Chih‐Ping Wang

To understand the favorable local and external conditions under which particle fluxes of warm ions (∼10 eV–1 keV) in the near‐Earth plasma sheet can acquire higher field‐aligned intensities (FAIs), we statistically analyzed 19 years of satellite observations in the equatorial magnetosphere. FAIs can reach substantially higher values in the 0–6 MLT quadrant than others, and at locations of smaller radial distances. For the plasma sheet warm ions immediately outside the plasmapause, higher FAIs are more frequently observed when the plasmapause is smaller and thus the plasma sheet is located closer to the Earth, where the local conditions show larger magnetic field strengths, shorter field‐line lengths, and higher plasma sheet electron fluxes, while the external conditions show the solar wind conditions being favorable for driving more active geomagnetic conditions, as indicated by lower SYM‐H and higher AE* values. The changes of warm ion fluxes leading to higher FAI values are due to increases in phase space densities along field‐aligned directions, as well as adiabatic effects associated with changes in magnetic field strength and field‐line length. The dominant particle source providing such an increase in field‐aligned warm ions during stronger geomagnetic conditions is likely to be an enhancement in ionospheric outflow coming out of auroral latitudes in response to intensified electron precipitation.

Cold protons play an important role in the Earth’s magnetosphere by modifying the dispersion relation of plasma waves. Their energy flux can be enhanced through a nonresonant response to electromagnetic ion cyclotron (EMIC) waves. This study combines Magnetospheric Multiscale (MMS) observations and a hybrid simulation to investigate cold proton dynamics during this nonresonant process. It is found that the energy flux of cold protons with kinetic energies below ~ 200 eV increases due to the bulk flow induced by EMIC waves, and this enhancement becomes stronger and extends to higher energies at higher magnetic latitudes. Despite the flux enhancement, the temperature and number density of cold protons remain constant. Moreover, we identify the formation mechanism of proton phase-bunching distribution in stationary gyrophase. During the nonresonant process, particles bunch in anti-phase with the wave magnetic fields, without exchanging energy with waves. In contrast, during the resonant process, particles bunch in anti-phase with the wave electric fields, facilitating energy transfer from particles to waves. We demonstrate that particle phase-bunching is a necessary but not sufficient condition for identifying resonant interactions with waves, and that the occurrence of energy transfer determines whether the response is nonresonant or resonant. Graphical Abstract

Microinjections are periodic enhancements of 10 s–100 s keV electron fluxes at a cadence of several minutes on the dawnside or duskside magnetosphere. Here, we extend previous studies by comprehensively examining the plasma and wave observations provided by the Magnetospheric Multiscale (MMS) mission. We report an event study showing that microinjection oscillates in specific phase relations relative to lower‐energy (1 keV) plasmas (180° out of phase) and the electromagnetic waves and the corresponding E × B velocity (90° out of phase). The phase relations can be explained by the following simple scenario: A plasma boundary, which separates the low‐energy plasmas and the plasma accessing the 10 s–100 s keV electrons, oscillates over the quasi‐stationary MMS satellites due to the Ultra‐Low Frequency (ULF) waves. The phase relations and the simple explanation suggest that the period of microinjection may be apparent, that is, the 10 s–100 s keV electron flux could be stable in the rest frame of its local plasma.

We present an event of Dispersive Field‐Aligned Warm Ion (∼10–1,000 eV) Flux Enhancements (DFAWIFEs) observed by multiple satellites in the dayside equatorial plasma sheet ( r > ∼10 R _E ) driven by transient (∼5–10 min) magnetopause inward movements (intrusion). The dispersion, which appeared a few minutes after a magnetopause intrusion, exhibited a feature of decreasing energy over a timescale of several tens of minutes. Comparing the observed dispersion profiles with predictions indicates that these warm ions were dominantly H ⁺ ions coming from a warm ion source in the topside ionosphere (∼1 R _E altitude). DFAWIFEs were observed at locations closer to the magnetopause where shear Alfvén waves generated by the intrusion were more intense, suggesting a connection between the wave powers and the warm ion source. Additionally, preexisting cold (<10 eV) ions were observed in the plasma sheet, suggesting cold ion outflows in the topside ionosphere as the seed population for the warm ion source. We establish a scenario to connect these observations to explain DFAWIFEs. A magnetopause intrusion can generate shear Alfvén waves propagating toward the ionosphere, providing the energy for generating a transient warm ion source by energizing the preexisting cold outflow in the topside ionosphere. The larger wave powers closer to the magnetopause are more likely to provide the needed energy for generating such a source. The warm ions from the source then travel along magnetic field lines to the equatorial plasma sheet with higher energy ions arriving first, resulting in the observed dispersion signature.

Solar wind directional discontinuities, such as rotational discontinuities (RDs), significantly influence energy and transport processes in the Earth's magnetosphere. A recent observational study identified a long‐lasting double cusp precipitation event associated with RD in solar wind on 10 April 2015. To understand the magnetosphere‐ionosphere response to the solar wind RD, a global hybrid simulation of the magnetosphere was conducted, with solar wind conditions based on the observation event. The simulation results show significant variations in the magnetopause and cusp regions caused by the passing RD. After the RD propagates to the magnetopause, ion precipitation intensifies, and a double cusp structure at varying latitudes and longitudes forms near noon in the northern hemisphere, which is consistent with the satellite observations by Wing et al. (2023, https://doi.org/10.1029/2023gl103194 ). Regarding dayside magnetopause reconnection, the simulation reveals that the high‐latitude reconnection process persists during the RD passing, regardless of whether the interplanetary magnetic field (IMF) with a high B _y / B _z ratio has a positive or negative B _z component, and low‐latitude reconnection occurs after the RD reaches the magnetopause at noon when the IMF turns southward. By examining the ion sources along the magnetic field lines, a connection is found between the single‐ or double‐cusp ion precipitation and the solar wind ions entering from both high‐latitude and low‐latitude reconnection sites. This result suggests that the double‐cusp structure can be triggered by magnetic reconnection occurring at both low latitudes and high latitudes in the opposite hemispheres, associated with a large B _y / B _z ratio of the IMF around the RD.

