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We examine the dynamics of protons during tail‐like to dipole‐like reconfigurations of Mercury's magnetosphere. Such reconfigurations that frequently occur in the highly dynamical Hermean environment are accompanied by induced electric fields leading to short‐lived convection enhancements. Using test particle calculations, we show that, under the effect of such induced electric fields, protons may be subjected to prominent energization while being injected into the inner magnetosphere. We demonstrate that this energization occurs in a nonadiabatic manner and can reach several tens of keV, possibly leading to particle trapping around the planet. Recent observations from BepiColombo during Mercury's third flyby provide evidences of energetic protons drifting in the vicinity of the planet. The present impulsive energization process is a possible mechanism for the build‐up of such populations. Plain Language Summary: The electric field induced by short‐lived relaxation of the magnetospheric field lines at Mercury may lead to prominent energization (tens of keV) of protons. As a result, these ion populations may be injected into the ring current in the inner magnetosphere. Key Points: Observational evidences of energetic protons (tens of keVs) in the inner magnetosphere of MercuryElectric field induced by rapid reconfigurations of the magnetosphere may lead to proton energization up to this energy rangeThis energization occurs in a nonadiabatic manner and contributes to ring current trapping in the inner magnetosphere [ABSTRACT FROM AUTHOR]

Plasmaspheric density structures are considered to control the propagation trajectories of fast magnetosonic (MS) waves in the inner magnetosphere. However, whether the plasmaspheric plume can effectively alter the propagation of MS waves remains unknown. Based on the analytical model of plasma density, ray tracing simulations are performed to investigate the propagation of exactly perpendicular MS waves in the equatorial plane in the magnetosphere containing a plasmaspheric plume. We find that plasmatrough and plume MS waves propagating toward the plasmaspheric plume can be reflected into the plasmaspheric core by the plume, then potentially migrating globally and thus quasi‐trapped inside the plasmaspheric core. The simulations also indicate that lower‐frequency MS waves approaching the plasmaspheric plume are more easily reflected and quasi‐trapped inside the plasmaspheric core. Our findings illustrate a previously unexplored way that plasmatrough MS waves could access and be trapped inside the plasmaspheric core via azimuthal plasmaspheric density structures. Plain Language Summary: Fast magnetosonic (MS) waves are one of the most common and intense electromagnetic emissions observed both inside and outside the plasmasphere. Portions of MS waves observed inside the plasmasphere are supposed to come from the propagation of those waves outside the plasmasphere. It is well known that the radial plasmaspheric density structures can strongly alter the propagation trajectories of MS waves in the inner magnetosphere. However, a typical azimuthal plasmaspheric density structure, the plasmaspheric plume, can form in the magnetosphere during the geomagnetic disturbed times. How the plasmaspheric plume affects the propagation of MS waves remains unknown. Based on the ray tracing simulations, we show that MS waves generated outside the plasmapause and inside the plume can be reflected into the plasmaspheric core by the plasmaspheric plume and a few of the reflected waves can migrate globally inside the plasmaspheric core. Our findings provide a new potential way for MS waves to access and be trapped inside the plasmaspheric core. Key Points: Propagation of magnetosonic waves in the magnetosphere containing a plasmaspheric plume is examined using the ray tracing simulationsPlasmatrough and plume magnetosonic waves can be reflected into the plasmaspheric core by the plasmaspheric plumeThose reflected magnetosonic waves can be quasi‐trapped inside the plasmaspheric core or propagate radially down to low altitudes [ABSTRACT FROM AUTHOR]

Flux ropes are ubiquitous at Earth's magnetopause and play important roles in energy transport between the solar wind and Earth's magnetosphere. In this study, structure and coalescence of the magnetopause flux ropes formed by multiple X line reconnection in cases with different southward interplanetary magnetic field (IMF) clock angles are investigated by using three-dimensional global hybrid simulations. As the IMF clock angle decreases from 180°, the axial direction of the flux ropes becomes tilted relative to the equatorial plane, the length of the flux ropes gradually increases, and core field within flux ropes is formed by the increase in the guide field. The flux ropes are formed mostly near the subsolar point and then move poleward toward cusps. The flux ropes can eventually enter the cusps, during which their helical structure collapses, their core field weakens gradually, and their axial length decreases. When the IMF clock angle is large (i.e., the IMF is predominantly southward), the flux ropes can coalesce and form new ones with larger diameter. The coalescence between flux ropes can occur both near the subsolar point when they are newly formed and away from the subsolar point (e.g., in the southern hemisphere) when they move toward cusps. However, when the IMF clock angle is small (≤135°), we do not find coalescence between flux ropes. [ABSTRACT FROM AUTHOR]

Using the Arase and Van Allen Probes satellite observations, we investigate the nonlinear electromagnetic ion cyclotron (EMIC) rising‐tone (RT) emissions with an increase of the solar wind dynamic pressure in the dayside magnetosphere. We find that EMIC RT emissions are accompanied by the extended dayside uniform zone (DUZ) over |MLAT| < 25° due to the dayside magnetospheric compression by an increase in Pdyn. Using the observed plasma and magnetic field data, we modeled the threshold amplitude for the nonlinear EMIC waves and compared it with the observation. The small gradient of the ambient magnetic field strongly contributes to the reduction in the threshold amplitude of nonlinear wave growth compared to other parameters. When the threshold amplitude falls to comparable level of pre‐existing EMIC waves, EMIC RT emissions are immediately triggered, suggesting direct evidence that the DUZ is the preferred condition to cause the nonlinear EMIC RT emission in the dayside magnetosphere. Plain Language Summary: Electromagnetic ion cyclotron (EMIC) waves play an important role in controlling the dynamics of charged particles in the inner magnetosphere. Especially, nonlinear EMIC rising‐tone (RT) emissions can cause the rapid loss of relativistic electrons and ring current ions. Here, we present direct evidence demonstrating that the distortion of the dayside magnetic field causes nonlinear EMIC RT emission in response to the intensification of the solar wind dynamic pressure. Remarkably, these nonlinear EMIC waves are generated through a reduction in the threshold wave amplitudes by the distortion of the magnetic fields, even in the absence of any significant change in the pre‐existing EMIC wave amplitude. The present result provides new insights into a triggering process of nonlinear plasma waves in the magnetosphere. Key Points: Electromagnetic ion cyclotron (EMIC) waves with rising‐tone (RT) elements were observed in the dayside magnetosphere during an increase in the solar wind dynamic pressureIncreasing solar wind dynamic pressure extends the dayside uniform zone of the magnetic field to higher magnetic latitudesThe uniform zone leads to the reduction of the nonlinear threshold wave amplitude, which triggers nonlinear EMIC RT emissions [ABSTRACT FROM AUTHOR]

Reconnection at Earth's magnetopause drives magnetospheric convection and provides mass and energy input into the magnetosphere/ionosphere system thereby driving the coupling between solar wind and terrestrial magnetosphere. Despite its importance, the factors governing the location of dayside magnetopause reconnection are not well understood. Though a few models can predict X‐line locations reasonably well, the underlying physics is still unresolved. In this study we present results from a comparative analysis of 274 magnetic reconnection events as observed by the Magnetospheric Multiscale (MMS) mission to determine what quantities affect the accuracy of such models and are most strongly associated with the occurrence of dayside magnetopause reconnection. We also attempt to determine under what upstream solar wind conditions each global X‐line model becomes least reliable. Plain Language Summary: Magnetic reconnection is an energy transferring process that happens at the Earth's magnetopause, the boundary between Earth's local plasma environment and the Sun's interplanetary environment. Reconnection helps move energy and particles into the region of space surrounding the earth called the magnetosphere. Dayside reconnection is important because it is a defining element to the amount of energy deposited into our magnetosphere. Despite its importance, scientists still do not completely understand how it works and where it happens. This study looks at many observations of magnetic reconnection to try and understand its precise location and figure out under which conditions it is more likely to occur. This paper compares in situ data with different models for predicting reconnection locations. Key Points: Compared four different reconnection line modelsOn average, the Maximum Bisection Field Model gives the best prediction of reconnection line locationThe Maximum Exhaust Velocity model gives the worst prediction of the reconnection line location [ABSTRACT FROM AUTHOR]

Simultaneous observations of whistler‐mode chorus and magnetosonic (MS) waves are reported within a magnetic peak in the inner magnetosphere for the first time. During the magnetic peak, Van Allen Probe A observes flux enhancements of both ring current electrons and relativistic electrons, while medium‐energy proton fluxes are reduced but high‐energy proton fluxes are raised. Those flux variations of ring current particles are almost concentrated in the perpendicular direction of the background magnetic field, leading to enhancements of the electron temperature anisotropy and a positive gradient of proton velocity distributions. The calculated linear growth rates show that unstable electrons with temperature anisotropic instabilities and unstable protons with ring distribution during the magnetic peak structure can provide free energy for the excitation of whistler‐mode chorus and MS waves, respectively. Our findings suggest that injected particles encountering the magnetic peak structure may form more complex unstable electron and ion distributions driving multiple instabilities simultaneously. Plain Language Summary: Recently, dipolarizing flux bundles have also been observed in the inner magnetosphere. These magnetic field structures are usually accompanied by dispersion of particle energies and pitch angles, resulting in complex and unstable particle distributions during magnetic structures, which may provide free energy for wave excitation. Previous observed waves during magnetic structures belong to single instability (electron type or proton type), rarely taking into account waves driven by both proton and electron instabilities. In this letter, simultaneous observations of chorus and magnetosonic (MS) waves are reported during a magnetic peak in the inner magnetosphere for the first time. Our results show that the complex distribution in the velocity phase space of ring current electrons and protons during the magnetic peak in the inner magnetosphere has the potential to simultaneously trigger chorus and MS waves. Key Points: Whistler‐mode chorus and fast magnetosonic (MS) waves are simultaneously observed during the magnetic peak in the inner magnetosphereEnhancements of ring current particle instabilities during the magnetic peak cause the excitation of whistler‐mode chorus and MS wavesOur results contribute to understanding the evolution of injected electrons and protons encountering the magnetic peak [ABSTRACT FROM AUTHOR]

