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Abstract 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 highB_y/B_zratio has a positive or negativeB_zcomponent, 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 largeB_y/B_zratio of the IMF around the RD. 

Abstract 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. 

Abstract We conduct a global hybrid simulation of an observation event to affirm that an interplanetary (IP) shock can drive significant suprathermal (tens to hundreds of eV) H⁺outflows from the polar cap. The event showed that a spacecraft in the lobe at ∼6.5 R_Ealtitude above the polar cap observed the appearance of suprathermal outflowing H⁺ions about 8 min after observing enhanced downward DC Poynting fluxes caused by the shock impact. The simulation includes H⁺ions from both the solar wind and the ionospheric sources. The cusp/mantle region can be accessed by ions from both sources, but only the outflow ions can get into the lobe. Despite that upward flowing solar wind ions can be seen within part of the cusp/mantle region and their locations undergo large transient changes in response to the magnetosphere compression caused by the shock impact, the simulation rules out the possibility that the observed outflowing H⁺ions was due to the spacecraft encountering the moving cusp/mantle. On the other hand, the enhanced downward DC Poynting fluxes caused by the shock impact drive more upward suprathermal outflows, which reach higher altitudes a few minutes later, explaining the observed time delay. Also, these simulated outflowing ions become highly field‐aligned in the upward direction at high altitudes, consistent with the observed energy and pitch‐angle distributions. This simulation‐observation comparison study provides us the physical understanding of the suprathermal outflow H⁺ions coming up from the polar cap. 

Abstract To better understand how sharp changes in the solar wind and interplanetary magnetic field conditions affect the ionosphere outflows at high latitudes, we analyze an event observed on 17 July 2002 showing suprathermal (tens to hundreds of eV) outflowing H⁺ions in the lobe driven by the impact of an interplanetary (IP) shock. A spacecraft in the lobe at altitudes of ∼6.5R_Efirst observed enhanced downward DC Poynting fluxes ∼2 min after the shock impact and then, another 8 min later, the appearance of suprathermal outflowing H⁺ions as ion beams and ion conics. The increasing downward DC Poynting fluxes and the increasing outflowing H⁺fluxes that appeared later were highly correlated because they shared a similar increasing trend with a time scale of ∼5 min. To explain such time delay and correlation, we conclude that a plausible scenario was that the enhanced DC Poynting fluxes reached down to lower altitudes, drove processes to accelerate the pre‐existing polar wind ions to ion beams and ion conics, and then these newly generated suprathermal ions flowed upward to the spacecraft altitudes. This event indicates that an IP shock can drive a significant amount of suprathermal H⁺outflows from the polar cap. 

Abstract Chorus waves are intense electromagnetic emissions critical in modulating electron dynamics. In this study, we perform two‐dimensional particle‐in‐cell simulations to investigate self‐consistent wave‐particle interactions with oblique chorus waves. We first analyze the electron dynamics sampled from cyclotron and Landau resonances with waves, and then quantify the advection and diffusion coefficients through statistical studies. It is found that phase‐trapped cyclotron resonant electrons satisfy the second‐order resonance condition and gain energy from waves. While phase‐bunched cyclotron resonant electrons cannot remain in resonance for long periods. They transfer energy to waves and are scattered to smaller pitch angles. Landau resonant electrons are primarily energized by waves. For both types of resonances, advection coefficients are greater than diffusion coefficients when the wave amplitude is large. Our study highlights the important role of advection in electron dynamics modulation resulting from nonlinear wave‐particle interactions. 

Abstract Chorus subpackets are the wave packets with modulated amplitudes in chorus waves, commonly observed in the magnetospheres of Earth and other planets. Nonlinear wave‐particle interactions have been suggested to play an important role in subpacket formation, yet the corresponding electron dynamics remain not fully understood. In this study, we have investigated the electron trapping through cyclotron resonance with subpackets, using a self‐consistent general curvilinear plasma simulation code simulation model in dipole fields. The electron trapping period has been quantified separately through electron dynamic analysis and theoretical derivation. Both methods indicate that the electron trapping period is shorter than the subpacket period/duration. We have further established the relation between electron trapping period and subpacket period through statistical analysis using simulation and observational data. Our study demonstrates that the nonlinear electron trapping through cyclotron resonance is the dominant mechanism responsible for subpacket formation. 

Abstract A 2‐D GCPIC simulation in a dipole field system has been conducted to explore the excitation of oblique whistler mode chorus waves driven by energetic electrons with temperature anisotropy. The rising tone chorus waves are initially generated near the magnetic equator, consisting of a series of subpackets, and become oblique during their propagation. It is found that electron holes in the wave phase space, which are formed due to the nonlinear cyclotron resonance, oscillate in size with time during subpacket formation. The associated inhomogeneity factor varies accordingly, giving rise to various frequency chirping in different phases of subpackets. Distinct nongyrotropic electron distributions are detected in both wave gyrophase and stationary gyrophase. Landau resonance is found to coexist with cyclotron resonance. This study provides multidimensional electron distributions involved in subpacket formation, enabling us to comprehensively understand the nonlinear physics in chorus wave evolution. 

Abstract An important discovery of MESSENGER is the occurrence of dayside disappearing magnetosphere (DDM) events that occur when the solar wind dynamic pressure is extremely high and the interplanetary magnetic field (IMF) is both intense and southward. In this study, we investigate the DDM events at Mercury under extreme solar wind conditions using a three‐dimensional (3‐D) global hybrid simulation model. Our results show that when the solar wind dynamic pressure is 107 nPa and the magnitude of the purely southward IMF is 50 nT, most of the dayside magnetosphere disappears within 10 s after the interaction between the solar wind and the planetary magnetic field starts. During the DDM event, the ion flux is significantly enhanced at most of the planetary dayside surface and reaches its maximum value of about 10¹⁰ cm⁻² s⁻¹at the low‐latitude surface, which is much larger than that under normal solar wind conditions. During the DDM events, the dayside bow shock mostly disappears for about 9 s and then reappears. Moreover, the time evolution of magnetopause standoff distance under different solar wind conditions is also studied. When the solar wind dynamic pressure exceeds 25 nPa and the IMF is purely southward, a part of the dayside magnetosphere disappears. Under the same IMF, the higher the solar wind dynamic pressure, the faster the magnetopause standoff distance reaches the planetary surface. When the solar wind conditions are normal (with a dynamic pressure of 8 nPa) or the IMF is purely northward, the dayside magnetosphere does not disappear. The results provide a clear physical image of DDM events from a 3‐D perspective. 

Key Points Enhancement of field‐aligned warm ions observed in the plasma sheet was energy‐dispersive with increasing energy from 20 eV to >100 eV The probe at larger r observed the energy‐dispersive enhancements 20 min earlier than did the probe at smaller r The enhancements were likely caused by enhanced convection and the dispersion was likely due to acceleration by field‐aligned potential