the plasmasphere. Results suggest that the ring current heats plasmasphere electrons, plasmasphere H+, and ionosphere O+. The heated O+ flows upward along the geomagnetic field, forming the "oxygen torus" that has been observed by numerous spacecraft. Key Points: Simulated ring current heating produces an outflow of "cold" (<1 eV) oxygen ions into the magnetosphereModel oxygen ion densities and positions are similar to observations of the so‐called "oxygen torus"Model oxygen ion enhancements resemble a partial toroidal shell that extends outwards along the geomagnetic field [ABSTRACT FROM AUTHOR]

Ultra‐low frequency (ULF) waves can effectively accelerate ring current ions through drift‐bounce resonance. Conventional theories have been developed to understand this resonance since the dawn of the space age, which usually assume ULF waves are quasi‐steady in time. However, ULF waves would experience growth and damping stages in the real magnetosphere, which requires an adjustment of the conventional theories. Here, we attempt to reveal how ions respond to evolving ULF waves. By analyzing an event observed by Van Allen Probe A and reproducing the observations via simulation, we find the phase shift of ion flux oscillation across resonant pitch angles varies with time, when wave growth and damping cannot be neglected. As a result, the inclination angle of stripes in pitch angle spectrogram varies with time, causing "fishbone‐like" pitch angle structures. One should take into account such ion behaviors when considering the role of drift‐bounce resonance in the real magnetosphere. Key Points: The responses of ring current ions to evolving ULF waves are observed by Van Allen Probe A and reproduced via simulationThe observed phase shift across resonant pitch angle changes with time, manifesting as increasingly inclined or vertical pitch angle stripesSimulation suggests the growth and damping of ULF waves are responsible for the time‐varying phase shift [ABSTRACT FROM AUTHOR]

We show that broadband low‐frequency Alfvénic field variations observed in the inner‐magnetosphere from the Van Allen Probes during geomagnetically disturbed intervals are composed of multiscale current sheets, current filaments, flow shears, and vortices. These observations pertain to spacecraft frame frequencies (fsc) over the range 0.1 < fsc < 10 Hz and correspond to structure advected over the spacecraft on scales (λ) over the range 0.1 ≲ λ/ρi ≲ 10. where ρi is the average ion gyroradius. The topology of the current and flow is derived via time domain and spectral approaches adapted for the interpretation of advected structure. The results reveal the action of magnetic and convective nonlinearities in the generation of broadband Alfvénic fluctuations prevalent in the inner magnetosphere during active times. The observed current filamentation and rotational vorticity is suggestive of how broadband Alfvénic field variations on kinetic scales are driven and support geophysically significant particle acceleration in the inner magnetosphere. Key Points: Broadband inner magnetospheric Alfvén waves are composed of multiscale sheet and filamentary currents, flow shears, and vorticesThe field and flow topology in broadband Alfvénic fluctuations is symptomatic of the action of magnetic and convective nonlinearitiesThe multiscale rotational field and flow topology suggests that cross‐scale energy transport supports particle acceleration [ABSTRACT FROM AUTHOR]

Molecular ions in the magnetosphere can be a tracer of fast ion outflows from the deep ionosphere. Statistical properties of molecular ions (O2+/NO+/N2+) in the ring current are investigated based on ion composition measurements (<180 keV/q) by medium‐energy particle experiments‐electron analyzer and low‐energy particle experiments‐ion mass analyzer instruments on board the Arase (Exploration of energization and Radiation in Geospace, ERG) satellite. The investigated period from late March to December 2017 includes 11 geomagnetic storms with the minimum Dst index less than −40 nT. The molecular ions are observed in the region of L = 2.5–6.6 and clearly identified at energies above ~12 keV during most magnetic storms. During quiet times, molecular ions are not observed. The average energy density ratio of the molecular ions to O+ is ~3%. The ratio tends to increase with the size of magnetic storms. Existence of molecular ions even during small magnetic storms suggests that the fast ion outflow from the deep ionosphere occurs frequently during geomagnetically active periods. Plain Language Summary: Molecular ions usually exist only in the low‐altitude (< 300 km) deep ionosphere and cannot escape to space without a fast ion outflow to overcome a rapid loss due to dissociative recombination. Thus, molecular ion escape from the terrestrial atmosphere to space can be used as a tracer of effective ion loss from the deep ionosphere. Here we report new observations by Arase satellite that enables definitive identification of molecular ions by frequent time‐of‐flight mode observations. The statistical analysis shows that molecular ions exist in near‐Earth space during most magnetic storms, while they are not detected during geomagnetically quiet periods. The existence of molecular ions even during small magnetic storms suggests that the magnetic storm is an effective driver of the ion loss from the deep terrestrial ionosphere. Key Points: Frequent TOF‐mode ion composition measurements by Arase revealed statistical properties of molecular ions in the ring currentThe molecular (O2+/NO+/N2+) ions are commonly observed during geomagnetically active periods even during small magnetic stormsThe results suggest that the rapid ion outflow from the deep ionosphere occurs frequently during active periods [ABSTRACT FROM AUTHOR]