Geomagnetic indices have been used as a proxy for studying electromagnetic ion cyclotron (EMIC) wave occurrences under different geomagnetic conditions. However, the drivers of EMIC waves are different during non‐storm, storm time and during individual storm phases. Using ∼7 years of data from the twin Van Allen Probes, we demonstrate that the occurrence probability of EMIC waves are not well captured by a specific geomagnetic activity index alone, but is rather well manifested by considering individual storm phases. We show EMIC wave occurrence statistics during different storm phases (pre‐onset, main and recovery) for geomagnetic activity indices Sym‐H, AE, and Kp and solar wind dynamic pressure Pdyn, illustrating that the occurrence rates vary significantly during different storm phases even for a given geomagnetic index. We also utilize this large database to show EMIC wave occurrence distribution, and how various wave and plasma parameters behave under different geomagnetic conditions. EMIC waves occur 2.9 times more often during geomagnetic storms than during non‐storm times. The majority (72%) of storm time EMIC waves occur during the recovery phase due to long recovering time, while the highest occurrence rates are in the pre‐onset phase, followed by main and recovery phases. EMIC waves in the main phase have occurrence peaks in the dusk to pre‐midnight sectors while recovery phase events spread to more Magnetic Local Time (MLT) sectors with peaks in the morning sector. Wave amplitudes are found to be evenly distributed across different MLT sectors during all geomagnetic conditions. Key Points: Geomagnetic storm phases, rather than geomagnetic indices alone, portray storm time electromagnetic ion cyclotron (EMIC) waves more effectivelyEMIC waves occur 2.9 times more frequently during storms than during non‐storm timesThe storm time occurrence rate is highest during the pre‐onset phase, followed by the main and recovery phases [ABSTRACT FROM AUTHOR]

This study presents a re‐evaluation of the Kullen and Janhunen (2004, https://doi.org/10.5194/angeo-22-951-2004) global northward interplanetary magnetic field (IMF) simulation, using the Grand Unified Magnetosphere–Ionosphere Coupling Simulation version 4 (GUMICS‐4), a global MHD model. We investigate the dynamic coupling between northward IMF conditions and the Earth's magnetotail and compare the results to observation‐based mechanisms for the formation of transpolar arcs. The results of this study reveal that under northward IMF conditions (and northward IMF initialization), a large closed field line region forms in the magnetotail, with similarities to transpolar arc structures observed from spacecraft data. This interpretation is supported by the simultaneous increase of closed flux measured in the magnetotail. However, the reconnection configuration differs in several respects from previously theorized magnetotail structures that have been inferred from both observations and simulations results and associated with transpolar arcs. We observe that dawn–dusk lobe regions form as a result of high‐latitude reconnection during the initialization stages, which later come into contact as the change in the IMF By component causes the magnetotail to twist. We conclude that in the GUMICS simulation, transpolar arc‐like structures are formed as a result of reconnection in the magnetotail, rather than high‐latitude reconnection or due to the mapping of the plasma sheet through a twisted magnetotail as interpreted from previous analysis of GUMICS simulations. Plain Language Summary: When the magnetic field associated with the solar wind is directed "northward," the Earth's aurora can adopt a formation where a "bar" of emission crosses the otherwise dim region that lies poleward of the usual auroral emissions. These phenomena are called "theta auroras" (due to their resemblance to the Greek letter), or "transpolar arcs," and have been observed from spacecraft observing the auroral regions. As spacecraft cannot directly observe the global configuration of magnetic field lines within planetary magnetospheres, simulations can be useful to gain insight into the possible structures that occur during different solar wind conditions. We use a global simulation to model the interaction between the solar wind and the Earth's magnetosphere (the region of space around our planet) and see that a large‐scale field line structure can form during certain northward‐directed magnetic field conditions. When we trace these field lines to the ionospheric boundary, we find that they resemble the global structures that have been observed for auroral arcs that stretch across the polar cap (also named theta aurora or transpolar arcs). We also note some key differences in their formation process from observational results. Key Points: Signatures consistent with magnetotail reconnection are observed in a simulation driven by northward interplanetary magnetic field conditionsThe simulation results in the production of a closed magnetotail structure and polar cap arcsThe above structure occurs within a magnetotail that is highly twisted such that open lobe regions cross the equatorial plane [ABSTRACT FROM AUTHOR]

Magnetosonic waves inside and outside the plasmasphere differ statistically in occurrence rate, frequency, and intensity. How the density interface separates magnetosonic waves inside and outside the plasmasphere remains not fully understood. Here we report an experimental test made with the Van Allen Probes mission from the plasmaspheric plume through the low‐density channel to the plasmaspheric core. Our linear instability analysis and two‐dimensional full‐wave modeling support that the magnetosonic waves propagate from elsewhere to the channel, undergo reflection and transmission at the flanking plasmaspheric density interfaces and eventually exhibit drastic differences in intensity and frequency coverage between neighboring regions. Such a mesoscale (tens of wavelength wide) interface with a strong refractive index gradient allows the transformation of incident waves to surface waves and consequently filters waves in both frequency and orientation. This unexpected filtering pattern could commonly occur at the plasmaspheric boundary and eventually affect the global distribution of magnetosonic waves. Plain Language Summary: Magnetosonic waves play an important role in the coupling between the ionosphere and magnetosphere and the evolution of the magnetospheric radiation environment. Previous global surveys have shown that magnetosonic waves inside and outside the plasmasphere differ significantly in occurrence rate, frequency, and intensity. In fact, magnetosonic waves are not confined near the source region but are able to propagate over a broad range of radial distances and magnetic local times. An obvious question arises as to how the density interface separates magnetosonic waves inside and outside the plasmasphere. Previous modeling and analysis suggested that the mesoscale (tens of wavelengths) density interface allowed the free penetration of magnetosonic waves from outside to inside the plasmasphere. In contrast, our data and modeling here show that such a mesoscale interface with a strong refractive index gradient allows the transformation of incident waves to surface waves and consequently filters waves in both frequency and incident angle. This unexpected filtering pattern could commonly occur at the plasmaspheric boundary and eventually affect the global distribution of magnetosonic waves. Key Points: A tens of wavelength wide plasmaspheric density interface separated the magnetosonic waves with differing intensity and frequency coverageThe local proton Bernstein instabilities are unable to explain the drastic differences between magnetosonic waves in neighboring regionsThe mesoscale plasmaspheric density interfaces can filter the inward penetrating magnetosonic waves in both frequency and incident angle [ABSTRACT FROM AUTHOR]

On 7 January 2014, a solar storm erupted, which eventually compressed the Earth's magnetosphere leading to the generation of chorus waves. These waves enhanced local wave‐particle interactions and led to the precipitation of electrons from 10 s eV to 100 s keV. This paper shows observations of a low energy cutoff in the precipitation spectrum from Van Allen Probe B Helium Oxygen Proton Electron measurements. This low energy cutoff is well replicated by the predicted loss calculated from pitch angle diffusion coefficients from wave and plasma observations on Probe B. To our knowledge, this is the first time a single spacecraft has been used to demonstrate an accurate theoretical prediction for chorus wave‐induced precipitation and its low energy cutoff. The specific properties of the precipitating soft electron spectrum have implications for ionospheric activity, with the lowest energies mainly contributing to thermospheric and ionospheric upwelling, which influences satellite drag and ionospheric outflow. Plain Language Summary: On 7 January 2014, a large storm erupted from the Sun. This storm encountered the Earth and compressed the magnetosphere a few days later. The compression of the magnetosphere led to the creation of chorus waves, a wave‐type known to interact only with electrons with specific energies. In this case, the waves interacted with electrons in the magnetosphere's outer radiation belt. They caused the loss of electrons from 10 s eV to 100 s keV into the ionosphere and upper atmosphere. This paper uses theory to determine which energies we expect will interact with the observed chorus wave. We use the Helium Oxygen Proton Electron instrument from the Van Allen Probes to see if our predictions are correct. We care about these processes because the loss of these electrons can affect ionospheric activity. Key Points: Upper band chorus waves can have a minimum resonant energy in the 10 s eV energy rangeChanges in the minimum resonant energy can change the cut off for what lower energy particles will be lostThe lower energy cut off can be observed in the Van Allen Probes Helium Oxygen Proton Electron data [ABSTRACT FROM AUTHOR]

Field‐aligned distributions of H+ with energy less than 100 eV in the magnetosphere originate from the ionosphere through ionospheric outflow. Recently, a statistical interhemispheric asymmetry in this ion outflow was reported. In this study, we investigate a case study of asymmetric field‐aligned distributions (E < 100 eV) resulting from asymmetric outflow observed on 13 August 2019 by the Hot Plasma Composition Analyzer onboard the Magnetospheric Multiscale mission. During the reported event, the average phase space density in the parallel direction (pitch angle < 45°) is about 4.7 times higher than in the anti‐parallel direction (pitch angle > 135°), indicating greater outflow from the southern hemisphere. In this event, electron precipitation fluxes are also asymmetric between hemispheres, with more electrons traveling to the southern hemisphere than to the northern hemisphere. Global MHD simulation results (SWMF/BAT‐S‐RUS available from the CCMC) confirm that several ionospheric parameters (joule heating, convection velocities, and energy fluxes of precipitating electrons) are more enhanced in the southern hemisphere. We suggest that these interhemispheric asymmetries might cause a stronger outflow from the southern hemisphere than from the northern hemisphere, to produce the observed asymmetric field‐aligned distributions for the low‐energy H+. Key Points: Low‐energy (E < 100 eV) H+ field‐aligned distributions are asymmetric in the outer magnetosphereConditions in the southern hemisphere ionosphere are favorable and result in asymmetric outflowThe observed low‐energy H+ pitch‐angle distributions are consistent with asymmetric outflow as the cause [ABSTRACT FROM AUTHOR]

