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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 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 Chorus subpackets/subelements are the wave packets occurring at intervals of ∼10–100 msec and are suggested to play a crucial role in the formation of substructures within pulsating aurora. In this study, we investigate the evolution of subpackets from the upstream to downstream regions. Using Van Allen Probe A measurements, we have found that the frequency of the upstream subpackets increases smoothly, but that of the downstream subpackets remains almost unchanged. Through a simulation in the real‐size magnetosphere, we have reproduced the subpackets with characteristics similar to those in observations, and revealed that the frequency chirping is influenced by both resonant current of electrons and wave amplitude due to nonlinear physics. Although the resonant currents in the upstream and downstream regions are comparable, the wave amplitude increases significantly during evolution, resulting in lower sweep rate in the downstream region. Our findings provide a fresh insight into the evolution of chorus subpackets.