Electromagnetic ion cyclotron (EMIC) waves and magnetosonic waves are commonly observed in the Earth's magnetosphere associated with enhanced ring current activity. Using wave and ion measurements from the Van Allen Probes, we identify clear correlations between the hydrogen‐ and helium‐band EMIC waves with the enhancement of trapped helium and oxygen ion fluxes, respectively. We calculate the diffusion coefficients of different ion species using quasi‐linear theory to understand the effects of resonant scattering by EMIC waves. Our calculations indicate that EMIC waves can cause pitch angle scattering loss of several keV to hundreds of keV ions, and heating of tens of eV to several keV helium and oxygen ions by hydrogen‐ and helium‐band EMIC waves, respectively. Moreover, we found that magnetosonic waves can cause the resonant heating of thermal protons. Our study indicates the importance of energy transfer from the EMIC and magnetosonic waves to ions with different species at thermal energies. Plain Language Summary: Electromagnetic ion cyclotron (EMIC) waves and magnetosonic waves are plasma waves commonly observed in the Earth's magnetosphere. The EMIC waves and magnetosonic waves are associated with the ions drifting around the Earth. We use satellite observations of the plasma waves and ions to study the relation between EMIC or magnetosonic waves with ions at thermal energies. The measurement clearly indicates the correlation between the enhancement of proton, helium, and oxygen fluxes with magnetosonic waves, hydrogen‐band EMIC waves, and helium‐band EMIC waves, respectively. We use the physics‐based model to quantify the possible heating of ions with different species by different wave modes. Our calculation suggests that the protons, helium, and oxygen ions at thermal energies could be effectively heated by magnetosonic waves, hydrogen‐band EMIC waves, and helium‐band EMIC waves, respectively. The model results explain the measured correlation between the ions and waves. Our study suggests that the EMIC and magnetosonic waves which are generated by the ions in ring current could transfer energy to the lower energy ions. Key Points: EMIC waves in different frequency bands and magnetosonic waves are correlated with the enhancement of ion fluxes of different speciesEMIC and magnetosonic waves could heat different ion species at thermal energies through resonant interactionField‐aligned thermal ions are accelerated to larger pitch angles and trapped ion fluxes increase due to the EMIC and magnetosonic waves [ABSTRACT FROM AUTHOR]

Near equatorial (fast) magnetosonic waves, characterized by high magnetic compressibility, are whistler‐mode emissions destabilized by proton shell/ring distributions. In the past, substorm proton injections are widely known to intensify magnetosonic waves in the inner magnetosphere. Here we report the unexpected observations by the Van Allen Probes of the magnetosonic wave quenching associated with the substorm proton injections under both high‐ and low‐density conditions. The enhanced proton thermal pressure distorted the background magnetic field configuration and the cold plasma density distribution. The reduced phase velocities locally allowed the weak growth or even damping of magnetosonic waves. Meanwhile, the spatially irregularly varying refractive indices might suppress the cumulative growth of magnetosonic waves. For intense injections, this wave quenching region could extend over 2 hr in magnetic local time and 0.5 Earth radii in radial distance. These results provide a new understanding of the generation and distribution of magnetosonic waves. Plain Language Summary: Magnetosonic waves are the near‐equatorially confined electromagnetic emissions between the proton gyrofrequency and the lower hybrid frequency in the magnetosphere. Theoretical and observational studies have demonstrated the potential of the magnetosonic waves to accelerate the radiation belt electrons. It is therefore important to understand the generation process and the spatiotemporal distribution of the magnetosonic waves. The substorm activities injecting hot protons into the inner magnetosphere are conventionally considered to intensify the magnetosonic waves. By analyzing the wave and particle data of the Van Allen Probes, we find a magnetosonic wave quenching region related to the distortion of the background magnetic field configuration and the cold plasma density distribution closely following the substorm injection front. We propose two possible causes for this phenomenon: (1) local weak growth or even damping of the magnetosonic waves with the reduced phase velocities and (2) suppression of the magnetosonic wave cumulative growth in the disturbed medium. This new finding may help refine the modeling of the magnetosonic waves and then the radiation belt electrons. Key Points: The enhanced thermal pressure of hot protons can distort the geomagnetic field configuration and the cold plasma density distributionThe magnetosonic wave quenching region can emerge closely following the substorm injection front under both high‐ and low‐density conditionsThe magnetosonic wave quenching region can extend over 2 hr in magnetic local time and 0.5 Earth radii in radial distance [ABSTRACT FROM AUTHOR]

Abstract: Magnetosonic waves play a potentially important role in the complex evolution of the radiation belt electrons. These waves typically appear as discrete emission lines along the proton gyrofrequency harmonics, consistent with the prediction of the local Bernstein mode instability of hot proton ring distributions. Magnetosonic waves are nearly dispersionless particularly at low harmonics and therefore have the roughly unchanged frequency time structures during the propagation. On the basis of Van Allen Probes observations, we here present the first report of magnetosonic harmonic falling and rising frequency emissions. They lasted for up to 2 hr and occurred primarily in the dayside plasmatrough following intense substorms. These harmonic emission lines were well spaced by the proton gyrofrequency but exhibited a clear falling (rising) frequency characteristic in a regime with the temporal increase (decrease) of the proton gyrofrequency harmonics. Such unexpected structures might be produced by the nonlinear interactions between the locally generated magnetosonic waves at the proton gyrofrequency harmonics and a constant frequency magnetosonic wave propagating away from the Earth. [ABSTRACT FROM AUTHOR]