Magnetosonic waves (MSWs) can be discrete or continuous in a time‐frequency spectrogram. For discrete MSWs that satisfy the Chirikov resonance overlap criterion (CROC), the spectrum can be regarded as continuous and hence the quasi‐linear theory can be employed to calculate the corresponding electron diffusion coefficient; otherwise, the contribution of each harmonic emission to the diffusion coefficient should be calculated separately. From the Van Allen Probe‐A (RBSP‐A) burst mode observations taken between January 2014 to December 2017, a total of 21,766 discrete MSW events were selected for statistical analysis, among which 314 events (1.44%) are found to not satisfy the CROC. The MSW events not satisfying the CROC are mainly distributed in the range between L ∼ 4 and 6 outside the plasmapause, they all have a frequency gap (e.g., a frequency difference ≥2 fcH between two discrete bands). Plain Language Summary: Radiation belt electrons resonate with magnetosonic waves (MSWs) through Landau and bounce resonances. Quasi‐linear diffusion theory is widely used in describing the interactions between charged particles and small amplitude, sufficiently broadband waves. However, the diffusion coefficients incorporated into numerical codes aiming to describe the inner magnetosphere dynamics are typically computed under the assumption that the MSW spectrum is continuous in frequency space. In this letter, based on high‐resolution Van Allen Probe‐A (RBSP‐A) burst mode data, two typical events in which discrete spectrum MSWs do not satisfy the Chirikov resonance overlap criterion (CROC) are reported. To analyze the global distribution and harmonic characteristics of MSWs that do not satisfy the CROC, 314 MSW events that do not satisfy the CROC are screened for statistical study. Key Points: Among 21,766 discrete magnetosonic waves (MSW) events, 314 events (1.44%) do not satisfy the Chirikov resonance overlap criterion (CROC) while 21,452 events (98.56%) satisfy the CROCOf 1,057 discrete MSW events with a frequency gap greater than 7 fcH, 281 events (26.58%) do not satisfy the CROCQuasi‐linear diffusion theory can be applied to all discrete spectrum MSWs with continuous harmonics [ABSTRACT FROM AUTHOR]

We investigated generation processes of field‐aligned currents (FACs) that are abruptly intensified at the beginning of the substorm expansion phase by tracing a packet of the Alfvén wave backward in time from the onset position in the ionosphere in the global magnetohydrodynamics (MHD) simulation. The generation region is found in the near‐Earth plasma sheet, in which (a) azimuthally moving plasma pulls the magnetic field line, and performs negative work against the magnetic tension force to excite the Alfvén waves, (b) FACs are generated from the requirement of Ampère and Faraday laws, and (c) field‐perpendicular current is converted to FACs. We call this near‐Earth FAC dynamo. The plasma involved originates in the tail lobe region. When near‐Earth reconnection occurs in the plasma sheet, the plasma is accelerated earthward by the Lorentz force, and decelerated by the plasma pressure gradient force, followed by the Lorentz force. The flow is deflected to the west and east directions by the plasma pressure gradient force and the Lorentz force, resulting in the excitation of Alfvén waves and FACs. The Alfvén waves propagate along the magnetic field in the rest frame of the moving plasma. When it arrives at the ionosphere, the auroral electrojet starts developing and the substorm expansion phase begins. The near‐Earth FAC dynamo can be distinguished from the near‐Earth dynamo (J · E < 0, where J is the current density and E is the electric field). We suggest that the evolution of the substorm can be understood in terms of the development of FACs. Plain Language Summary: Abrupt intensification of field‐aligned currents (FACs) is one of the manifestations of substorms, which result in bright aurora and auroral electrojet. The FACs are also important to supply energy to the polar ionosphere, in which a large amount of energy (∼1011 W) is consumed during the substorms. The origin of the substorm‐associated FAC is a long‐lasting issue, and not well understood. To identify the processes involved in the generation of the substorm‐associated FACs, we performed global magnetohydrodynamics (MHD) simulation, and traced a packet that is supposed to carry the perturbations associated with the FACs. With this new method, we found that the substorm‐associated FACs are generated in the near‐Earth region near midnight where the plasma pulls the magnetic field lines to excite the Alfvén waves carrying FACs. A series of processes from the near‐Earth reconnection to the sudden intensification of the auroral electrojet (auroral breakup) becomes clearer than ever. Key Points: We identify the origin of substorm‐associated field‐aligned current (FAC) by tracing a packet of Alfvén waves in a global MHD simulationThe FAC is generated in near‐earth plasma sheet where azimuthally moving plasma pulls magnetic field lines (near‐Earth FAC dynamo)The evolution of the substorm is governed by FAC dynamos; flank FAC dynamo for growth phase and near‐Earth FAC dynamo for expansion phase [ABSTRACT FROM AUTHOR]

Pc1 geomagnetic pulsations correspond to electromagnetic ion cyclotron (EMIC) waves in the magnetosphere and are excited there with frequencies of 0.2–5 Hz. The instantaneous longitudinal extent of Pc1 waves on the ground has not been estimated yet. In this study, we analyze the Pc1 pulsations observed at seven longitudinally‐distributed ground stations at subauroral latitudes at ∼60° magnetic latitude for 1 year from July 2018 to June 2019. The hourly occurrence rates of Pc1 pulsations at all 7 stations have a peak (14%–39.6%) in the post‐noon sector and a local minimum (4.1%–8.1%) at midnight. The average frequencies become highest (0.6–1.1 Hz) after midnight and lowest (0.3–0.5 Hz) after noon at all 7 stations. An increasing tendency of total Pc1 occurrence with respect to magnetic latitude was observed. Based on these observations, we obtained a peak of probability distribution of the instantaneous Pc1 longitudinal extent as ∼82.5° with a half maximum at ∼114°, though this probability distribution can be affected by the limitation of the number of the stations. We also made model calculations on the possible longitudinal extent using artificial random Pc1 waves with fixed extents. The comparison of the model results with the observation suggests longitudinal extent of 70°–86° comparable to the peak of probability distribution (∼82.5°). A superposed epoch analysis shows that the longitudinal extent of Pc1 waves tends to increase during recovery phase of geomagnetic storms. Key Points: We study the longitudinal extent of Pc1 pulsations using 1 year of data obtained at seven ground stations at subauroral latitudesThe occurrence rates of Pc1 pulsations at subauroral latitudes are highest in the post‐noon and lowest in the midnight sectorThe peak of probability distribution of the instantaneous Pc1 longitudinal extent is ∼82.5° with a half maximum at ∼114° [ABSTRACT FROM AUTHOR]

We present initial results of low‐energy ion observations from BepiColombo's first Mercury flyby. Unprecedentedly high time resolution measurements of low energy ions at Mercury by BepiColombo Mio reveal rapid (a few seconds) and large (1–2 orders of magnitude) fluctuations of ion flux around the magnetopause and within the magnetosphere. Around the magnetic equator in the pre‐midnight magnetotail, Mio observed plasma sheet ions consistent with previous observations. In the midnight magnetotail near the closest approach, Mio observed the co‐existence of high‐energy (∼keV/q) and low‐energy (<∼300 eV/q) ion components. The low‐energy component is inferred to be cold with a temperature well below 100 eV and have a major contribution to the total density as opposed to previously reported cold tenuous ions. Future observations by Mio will provide insights into the sources, transport, and acceleration of the newly identified ion components. Plain Language Summary: BepiColombo, launched on 20 October 2018, is now en route to Mercury. Along its 7‐year journey to the Mercury orbit insertion, BepiColombo conducted, and will conduct, nine planetary flybys in total. This paper presents brand‐new observations of ions (positively charged particles) in Mercury's magnetosphere by BepiColombo Mio during its first encounter with Mercury. Thanks to high‐time resolution measurements by Mio, it is revealed that ions around Mercury change dramatically and rapidly just over a few seconds. Mio also observed an unexpectedly dense ion population with low energies, for which we do not have a definitive explanation yet. Future flyby and in‐orbit observations by BepiColombo will provide key information for unraveling the dynamics and sources of ions in Mercury's magnetosphere. Key Points: We present initial reports on low energy ion observations during BepiColombo's first Mercury flybyMio observed large fluctuations of ion flux with time scales down to a few seconds around the magnetopause and within the magnetosphereIon energy spectra obtained in the midnight magnetotail suggest the presence of an unexpectedly dense cold component [ABSTRACT FROM AUTHOR]

The arc aurora can be considered as the visualization of the field‐aligned current (FAC). In various cases, the numerically reproduced FAC morphologically matches well with that of the observed arc aurora. Such example can be seen in the sun‐aligned arc, the fan arc, and the theta aurora under the northward interplanetary magnetic field (IMF), and the substorm, the quiet arc, and the initial brightening under the southward IMF. The FAC reproduced by the global simulation is based on the general principle of the magnetohydrodynamic (MHD). In such general principle, the FAC is transmission of net charge, net charge is divE, and E is equivalent to motion. After all, the FAC is transmission of convection motion. In order to transmit the FAC to the dissipative ionosphere based on the MHD, formations of the dynamo and shear that leads from the magnetosphere to the ionosphere are essential. The dynamo and shear are reproduced so as to properly match with each convection structure. The comparison under various magnetospheric topologies between the global simulation and the arc aurora observation indicates that the arc aurora and the FAC occurs inevitably controlled by convection structure. Such a principle of the FAC and convection holds even for the onset FAC. Key Points: Under any magnetospheric topologies, shear structure brings about morphologies of the field‐aligned current (FAC) and arc aurorasThe FAC occurs with the dynamo and shear following the MHD principle, and transmits convection from the magnetosphere to the ionosphereThe substorm CW that forms without considering the dynamo and shear is not reproduced in the global simulation [ABSTRACT FROM AUTHOR]

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]

Ground‐based facilities make an important contribution to our understanding of the magnetospheric system. Coordination with ground‐based facilities has been embedded within the Cluster mission since before launch, and this has given rise to a large number of studies which have exploited data from both Cluster and one or more of the diverse range of ground‐based instrumentation that have been available during the mission timespan. In this paper, we review the advances that have been made to our understanding of the magnetosphere‐ionosphere system, in which insight has been enhanced by combining ground‐based observations with observations from Cluster. Topics covered span from the bow shock to the magnetotail and down to the aurora. Key Points: Space‐ and ground‐based measurements strongly complement each other, and combining the two further enhances the insight they provideStudies exploiting ground‐based observations have been a key aspect of the Cluster mission, yielding approximately 100 papersWe review these studies, covering the full range of solar wind/magnetosphere/ionosphere coupling and dynamics [ABSTRACT FROM AUTHOR]