Dawnside auroral polarization streams (DAPS) are fast eastward flows in the dawn convection cell of Earth's ionosphere. With a steep flow gradient near the interface between Region 1 and 2 currents and a peak poleward of it, DAPS were suggested to be responsible for instabilities and dramatic events in the magnetosphere‐ionosphere (M‐I) system. To predict these events, it is important to investigate when and where DAPS prefer to occur and how they are related to other M‐I phenomena. We conduct this investigation statistically using 10 years of Swarm data and find that DAPS under sunlit and dark ionospheric conditions exhibit different dependencies on magnetic local times and geomagnetic activities, reflecting a complicated interplay between magnetotail dynamics and ionospheric conductance. The statistical results also reveal a strong correlation between DAPS and embedded Region 2 currents. These findings provide new insights into the DAPS generation mechanism.

Electromagnetic ion cyclotron (EMIC) waves are commonly observed in the Earth's magnetosphere and play a significant role in regulating relativistic electron fluxes. The waveform of EMIC waves comprises amplitude‐modulated wave packets, known as “subpackets.” Despite their prevalence, the underlying physics and associated particle dynamics for subpacket formation remain poorly understood. In this study, using Van Allen Probe A observations, we present several rising‐tone EMIC wave events to reveal the downward frequency chirping between adjacent subpackets. By performing a hybrid simulation, we demonstrate for the first time that these wave properties are associated with the oscillation of proton holes in the wave gyrophase space induced by cyclotron resonance. The oscillation modulates the energy transfer between waves and particles, establishing a direct link between subpacket formation in cyclotron waves and nonlinear wave‐particle interactions. This new understanding advances our knowledge of subpacket formation in general and its broader implications in space plasma physics.

Enhancement of currents in Earth's ionosphere adversely impacts systems and technologies, and one example of extreme enhancement is supersubstorms. Despite the name, whether a supersubstorm is a substorm remains an open question, because studies suggest that unlike substorms, supersubstorms sometimes affect all local times including the dayside. The spectacular May 2024 storm contains signatures of two supersubstorms that occurred successively in time with similar magnitude and duration, and we explore the nature of them by examining the morphology of the auroral electrojet, the corresponding disturbances in the magnetosphere, and the solar wind driving conditions. The results show that the two events exhibit distinctly different features. The first event was characterized by a locally intensified electrojet followed by a rapid expansion in latitude and local time. Auroral observations showed poleward expansion of auroras (or aurorae), and geosynchronous observations showed thickening of the plasma sheet, magnetic field dipolarization, and energetic particle injections. The second event was characterized by an instantaneous intensification of the electrojet over broad latitude and local time. Auroras did not expand but brightened simultaneously across the sky. Radar and LEO observations showed enhancement of the ionospheric electric field. Therefore, the first event is a substorm, whereas the second event is enhancement of general magnetospheric convection driven by a solar wind pressure increase. These results illustrate that the so‐called supersubstorms have more than one type of driver, and that internal instability in the magnetotail and external driving of the solar wind are equally important in driving extreme auroral electrojet activity.

We present multi‐platform observations of plasma cloak, O+ outflows, kinetic Alfven waves (KAWs), and auroral oval for the geomagnetic storm on 17 March 2015. During the storm's main phase, we observed a generally symmetric equatorward motion of the auroral oval in both hemispheres, corresponding to the plasmasphere erosion and inward motion of the plasma sheet. Consequently, Van Allen Probes became immersed within the plasma sheet for extended hours and repeatedly observed correlated KAWs and O+ outflows. The KAWs contain adequate energy flux toward the ionosphere to energize the observed outflow ions. Adiabatic particle tracing suggests that the O+ outflows are directly from the nightside auroral oval and that the energization is through a quasi‐static potential drop. The O+ outflows from the nightside auroral oval were adequate (‐ #/‐s) and prompt (several minutes) to explain the newly formed plasma cloak, suggesting that they were a dominant initial source of plasma cloak during this storm.

To understand the entry of the cool low‐latitude mantle ions into the tail plasma sheet near the flanks under persistent interplanetary magnetic field B_y, we evaluate the role of the cross‐field diffusive transport by kinetic Alfvén waves (KAWs) by investigating two events observed by multiscale (MMS) spacecraft. Around the separatrix between the open and closed field‐line regions, a two‐component mixing of hot plasma sheet ions of a few keV with cool mantle ions of a few hundred eV was observed, indicating transport across the separatrix. The waves observed between 0.01 and 10 Hz around the separatrix had characteristics consistent with those of KAWs. The consistency allowed us to estimate the wave vectors as a function of frequency by fitting KAW dispersion to the observations. Using the observed wave powers, plasma moments, and the estimated wave vectors, we computed the cross‐field diffusion rates associated with KAWs. The diffusion rates were found to be comparable to or larger than the Bohm diffusion rates during the intervals when the two‐component mixing was observed, indicating that the KAW diffusive transport can play a role in the entry of low‐latitude mantle ions into the plasma sheet.