Abstract: We present observational evidence of drift resonance between westward propagating odd mode standing ultralow frequency waves and energetic protons. Compressional ∼13 mHz (Pc4 band) waves and proton flux oscillations at >50 keV were detected at ∼03 hr magnetic local time by the Arase satellite on 15 April 2017. The azimuthal wave number (m number) is estimated to be ∼−50 from ground observations, while the theory of drift resonance gives m ∼− 49 for odd mode waves and ∼110‐keV protons, providing evidence that the drift resonance indeed took place in this event. We also found a steep earthward gradient of proton phase space density, which can quantitatively explain the wave excitation. The observed waves show typical features of giant pulsations (Pgs), regarding local time, m number, and flux oscillations. This study, therefore, has great implications to the field line mode structure and excitation mechanism of Pgs. [ABSTRACT FROM AUTHOR]

Abstract: The transport mechanism of the ring current ions differs among ion energies. Lower‐energy (≲150 keV) ions are well known to be transported convectively. Higher‐energy (≳150 keV) protons are reported to be transported diffusively, while there are few reports about transport of higher‐energy oxygen ions. We report the radial transport of higher‐energy oxygen ions into the deep inner magnetosphere during the late main phase of the magnetic storm on 23–25 April 2013 observed by the Van Allen Probes spacecraft. An enhancement of 1–100 mHz magnetic fluctuations is simultaneously observed. Observations of 3 and 30 mHz geomagnetic pulsations indicate the azimuthal mode number is ≤10. The fluctuations can resonate with the drift and bounce motions of the oxygen ions. The results suggest that the combination of the drift and drift‐bounce resonances is responsible for the radial transport of higher‐energy oxygen ions. [ABSTRACT FROM AUTHOR]

Abstract: Magnetosonic waves are highly oblique whistler mode emissions transferring energy from the ring current protons to the radiation belt electrons in the inner magnetosphere. Here we present the first report of prompt disappearance and emergence of magnetosonic waves induced by the solar wind dynamic pressure variations. The solar wind dynamic pressure reduction caused the magnetosphere expansion, adiabatically decelerated the ring current protons for the Bernstein mode instability, and produced the prompt disappearance of magnetosonic waves. On the contrary, because of the adiabatic acceleration of the ring current protons by the solar wind dynamic pressure enhancement, magnetosonic waves emerged suddenly. In the absence of impulsive injections of hot protons, magnetosonic waves were observable even only during the time period with the enhanced solar wind dynamic pressure. Our results demonstrate that the solar wind dynamic pressure is an essential parameter for modeling of magnetosonic waves and their effect on the radiation belt electrons. [ABSTRACT FROM AUTHOR]

Hydrogen band electromagnetic ion cyclotron (EMIC) waves have received much attention recently because they are found to frequently span larger spatial areas than the other band EMIC waves. Using test particle simulations, we study the nonlinear effects of hydrogen band EMIC waves on ring current H+ ions. A dimensionless parameter R is used to characterize the competition between wave-induced and adiabatic motions. The results indicate that there are three regimes of wave-particle interactions for typical 35 keV H+ ions at L = 5: diffusive (quasi-linear) behavior when αeq ≤ 35° ( R ≥ 2.45), the nonlinear phase trapping when 35° < αeq < 50° (0.75 < R < 2.45), and both the nonlinear phase bunching and phase trapping when αeq ≥ 50° ( R ≤ 0.75). The phase trapping can transport H+ ions toward large pitch angle, while the phase bunching has the opposite effect. The phase-trapped H+ ions can be significantly accelerated (from 35 keV to over 500 keV) in about 4 min and thus contribute to the formation of high energy components of ring current ions. The results suggest that the effect of hydrogen band EMIC waves is not ignorable in the nonlinear acceleration and resonance scattering of ring current H+ ions. [ABSTRACT FROM AUTHOR]

The most important parameter driving the solar wind-magnetosphere interaction is the southward ( B z ) component of the interplanetary magnetic field (IMF). While the dawn-dusk ( B -dependence of the ring current and y index. Using satellite observations and a global magnetohydrodynamic model coupled with a ring current model, we show that for a fixed level of solar wind driving the flux of energetic magnetospheric protons and the growth-rate of the ring current are greater for B y -dependence of the ring current and Dst index. Using satellite observations and a global magnetohydrodynamic model coupled with a ring current model, we show that for a fixed level of solar wind driving the flux of energetic magnetospheric protons and the growth-rate of the ring current are greater for B y  < 0 ( B y  > 0) than for B y  > 0 ( B y  < 0) in Northern Hemisphere summer (winter). While the physical mechanism of this explicit B y -effect is not yet fully understood, our results suggest that IMF B y modulates magnetospheric convection and plasma transport in the inner magnetosphere., (© 2022 The Authors.)