This paper presents updated methods for locating the Poleward and Equatorward Auroral Luminosity Boundaries (PALB and EALB) directly from IMAGE Far UltraViolet (FUV) images of the Northern Hemisphere auroral oval. Separate boundaries are determined from images measured at different FUV wavelengths. In addition, new methods for indirectly estimating the Open‐Closed magnetic field line Boundary (OCB) and the Equatorward Precipitation Boundary (EPB) locations are presented; these new boundaries are derived from a combination of the auroral luminosity boundary estimates with statistical latitudinal offsets derived from comparisons with low‐altitude spacecraft Particle Precipitation Boundaries (PPBs). Subsequently, we derive new circle model fits for all these boundary data sets, as well as new quality control criteria for these model fits. The suitability of circle fits for each of the data sets is discussed, and the OCB and PALB circle fits are validated against the Convection Reversal Boundary (CRB), as measured by low‐altitude in situ spacecraft. All the new boundary data sets, covering the epoch May 2000 to October 2002, are freely available online. Plain Language Summary: The ability to measure and model near‐Earth space, its response to driving forces in the interplanetary solar wind, and the impact of these forces on the Earth's upper atmosphere is the basis for the understanding and forecasting of space weather. Because there is a strong physical connection between the Earth's magnetosphere (the region of near‐Earth space dominated by the Earth's magnetic field) and ionosphere (the ionized region of the upper atmosphere) near the poles, observations of the ionosphere provide information about the magnetospheric state. Dynamic processes within the magnetosphere accelerate electrons and ions within the space environment giving them the energy to precipitate in the upper atmosphere. These high‐energy charged particles excite atmospheric atoms and molecules, creating aurora. The light from the aurora can be observed at ultraviolet wavelengths by space‐based imagers. In this study we identify the higher and lower latitude edges of the aurora and determine their relationship to important particle boundaries. Circles are then fitted to the spatial boundary variations to make sure they are defined across all longitudes. These data sets can be used to identify and track the spatial extent and dynamics of magnetospheric regions and boundaries. Key Points: Improved poleward and equatorward auroral luminosity boundaries are determined from IMAGE FUV measurementsNew Open‐Closed magnetic field line Boundary (OCB) and Equatorward Precipitation Boundary (EPB) data sets are derived from these observationsNew circle model fits are derived for all the boundary data sets [ABSTRACT FROM AUTHOR]

The auroral acceleration region plays an important role in the magnetosphere‐ionosphere coupling system. In this study, signatures of an auroral U‐shaped potential structure were found for the first time in the near‐equatorial inner magnetosphere by the Arase satellite at ∼6.0 RE geocentric distance and 11° magnetic latitude. The observed magnetic and electric field variations corresponded to the equatorward motion of the upward field‐aligned current and converging perpendicular electric field. Examining the three‐dimensional velocity distribution function of H+ and O+ ions, we demonstrate that upflowing ion beams were significantly deflected in an east‐west direction with a perpendicular velocity up to ∼80 km/s, which is consistent with the ExB drift velocity. A simple particle drift model with the inferred auroral perpendicular potential presents a new kink‐like drift path of ions from the magnetotail, implying that the auroral potential structure has a great impact on particle dynamics in the near‐earth plasma sheet. Plain Language Summary: The auroral arc involves multiple physical processes over a wide altitude range in the upper atmosphere. While the primary energy sources are at high altitudes, the accelerated electrons that are directly responsible for auroral emissions are mainly produced at lower altitudes. In the region where the electrons are accelerated downward, known as the auroral acceleration region, a peculiar electric field structure is formed. We clarified whether this electric field structure exists up to near the energy source region and how it affects the plasma dynamics by an in situ observation by the Arase satellite. We found for the first time that ions accelerated from below the satellite were supplied to the energy source region, and were deflected by the observed auroral electric field structure. Our model calculation shows that the electric field structure can significantly meander the motion of the entire near‐Earth space plasma as it flows toward the Earth. Our result invites a new perspective that the structure of the electric field from low altitudes associated with the auroral arc has a significant impact on the dynamics of near‐Earth space plasmas. Key Points: Signatures of an auroral U‐shaped potential structure were found first in the near‐equatorial inner magnetosphere by the Arase satelliteUpflowing ion beams, especially O+ ion beams, were significantly deflected from field lines by a converging perpendicular electric fieldThe converging electric field and ion beam potentially affect the particle dynamics in the near‐Earth plasma sheet [ABSTRACT FROM AUTHOR]

Although commonly observed in the inner magnetosphere, fast magnetosonic (MS) waves are often owe to be excited by those proton rings resulting from the so‐called energy‐dependent drift path of ring current protons. In this letter, we report an interesting MS wave event in a dipolarizing flux bundle (DFB) observed by the Van Allen Probe A. In the DFB, perpendicular proton fluxes are found depleted at low and intermediate energies, while enhanced at high energies, which should be attributed to proton accelerations by DFB. As a result, proton ring could be locally formed. Using the observed plasma environment parameters and the modeled proton distributions, we have calculated the linear growth rates for MS waves and found that these MS waves might be locally excited. Our results provide insight into the generation of MS waves in a DFB, and provide a new channel to excite MS waves in the inner magnetosphere. Plain Language Summary: Particle acceleration has often been observed in dipolarization events in the magnetotail plasma sheet, which could be thought as a result of the energy conversion from magnetic field into particle kinetic energies in a mesoscale. These particles might drive various micros‐instabilities to release their energies. Recently, dipolarizing flux bundles (DFB) have also been observed in the inner magnetosphere. However, rare observations of waves in a kinetic scale in DFB in the inner magnetosphere (especially inside the geosynchronous orbit) are reported. In this letter, we first (to our knowledge) report an interesting event of magnetosonic (MS) waves observed in a dipolarizing flux bundle in the inner magnetosphere, where local proton rings could be formed due to the particle acceleration to drive instabilities of MS waves. Key Points: Fast magnetosonic (MS) waves with clearly discrete harmonics are observed in dipolarizing flux bundle (DFB) inside the geosynchronous orbitA local proton ring distribution due to the proton acceleration of the DFB is demonstrated to support the growth of fast magnetosonic wavesOur results provide one new channel to excite MS waves in inner magnetosphere and new insight into microscale energy rearrangements in DFB [ABSTRACT FROM AUTHOR]

Using observations of Van Allen Probes, we present a statistical study of plasmaspheric plumes in the inner magnetosphere. Plasmaspheric plumes tend to occur during the recovery phase of geomagnetic storms. Furthermore, the results imply that the occurrence rate of observed plasmaspheric plume in the inner magnetosphere is larger during stronger geomagnetic activity. This statistical result is different from the observations of the Cluster satellite with much higher L shells in most orbital periods, which suggests that the plasmaspheric plume near the magnetopause tends to be observed during moderate geomagnetic activity (Lee et al., 2016). In the following, the dynamic evolutions of plasmaspheric plumes during a moderate geomagnetic storm in February 2013 and a strong geomagnetic storm in May 2013 are simulated through group test particle simulation. It is obvious that the plasmaspheric particles drift out on open convection paths due to sunward convection during both geomagnetic storms. It seems that the outer plasmaspheric particles exhaust the energy available to them sooner, and the plasmasphere shrinks faster during strong geomagnetic storms. As a result, the longitudinal width of the plume is narrower, and the plume is limited to lower L shells during the recovery phase of strong geomagnetic storm. The simulated evolutions may provide a possible interpretation for the occurrence rates: Van Allen Probes tend to observe plumes during stronger geomagnetic storms, and the Cluster satellite with higher L shells tends to observe plumes during moderate geomagnetic storms. [ABSTRACT FROM AUTHOR]

A numerical model is used to study the possibility of a thin current sheet formation in the near-Earth magnetotail in the growth phase of a substorm for a wide range of parameters of longitudinal countermoving ion flows that create current sheet. The simulation results make it possible to conclude that the current sheet can be formed by oxygen ion flows of ionospheric origin in cases where the proton fluxes can be neglected or they are rather weak. Such conditions are realized in the Earth's magnetosphere during periods of increased geomagnetic activity. In addition, the influence of electron pressure anisotropy on the steady-state configuration of the considered current sheet is investigated. [ABSTRACT FROM AUTHOR]

To examine the species dependence of the sustained energy spectrum gaps, in this letter, we analyzed the magnetic local time distribution of sustained proton and oxygen gaps and find that there is a clear difference between proton and oxygen ions: proton sustained gaps are predominantly distributed near the prenoon sector, while oxygen sustained gaps are mostly in the afternoon sector with lower occurrence rate than that for protons. This is primarily due to the different charge exchange loss rates of protons and oxygen ions. This scenario is supported by particle‐tracing simulations, which reproduce most of the observed characteristics. Through model‐data comparison, we found that the spectral gaps at different local times are formed by two different types of mechanisms: (a) the gap is formed due to the extended drift time which is responsible for the gap appearing on the dayside for both proton and oxygen ions; (b) the gap is formed inside the Alfven layer, which accounts for the observed sustained gaps in proton energy spectrums but not in oxygen ions due to the different charge exchange losses at various energies on the preexisting ion populations that lie on closed drift paths. These findings may provide insight into our understanding of ion composition and possible wave generation in the inner magnetosphere. Plain Language Summary: The dynamic evolution of the Earth's magnetosphere is complicated, and the temporal and spatial variations of the ion populations provide unique insights into the underlying physics. In this study, we have investigated a peculiar signature in the energy spectrum of ions, in which their fluxes show a sustained gap within a very narrow energy range throughout a full orbit of Van Allen Probes. Through a statistical survey, we find that proton sustained gaps are predominantly distributed near the prenoon sector, while oxygen sustained gaps are mostly in the afternoon sector with the total occurrence rate much lower than that for protons. By using particle‐tracing simulations that reproduce most of the observed characteristics, we reveal that the difference in the magnetic local time distribution of proton and oxygen sustained gaps is primarily due to the different charge exchange loss rates between proton and oxygen ions of various energies. Key Points: The sustained spectral gaps show clear species dependence with higher occurrence rate for protons than for oxygen ionsProton sustained gaps are predominantly distributed near the prenoon sector while oxygen gaps are in the afternoon sectorThe different magnetic local time (MLT) distributions between proton and oxygen spectral gaps are primarily caused by the different charge exchange loss rates [ABSTRACT FROM AUTHOR]

The Kelvin‐Helmholtz instability at the magnetospheric boundary plays a crucial role in solar wind‐magnetosphere‐ionosphere coupling, particle entry, and energization. The full extent of its impact has remained an open question due, in part, to global models without sufficient resolution to capture waves at higher latitudes. Using global magnetohydrodynamic simulations, we investigate an event when the Magnetospheric Multiscale (MMS) mission observed periodic low‐frequency waves at the dawn‐flank, high‐latitude boundary layer. We show the layer to be unstable, even though the slow solar wind with the draped interplanetary magnetic field is seemingly unfavorable for wave generation. The simulated velocity shear at the boundary is thin (∼0.65RE) and requires commensurately high spatial resolution. These results, together with MMS observations, confirm for the first time in fully three‐dimensional global geometry that KH waves can grow in this region and thus can be an important process for energetic particle acceleration, dynamics, and transport. Plain Language Summary: The boundary separating magnetospheric plasma and the solar wind can become unstable due to the Kelvin‐Helmholtz instability, forming waves that can facilitate mass, momentum, and energy transfer into the magnetosphere. How prevalent Kelvin‐Helmholtz waves are along the magnetopause is therefore a fundamental question to understanding the magnetospheric response to the solar wind. Determining when and where these waves occur on the boundary has remained a challenge. The lower latitudes have been extensively studied while the Cluster mission has provided observations of KH at high‐latitudes. No global models have been capable of resolving the high‐latitude boundary layer, preventing numerical studies of waves within this region. We present a simulation that captures Kelvin‐Helmholtz waves at the high‐latitude boundary for the first time. The instability formed despite the magnetosphere being immersed in pristine slow wind, which reduced the velocity drop across the shear layer at the equator. The simulated period took advantage of an opportunity when the Magnetospheric Multiscale mission was located at the high‐latitude boundary and observed boundary oscillations. Together with the observations, we confirm that Kelvin‐Helmholtz waves can grow at high‐latitudes and thus be able to contribute to particle entry and energization in this region. Key Points: Global magnetohydrodynamic simulation of the magnetosphere captures unstable Kelvin‐Helmholtz waves at the high‐latitude boundary layer for the first timeThe growth of the surface waves occurs despite the stabilizing slow solar wind and draped magnetic field near the high‐latitude cuspThe fastest growing wave mode resolved by the simulation is consistent with Magnetospheric Multiscale mission observations of the same event [ABSTRACT FROM AUTHOR]

Cluster was the first mission in the terrestrial magnetosphere to involve four spacecraft in a tetrahedral configuration, providing three‐dimensional measurements of the space plasma parameters. Cluster was also equipped with a very comprehensive instrumentation, allowing the measurement of the ion populations outflowing from the ionosphere, their circulation in the magnetosphere, and their eventual escape to outer space. The observations of the outflowing and escaping ion populations performed by Cluster are reviewed and the most prominent results highlighted. These show the dominance in the magnetotail lobes of cold plasma outflows originating from the polar caps. For the energetic heavy ion outflow, the cusps constitute the main source. Their transport and acceleration through the polar cap into the lobes and then into the plasma sheet has been characterized. The dependence of the polar outflow on the solar wind parameters and on the geomagnetic activity has been evaluated for both cold ion populations and heavy energetic ions. For the latter, outflow has been observed during all periods but an increase by two orders of magnitude has been shown during extreme space weather conditions. This outflow is adequate to change the composition of the atmosphere over geological timescales. At lower latitudes, the existence of a plasmaspheric wind, providing a continuous leak from the plasmasphere, has been demonstrated. The general scheme of the outflowing ion circulation in the magnetosphere or escape, and its dependence on the IMF conditions, has been outlined. However, several questions remain open, waiting for a future space mission to address them. Plain Language Summary: The Earth's upper atmosphere is slowly escaping to space, either in the form of light electrically neutral atoms (mainly hydrogen) or in the form of ions (all species). The Cluster mission, launched in year 2000 and consisting of four spacecraft orbiting the Earth, has greatly advanced our understanding of how the ions are outflowing from the upper atmosphere, can be accelerated by the electric and magnetic fields present there, and can then eventually escape to outer space. The results show how this escape is highly sensitive to the level of solar activity and geomagnetic activity. Although escape occurs under all activity conditions, during space storms it dramatically increases, and during extreme space weather events this increase in the escape rate can be by almost two orders of magnitude. This escape in the long‐term (few billion years) is adequate to change the composition of the Earth's atmosphere. Key Points: Cluster has advanced our understanding of how the atmosphere slowly escapes to space and has revealed previously hidden escaping populationsThe outflowing ion populations cover a very wide energy domain, from cold plasma to energetic ions, and contain both light and heavy ionsIon outflow occurs during all activity levels, but it can increase by two orders of magnitude during extreme space weather events [ABSTRACT FROM AUTHOR]

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

In this work, we review the results of observations by the Cluster/Research with Adaptive Particle Imaging Detectors (RAPID) energetic charged particle detector (∼>40 keV–∼1 MeV). The origin of energetic ions in the region upstream of the Earth's bow shock is compared under different solar wind conditions. We give a brief overview of the acceleration mechanisms of charged particles in the plasma sheet. Effective acceleration is associated with (multiple) interaction(s) of charged particles with multiscale magnetic structures and/or electromagnetic fluctuations, which are the consequences of reconnection or other plasma instabilities. The necessity of such acceleration mechanisms to reproduce the distributions of energetic particles on closed magnetic field lines is discussed. We review empirical models describing the distributions of charged particles. Several aspects of the dynamics of energetic particles during substorms and their influence on the dynamics of magnetic storms are shown. We advise to consider including the energetic particles at >40 keV in calculations of the plasma temperature and pressure during these dynamic processes. Plain Language Summary: A fundamental scientific question is how plasma in the universe is heated up and accelerated. Understanding acceleration at supernovae shocks, of cosmic jets or laboratory plasmas still has many open questions. Near‐Earth space environment is an excellent laboratory to investigate plasma dynamics and to reveal the fundamental laws it obeys. Energetic plasmas are also hazardous for space satellites and play a key role in space weather. It is, therefore, necessary to study the energization of space plasmas, their distribution and consequences on the magnetospheric dynamics. In situ observations in the near‐Earth space by the Cluster satellites and the energetic particle detector, Research with Adaptive Particle Imaging Detectors (RAPID), reveal new insights in plasma acceleration, unexpected features in its distributions, and effects on substorm and geomagnetic storm dynamics. These observations also help space weather applications to determine the level of energetic particle intensities. Key Points: Energetic particles at energies >40 keV have to be considered for the plasma temperature and pressure calculationsEffective acceleration is due to interaction of charged particles with multiscale magnetic structures and/or electromagnetic fluctuationsDirection of Interplanetary Magnetic Field leads to ion distribution asymmetries between Northern and Southern hemispheres [ABSTRACT FROM AUTHOR]

The investigation of the magnetospheric polar cusps was one of the main objectives of the Cluster mission. The four satellites have crossed those regions numerous times over the years and, with their suitable instrumentation, favorable orbits, and unique multipoint measurements, many aspects of the cusp have been unveiled. The first of those is its highly dynamic nature. With four satellites, whatever their altitude and thus their configuration, the spatial/temporal ambiguity has been lifted and a completely new insight into the motion of the cusp and evolution of the transient small‐scale structures within it was given. Second, the high altitude or exterior cusp, its properties and dynamics, magnetic configuration, the energetic particles observed within, and its boundary with the magnetosheath were characterized in a number of case studies and on a statistical basis. Third, the Cluster cusp observations brought a fresh eye on the reconnection process at the magnetopause, including its location and spatio‐temporal variability. Finally, the complete payload with particle, field and wave detectors have made it possible to treat cusp as a plasma physics laboratory to progress our understanding of wave‐particle interaction and turbulence within the cusp. We have no choice but to note that Cluster has been extremely successful for all these matters. Key Points: We review 20 years of cusp research with one of ESA's milestones: the Cluster missionA multipoint capability, appropriate orbits, and a comprehensive instrumentation made Cluster the ideal space mission to study the cuspCluster enabled to address key questions including cusp dynamics, ion dispersions and their consequences on merging, and waves and turbulence [ABSTRACT FROM AUTHOR]

The properties of cold, dense, low energy (<150 eV) ions within Earth's magnetosphere between 6 and 14 RE distance are examined using data sampled by Time History of Events and Macroscale Interactions during Substorms spacecraft during a new low‐energy plasma mode that operated from June 2016 to July 2017. These ions are a persistent feature of the magnetosphere during enhanced solar wind dynamic pressure and/or magnetospheric activity. These ions have densities ranging from 0.5 to tens of cm−3, with a mean of ∼1 cm−3 and temperatures of a few to tens of eV, with a mean of ∼13 eV. These yield cold to hot ion density and temperature ratios that are 4.4 and 4×10−3, respectively. Comparisons reveal that the cold ion densities are positively correlated with solar wind dynamic pressure. These ions are organizable, according to their pitch‐angle distribution, as being transverse/convection dominated (interpreted as plume plasma) or magnetic field‐aligned (FAL) (uni‐ or bi‐directional characteristic of ion outflow or cloak plasma). Transverse ions preferentially occur in the prenoon to dusk sectors during sustained active magnetospheric conditions driven by enhanced solar wind dynamic pressure under southward Bz and westward By IMF orientations. Transverse ion velocities (reaching several tens of km/s) have a westward directed tendency with a slight radially outward preference. In contrast FAL ions preferentially occur from morning to noon during northward IMF orientations, enhanced solar wind dynamic pressure, and quiet magnetospheric conditions within several hours after moderate to strong activity. The FAL ions also have bulk velocities ≲30 km/s, with an eastward and radially outward tendency. Key Points: Dense low energy ions are a persistent feature of the magnetosphere during enhanced solar wind dynamic pressure and/or KpLow energy transverse ions tend to occur in duskside magnetosphere under southward Bz and westward By IMF conditions, with westward flowLow energy magnetic field‐aligned (uni‐ or bi‐directional) ions tend to occur in the dawnside under northward IMF, with an eastward flow [ABSTRACT FROM AUTHOR]

We use 214,410 hourly observations of transpolar voltage, ΦPC, from 25 years' observations by the Super Dual Auroral Radar Network radars to confirm the central tenet of the expanding‐contracting polar cap model of ionospheric convection that ΦPC responds to both dayside and nightside reconnection voltages (ΦD and ΦN). We show that ΦPC not only increases at a fixed level of the nightside auroral electrojet AL index with increasingly southward interplanetary magnetic field (IMF) (identifying the well‐known effect of ΦD on ΦPC) but also with increasingly negative AL at a fixed southward IMF (identifying a distinct effect of ΦN on ΦPC). We also study the variation of ΦPC with time elapsed since the IMF last pointed southward, Δt, and show that low/large values occur when (−AL) is small/large. Lower numbers of radar echoes, ne, mean that the "map‐potential" reanalysis technique used to derive ΦPC is influenced by the model used: we present a sensitivity study of the effect of the threshold of ne required to avoid this. We show that for any threshold ne, ΦPC falls to about 15 kV for Δt greater than about 15 h, indicating any viscous‐like voltage ΦV is considerably smaller than this. It is shown that both ΦPC and (−AL) increase with increased solar wind dynamic pressure pSW, but not as much as the midlatitude geomagnetic index am. We conclude pSW increases both ΦD and ΦN through increasing the magnetic shear across the relevant current sheet but has a larger effect on midlatitude geomagnetic indices because of the effect of additional energy stored in the tail lobes. Plain Language Summary: Large‐scale circulation of the ionized upper atmosphere over the poles is driven by the solar wind flow by the process of magnetic reconnection that interconnects the magnetic field of Earth with that embedded in the solar wind flow. In long‐term averages, the rate at which this "open" magnetic flux is transferred away from the Sun and in the polar ionospheres is the same, but on shorter timescales the field can stretch between the two locations and this decouples the transfer rates. The expanding‐contracting polar cap model has been very useful in predicting the faster changes that can occur in the ionosphere, changes that can have an influence on a wide range of modern operational systems from satellites to power grids. Two key concepts that the model is based on were obtained from only very small data sets and here we use 214,410 hourly observations from 25 years of observations by the Super Dual Auroral Radar Network to show that the concepts do apply all the time. Key Points: Transpolar voltage depends on both dayside and nightside reconnection voltages and both are enhanced by larger solar wind dynamic pressureLarge voltages seen when the interplanetary magnetic field (IMF) points northward decay away with elapsed time since the IMF last pointed southwardOngoing nightside reconnection mimics the effect of proposed viscous‐like interactions which are shown to cause considerably less than 15 kV of voltage [ABSTRACT FROM AUTHOR]

Ionospheric ions (mainly H+, He+, and O+) escape from the ionosphere and populate the Earth's magnetosphere. Their thermal energies are usually low when they first escape the ionosphere, typically a few electron volt to tens of electron volt, but they are energized in their journey through the magnetosphere. The ionospheric population is variable, and it makes significant contributions to the magnetospheric mass density in key regions where magnetic reconnection is at work. Solar wind--magnetosphere coupling occurs primarily via magnetic reconnection, a key plasma process that enables transfer of mass and energy into the near-Earth space environment. Reconnection leads to the triggering of magnetospheric storms, auroras, energetic particle precipitation and a host of other magnetospheric phenomena. Several works in the last decades have attempted to statistically quantify the amount of ionospheric plasma supplied to the magnetosphere, including the two key regions where magnetic reconnection occurs: the dayside magnetopause and the magnetotail. Recent in situ observations by the Magnetospheric Multiscale spacecraft and associated modeling have advanced our current understanding of how ionospheric ions alter the magnetic reconnection process, including its onset and efficiency. This article compiles the current understanding of the ionospheric plasma supply to the magnetosphere. It reviews both the quantification of these sources and their effects on the process of magnetic reconnection. It also provides a global description of how the ionospheric ion contribution modifies the way the solar wind couples to the Earth's magnetosphere and how these ions modify the global dynamics of the near-Earth space environment. [ABSTRACT FROM AUTHOR]

Equatorial magnetosonic waves (EMS), together with chorus and plasmaspheric hiss, play key roles in the dynamics of energetic electron fluxes in the magnetosphere. Numerical models, developed following a first principles approach, that are used to study the evolution of high energy electron fluxes are mainly based on quasilinear diffusion. The application of such numerical codes requires statistical models for the distribution of key magnetospheric wave modes to estimate the appropriate diffusion coefficients. These waves are generally statistically modeled as a function of spatial location and geomagnetic indices (e.g., AE, Kp, or Dst). This study presents a novel dynamic spatiotemporal model for EMS wave amplitude, developed using the Nonlinear AutoRegressive Moving Average eXogenous machine learning approach. The EMS wave amplitude, measured by the Van Allen Probes, are modeled using the time lags of the solar wind and geomagnetic indices as inputs as well as the location at which the measurement is made. The resulting model performance is assessed on a separate Van Allen Probes data set, where the prediction efficiency was found to be 34.0% and the correlation coefficient was 56.9%. With more training and validation data the performance metrics could potentially be improved, however, it is also possible that the EMS wave distribution is affected by stochastic factors and the performance metrics obtained for this model are close to the potential maximum. [ABSTRACT FROM AUTHOR]

In addition to wave particle scattering in the magnetosphere, atmospheric backscatter of magnetospheric electrons is an important process that contributes to the formation of the precipitated electrons in the region of diffuse aurora. Two magnetically conjugate regions are involved in a complex magnetosphere‐ionosphere (MI) particle and energy interplay. Based on synthesizing previous theoretical/modeling studies and experimental evidence, we demonstrate the need for improving the quantification of magnetospheric electrons backscatter processes that can affect inner magnetospheric electrodynamics, transport and loss in a way that is not easily predicted. We discuss how these complex and energy‐dependent MI coupled processes can be treated in magnetospheric modeling. Key Points: Atmospheric collisional energy degradation significantly affects precipitating electron distributionsBackscatter influences ionospheric conductance and inner magnetospheric electrodynamicsA framework for treating atmospheric backscatter in magnetospheric models is presented [ABSTRACT FROM AUTHOR]

The Feature Tracking of Aurora (FTA) model was constructed using 1.5 years of Polar Ultraviolet Imager data and is based on tracking a cumulative energy grid in 96 magnetic local time (MLT) sectors. The equatorward boundary, poleward boundary, and 19 cumulative energy bins are tracked with the energy flux and the latitudinal position. With AE increasing, the equatorward boundary moves to lower latitudes everywhere, while the poleward boundary moves poleward in the 2300-0300 MLT region and equatorward in other MLT sectors. This results in the aurora getting wider on the nightside and becoming narrower on the dayside. The peak intensity of the aurora in each MLT sector is almost linearly related to AE, with the global peak moving from pre-midnight to post-midnight as geomagnetic activity increases. Ratios between the Lyman-Birge-Hopfield-long and -short models allow the average energy to be calculated. Predictions from the FTA and two other auroral models were compared to the measurements by the Defense Meteorological Satellite Program Special Sensor Ultraviolet Spectrographic Imagers (SSUSI) on March 17, 2013. Among the three models, the FTA model specified the most confined patterns with the highest energy flux, agreeing with the spatial and temporal evolution of SSUSI measurements better and predicted auroral power (AP) better during higher activity levels (SSUSI AP > 20 GW). The Fuller-Rowell and Evans (1987) and FTA models specified very similar average energy compared with SSUSI measurements, doing slightly better by ~1 keV than the OVATION Prime model. [ABSTRACT FROM AUTHOR]

Magnetosonic (MS) waves are electromagnetic waves that play important roles in the acceleration and scattering of radiation belt electrons. However, previous statistical analyses of the global MS wave distribution were mainly restricted to magnetic field measurements. In this study, we first report a low‐harmonic MS wave event observed only by the electric field instrument in Van Allen Probes. The MS wave frequencies follow the local proton gyrofrequency (fcp), which suggests the characteristics of nearly local generation. We further statistically investigate similar wave events using Van Allen Probes data. The identified MS wave power exhibits peaks between 4fcp and 10fcp, regardless of the L‐shell, but it shows a dependence on magnetic local time. This work is supplemental to previous MS wave frequency spectra and provides new insights to better understand the source region of MS waves in the Earth's magnetosphere. Plain Language Summary: MS waves have been extensively studied, but previous statistical analyses of the global MS wave distribution using Van Allen Probes mostly utilized only magnetic field data, and unintended bias might have been introduced due to the noise floor of the magnetometer. Here, we report an example case study of low‐harmonic MS waves detected only by electric field measurements. Furthermore, we statistically investigate the missed emissions with frequencies at multiples of the proton gyrofrequency (fcp); these observations occurred outside the plasmasphere in the dayside region. High‐sampling‐rate burst mode data are also used to analyze the MS wave frequency spectra and wave power peaks between 4fcp and 10fcp. These results, which complement previous statistics, help us better understand the MS wave frequency spectra and evaluate the wave effects on particle dynamics. Key Points: Van Allen Probes EFW instrument observed low‐harmonic magnetosonic waves with electric field power which could not be captured by MAGThe low‐harmonic magnetosonic waves are important for the electron acceleration at higher energies than the high‐harmonic wavesOur statistics indicates that the wave magnetic field power spectral density peaks within 4–10fcp and is MLT dependent [ABSTRACT FROM AUTHOR]

The magnetic reconnection process at the Earth's magnetopause is observed at various times and places as either anti-parallel or component reconnection. The two scenarios are combined in a model known as the Maximum Magnetic Shear model, which predicts the location of long and continuous reconnection X-lines across the dayside magnetopause along the ridge of maximum magnetic shear. This study investigates a Magnetospheric Multiscale Mission (MMS) magnetopause encounter where the spacecraft had repeated contact with the boundary layers for about 40 min. While in contact with the magnetopause boundary layers, the Interplanetary Magnetic Field (IMF) slowly and continuously rotated, magnetically connecting the MMS spacecraft to different sections along an extended dayside reconnection X-line. These magnetic connection points slide along component reconnection tilted X-lines and also connect several times to the anti-parallel section of the magnetopause X-line. Despite the continuously changing IMF conditions, each magnetopause boundary layer encounter shows accelerated ion beams, indicating an active reconnection X-line at the magnetopause that covers a length of about 20 RE. [ABSTRACT FROM AUTHOR]

Inner magnetospheric magnetosonic waves are usually considered to grow from the near‐equatorial ion Bernstein instability below the lower hybrid resonance frequency and contribute significantly to the radiation belt electron dynamics. We here present unusual observations of magnetosonic waves extending above the local lower hybrid resonance frequency in the midlatitude ∼15° plasmasphere after the substorm proton injection. Linear instability analyses and ray‐tracing simulations show that, because of the latitudinal variation of normal angles, these waves have little growth near the equator but are amplified accumulatively to the observable level over broad latitudes |λ| < 30° during the bounce‐drift propagation. Our data and modeling illustrate a previously unexplored scenario that off‐equatorial proton ring distributions could be a significant source of magnetosonic waves in the inner magnetosphere. Such off‐equatorially generated magnetosonic waves might differ from the near‐equatorially generated ones in latitudinal coverage, wavevector distribution, and then effect on the radiation belt electrons. Plain Language Summary: Magnetosonic waves refer to the nearly linearly polarized, compressional, electromagnetic emissions in the inner magnetosphere. They have recently received an increased interest for their contributions to the Van Allen radiation belt electron dynamics. A fundamental question has been where and how the magnetosonic waves are generated. In contrast to the traditional view that magnetosonic waves grow from ion Bernstein instability near the magnetic equator, we here propose the possibility of off‐equatorial sources for magnetosonic waves on the basis of the analysis of the unusual magnetosonic waves extending above the local lower hybrid resonance frequency in the midlatitude plasmasphere. Our modeling shows that these waves have little growth near the equator but are amplified accumulatively to the observable level by substorm‐injected protons over latitudes |λ| < 30° during the bounce‐drift propagation. Because of the differences in latitudinal coverage and wavevector distribution, the off‐equatorially and near‐equatorially generated magnetosonic waves are expected to have different effects on the radiation belt electrons. Key Points: Magnetosonic waves can extend above the lower hybrid resonance frequency in the off‐equatorial high‐density plasmasphereThe near‐equatorial instabilities of hot protons are difficult to explain the observed frequency extension of magnetosonic wavesThe unusual magnetosonic waves are more likely to grow accumulatively over latitudes |λ| < 30° during the bounce‐drift propagation [ABSTRACT FROM AUTHOR]

In this study, we investigate ionospheric responses, including currents and aurorae, to solar wind dynamic pressure (SW Pdyn) sudden increases, which are critical for understanding solar windmagnetosphere-ionosphere coupling. We focus on two similar SW Pdyn pulse events that occurred on January 24, 2012 and November 12, 2010. In both cases, equivalent ionospheric currents (EIC) vortices were generated within about 10 min after the pressure pulse arrival, with a counter-clockwise rotating vortex (viewed from above) observed on the dusk side in the former case, and a clockwise vortex observed on the dawn side in the latter. Simultaneous ground-based All-Sky Imager (ASI) observations in the vicinity of the observed EIC vortex in each case showed that aurorae intensified on the dusk side and diminished on the dawn side. These observations provide direct evidence of the scenario proposed by Shi et al. (2014) that magnetospheric flow vortices generated by a solar wind pressure pulse carry fieldaligned currents into the ionosphere and thereby modulate auroral activity. The dawn/dusk asymmetry in the auroral intensification is a direct result of the opposite sense of vortex rotation on the dawn and dusk sides, which generate oppositely directed field-aligned currents into/out of the ionosphere [ABSTRACT FROM AUTHOR]

Charged particles are observed to be injected into the inner magnetosphere from the plasma sheet and energized up to high energies over short distance and time, during both geomagnetic storms and substorms. Numerous studies suggest that it is the short-duration and high-speed plasma flows, which are closely associated with the global effects of magnetic reconnection and inductive effects, rather than the slow and steady convection that control the earthward transport of plasma and magnetic flux from the magnetotail, especially during geomagnetic activities. In order to include the effect of the inductive electric field produced by the temporal change of magnetic field on the dynamics of ring current, we implemented both theoretical and numerical modifications to an inner magnetosphere kinetic model—Hot Electron-Ion Drift Integrator. New drift terms associated with the inductive electric field are incorporated into the calculation of bounce-averaged coefficients for the distribution function, and their numerical implementations and the associated effects on total drift and energization rate are discussed. Numerical simulations show that the local particle drifts are significantly altered by the presence of inductive electric fields, in addition to the changing magnetic gradient-curvature drift due to the distortion of magnetic field, and at certain locations, the inductive drift dominates both the potential and the magnetic gradient-curvature drift. The presence of a self-consistent inductive electric field alters the overall particle trajectories, energization, and pitch angle, resulting in significant changes in the topology and the strength of the ring current. [ABSTRACT FROM AUTHOR]

We present the discovery of seven new flux transfer events (FTEs) at Saturn's dayside magnetopause by the Cassini spacecraft and analyze the observations of all eight known FTEs. We investigate how FTEs may differ at Saturn where the magnetopause conditions are likely to diamagnetically suppress magnetic reconnection from occurring. The measured ion-scale FTEs have diameters close to or above the ion inertial length di~1-27 (median and mean values of 5 and 8), considerably lower than typical FTEs found at Earth. The FTEs magnetic flux contents are 4--461 kWb (median and mean values of 16 and 77 kWb), considerably smaller (<0.1%) than average flux opened during magnetopause compression events at Saturn. This is in contrast to Earth and Mercury where FTEs contribute significantly to magnetospheric flux transfer. FTEs therefore represent a negligible proportion of the amount of open magnetic flux transferred at Saturn. Due to the likely suppression of the two main growth-mechanisms for FTEs (continuous multiple x-line reconnection and FTE coalescence), we conclude that adiabatic expansion is the likely (if any) candidate to grow the size of FTEs at Saturn. Electron energization is observed inside the FTEs, due to either Fermi acceleration or parallel electric fields. Due to diamagnetic suppression of reconnection at Saturn's magnetopause, we suggest that the typical size of FTEs at Saturn is most likely very small, and that there may be more di-1 FTEs present in the Cassini magnetometer data that have not been identified due to their brief and unremarkable magnetic signatures. [ABSTRACT FROM AUTHOR]

Energy and momentum can be transferred from the magnetosphere to the ionosphere through field-aligned currents (FACs), which show seasonal variations due to seasonal variation of the ionospheric conductance. Using Magnetospheric Multiscale Mission observations, we investigate the characteristics of kinetic-scale (close to local ion inertial length) FACs in the plasma sheet boundary layer (PSBL), such as occurrence rate, magnitude, and carriers. The results show that: (1) the occurrence rates of FACs exhibit a trident distribution; they are also larger in the northern hemisphere than in the southern hemisphere, especially for the earthward FACs. (2) Distribution patterns vs. plasma β are different for the beam-type and the flow-type FACs. Flow-type FACs are observed closer to the central plasma sheet (higher β), and their occurrence rate decreases linearly toward the tail lobe (lower β), while the beamtype FACs are mainly and equally distributed within 0.1 < β < 1.0. (3) The magnitude of FACs shows little dependence on β, and generally increases closer to the Earth. (4) Occurrence rate and magnitude of FACs both increase during active periods, consistent with ionospheric observations. (5) The main carriers for FACs in the PSBL are thermal electrons, while cold electrons also contribute occasionally, especially during active periods. Hemispheric asymmetry of occurrence rate and dependence on geomagnetic activity of the kinetic-scale FACs in the PSBL are consistent with FACs at ionospheric altitudes. This fact demonstrates that kinetic-scale FACs are significant in magnetosphere-ionosphere coupling and illustrates the possible ionospheric feedback effects to the magnetosphere in the nightside. [ABSTRACT FROM AUTHOR]

We have taken a key step in evaluating the importance of ionospheric outflows relative to electrodynamic coupling in the thermosphere’s impact on geospace dynamics. We isolated the thermosphere’s material influence and suppressed electrodynamic feedback in whole geospace simulations by imposing a time‐constant ionospheric conductance in the ionospheric Ohm’s law in a coupled model that combines the multifluid Lyon‐Fedder‐Mobarry magnetosphere model with the Thermosphere Ionosphere Electrodynamic General Circulation Model and the Ionosphere Polar Wind Model that includes both polar wind and transversely accelerated ion species. Numerical experiments were conducted for different thermospheric states parameterized by F10.7 for interplanetary driving representative of the stream interaction region that swept past Earth on March 27, 2003. We demonstrate that thermosphere through its regulation of ionospheric outflows influences magnetosphere‐ionosphere (MI) convection and the ion composition, symmetries, x‐line perimeter and magnetic merging of the magnetosphere. Feedback to the ionosphere‐thermosphere from evolving MI convection, and Alfvénic Poynting fluxes and soft (∼few 100 eV) electron precipitation originating in the magnetosphere, in turn, modify the evolving O+ outflow properties. The simulation results identify a variety of observed magnetospheric features that are attributable directly to the thermosphere’s material influence: Asymmetries in O+ outflow fluxes and velocities in the pre/postnoon low‐altitude magnetosphere, dawn/duskside lobes and pre/postmidnight plasmasheet; O+ distribution of the plasmasheet; magnetic x‐line location and reconnection rate along it. O+ outflows during solar maximum conditions (high F10.7) tend to counteract the plasmasheet’s pre/postmidnight asymmetries caused by the night‐to‐day gradient in ionospheric Hall conductance.Key Points: A global geospace model including ionospheric outflows was used to diagnose material impacts of the thermosphere on geospaceThermospheric states influence the distributions, fluxes and other properties of O+ outflows and their feedback on geospace dynamicsVariations in thermospheric EUV irradiance manifest in the ion composition, shape, symmetries, and magnetic merging of the magnetosphere [ABSTRACT FROM AUTHOR]

Direct comparisons between RBSP (Van Allen Probes or Radiation Belt Storm Probes) and TWINS (Two Wide‐angle Imaging Neutral‐atom Spectrometers) for the main phase of two storms, March 17 and October 7, 2015, showed agreement between the in situ ion measurements and the ion spectra from the deconvolved energetic neutral‐atom (ENA) measurements, except when O+ ions were significant. Spatial evolution of individual energy peaks in the ion spectra is studied using TWINS data. O+ ions are seen to result in intense peaks at 5–10 keV/amu in the TWINS ion spectra. These ion populations are confined to low L shells (L < 5) and localized in the premidnight sector. When H+ ions are significant, the low energy peaks (<25 keV/amu) are found to be less intense than the high energy peaks (>25 keV/amu), located at L > 4 and localized within the premidnight sector. During times of rapidly varying AE indices, two spatially distinct peaks, between 3–5RE and 6–8RE, are observed for the ions with energies >25 keV/amu. The outer peak appears for a few hours and fades while the inner peak is more stable. These structures are found to be consistent with particle injections observed in the RBSP data. When double peaked structures are swept off, low energy ions accumulate in the premidnight to midnight sectors whereas high energy ions are located premidnight to postmidnight sectors. Faster drift orbits of >25 keV/amu ions may cause this kind of distribution.Key Points: TWINS ion spectra agree with RBSP in situ H+ ion spectra during main phase of geomagnetic storms on March 17 and October 7, 2015O+ ions contribute to peaks at energies 5–10 keV/amu in TWINS ion spectra which spatially extend to L < 5 and in the premidnight MLT sectorsSpatial dynamics of TWINS ion spectra reveal signatures of varying convection rates, particle injections, and energy‐dependent drift [ABSTRACT FROM AUTHOR]

Fast magnetosonic (MS) waves are excited by the ring distribution of energetic protons preferably when the ring velocity (VR) is within a factor of 2 above or below the local Alfvén speed (VA). Here we examine the global distributions of MS waves and proton rings with 0.5VA = VR = 2VA based on 64 months (from October 25, 2012 to February 28, 2018) of Van Allen Probes observations. The statistical results show that MS waves are present over a broad region of L = 1.2-6.0 and 00-24 magnetic local time (MLT), with a higher occurrence rate at L = 2.5-5.5 on the dayside. Proton rings occur mainly on the dayside of L > 5.0. During active geomagnetic periods, both MS waves and proton rings occur more frequently and extend to low L-shells. The current results provide the further observational evidence that MS waves can be excited by proton rings at a distant region and propagate to low L-shells. [ABSTRACT FROM AUTHOR]

In this study, the excitation of narrowband fast magnetosonic (MS) waves near the lower hybrid resonance frequency (fLHR) has been investigated with observations from Van Allen Probes mission and linear growth theory. A typical wave event is first examined to show that these waves can be excited through linear instabilities driven by partial shell distributions of protons. Then it is found that these narrowband MS waves from 188 wave events observed by the Van Allen Probe A between January 1, 2013 to December 31, 2015 have central frequencies around 0.7fLHR with a bandwidth of 0.2fLHR. In addition, these waves are observed mainly in the dayside and dusk sectors outside the plasmapause, which is different from previously reported results. Moreover, the simultaneously observed energetic protons during wave activities show that the ratios of the ring speed Vr to the Alfvén speed VA mainly fall into the range of 0.8 < Vr/VA < 1, and this preferred condition for excitations of narrowband MS waves near fLHR is also verified by a parameter analysis of calculating linear wave growth rates combined with wave observations. Plain Language Summary Emissions around the lower hybrid resonance frequency (fLHR) in the outer magnetosphere have been speculated that they may play an important role in the process of plasma diffusion and coupling well to both electrons and ions. Theoretical calculations have predicted that fast magnetosonic (MS) waves near fLHR in the inner magnetosphere have similar wave characteristics with the emissions around fLHR in the outer magnetosphere. However, up to now, there are few reports about the excitation mechanism of narrowband MS wave near fLHR in the inner magnetosphere, especially the favorable generation condition has not been revealed. In this paper, a typical kind of narrowband MS wave near fLHR has been observed to be excited through linear instability by Van Allen Probe. Statistical observations for narrowband MS waves by Van Allen Probe-A spacecraft have shown that central frequencies are located around 0.7fLHR with a bandwidth of 0.2fLHR. The simultaneous observations of energetic ions have shown that the ratios of the ring speed Vr to the Alfvén speed VA mainly fall into the range of 0.8 < Vr/VA < 1. Finally, we have also theoretically confirmed that the observed range of Vr/VA is the favorable excitation condition of narrowband MS waves near fLHR. [ABSTRACT FROM AUTHOR]

The generation of Pc4‐5 ultralow frequency (ULF) waves after interplanetary shock‐induced electric field impulses in the Earth's magnetosphere is studied using Van Allen Probes measurements by investigating the relationship between the first impulses and subsequent resonant ULF waves. In the dayside, the relevant time scales of the first impulse is correlated better with local Alfvén speed than with local eigenfrequency, implying that the temporal scale of the first impulse is more likely related to fast‐mode wave propagation rather than local field line resonance. There are only 20 out of 51 events with narrow‐band poloidal ULF waves induced after the first impulse, showing a higher chance for ULF wave generation at the locations where the impulse equivalent frequency scale matches the local eigenfrequency. It is suggested that the shock‐related ULF wave can be excited in the magnetosphere on condition that shock‐induced impulse has large enough amplitude with its frequency matching the local eigenfrequency. Key Points: The 51 shock‐related impulse events are investigated based on Van Allen Probes measurement with 39% of them followed by poloidal ULF wave while 61% notThe time scale of shock‐induced electric field impulse is more related to fast‐mode wave propagation than field line resonanceThe shock‐related ULF wave is more likely excited when the shock‐induced impulse's equivalent frequency matches the local eigenfrequency [ABSTRACT FROM AUTHOR]

Magnetic reconnection is often suggested to be implicated in a variety of dynamic phenomena observed in the complex magnetosphere of Mars. We present a first global survey of ion jets within current sheets, which are thought of as a primary product of magnetic reconnection, with Mars Atmosphere and Volatile EvolutioN (MAVEN) data obtained on the dayside and nightside in the Martian magnetosphere. We develop a fully automated algorithm to efficiently and reliably identify ion flow velocity deviations during current sheet crossings recorded by MAVEN. The obtained statistical results show that (i) sunward and anti‐sunward ion jets embedded within current sheets are commonly and widely observed in the dayside and nightside magnetosphere of Mars, (ii) measured magnetic field profiles and inferred magnetic topology during the jet crossings are generally consistent with those expected for reconnecting current sheets, (iii) the jet occurrence is apparently independent of upstream driver conditions, and (iv) most of the identified current sheets are thin with a roughly ion scale half‐thickness and are embedded in low beta plasma, suggesting that they are seemingly capable of reconnection based on the known onset conditions. The widespread distributions and common detection of ion jets, as well as the ubiquitous presence of thin, low‐beta current sheets, imply that magnetic reconnection could operate frequently in many locations around Mars, thereby playing an important role in the dynamics of the Martian magnetosphere for most, if not all, of the solar wind conditions in the present epoch. Key Points: Both sunward and anti‐sunward ion jets embedded within current sheets are widely and commonly observed in the near Mars spaceMagnetic field configuration and topology around the jets are generally consistent with those expected from magnetic reconnectionThe jet occurrence appears independent of upstream drivers, possibly resulting from ubiquitous formation of low‐beta, thin current sheets [ABSTRACT FROM AUTHOR]

We present the first prediction of the ionospheric response to the 14 December 2020 solar eclipse using the SUPIM‐INPE model. Simulations are made for all known ionosonde stations for which solar obscuration is significant. The found response is similar to that previously reported for other eclipses, but it also shows a modification of the equatorial fountain transport that will impact the low latitudes after the event. In addition to the large reduction of electron concentration along the totality path (~4.5 TECu, ~22%), a significant electron and oxygen ion temperature cooling is observed (up to ~400 K) followed by lasting temperature increases. Changes of up to ~1.5 TECu (~5%) are also expected at the conjugate hemisphere. These predictions may serve as a reference for eventual ionospheric measurements of multiple instruments and are leading to a better understanding of the ionospheric response to solar eclipses. Key Points: The eclipse obscuration would lead to a TEC reduction of up to ~4.5 TECu (~22%)Electron and oxygen ion temperatures are expected to decrease up to ~400 (~22%) and ~200 K (~8%), respectivelyMagnetically conjugate reductions of up to ~1.5 TECu (~5%) are predicted [ABSTRACT FROM AUTHOR]

This study presents simulations of the coupled space environment during a geomagnetic storm that separates the different sources of near‐Earth plasma. These simulations include separate fluids for solar wind and ionospheric protons, ionospheric oxygen, and the plasmasphere. Additionally, they include the effects of both a hot ring current population and a cold plasmaspheric population simultaneously for a geomagnetic storm. The modeled ring current population represents the solution of bounce‐averaged kinetic solution; the core plasmaspheric model assumes a fixed temperature of 1 eV and constant pressure along the field line. We find that during the storm, ionospheric protons can be a major contributor to the plasmasheet and ring current and that ionospheric plasma can largely displace solar wind protons in much of the magnetosphere under certain conditions. Indeed, the ionospheric source of plasma cannot be ignored. Significant hemispheric asymmetry is found between the outflow calculated in the summer and winter hemispheres, consistent with past observations. That asymmetric outflow is found to lead to asymmetric filling of the lobes, with the northern (summer) lobe receiving more outflow that has a higher proportion of O+ and the southern (winter) lobe receiving less outflow with a higher proportion of H+. We moreover find that the inclusion of the plasmasphere can have a system‐wide impact. Specifically, when the plasmasphere drainage plume reaches the magnetopause, it can reduce the reconnection rate, suppress ionospheric outflow and change its composition, change the composition in the magnetosphere, and reduce the ring current intensity. Key Points: Ionospheric H+ is a critically important contributor to the magnetosphere during a stormSeasonal effect on outflow create asymmetric filling of the lobesThe inclusion of an additional plasmaspheric fluid has system‐wide effects [ABSTRACT FROM AUTHOR]