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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. Plain Language Summary Wave‐particle interactions can modulate electron distributions through advection and diffusion. Previous studies focusing on advection and diffusion primarily relied on test particle simulations, which uses a simplified model of wave evolution. In this study, we perform self‐consistent simulations to investigate the wave‐particle interactions with chorus waves and quantify the advection and diffusion coefficients of resonant electrons. It is found that advection coefficients are greater than diffusion coefficients in both cyclotron and Landau resonances, indicating the significant role of nonlinear wave‐particle interactions. The quantification of advection and diffusion coefficients in a self‐consistent system is important for understanding and predicting the loss and energization processes in radiation belt electrons. This study complements previous diffusion models that regarded the evolution of electron dynamics in wave‐particle interactions as a slow diffusive process. Key Points Electron advection and diffusion in wave‐particle interactions with chorus waves are investigated through self‐consistent simulations The second‐order time derivative of gyrophase angle is nearly zero for phase‐trapped electrons but is negative for phase‐bunched electrons The advection and diffusion coefficients for cyclotron and Landau resonant electrons interacting with chorus waves are quantified

Most of the energy in magnetospheric plasma is available in the form of low-frequency turbulence. In this paper, we have explored the possibility of pumping such low-frequency turbulence wave energy into high-frequency X-mode in the magnetosphere. We have considered the wave energy up-conversion process through the nonlinear wave-particle interaction of the ion-acoustic wave and the X-mode wave. In this model of wave energy up-conversion, we have considered a particle distribution of modified Maxwellian with the involvement of a gradient parameter associated with the spatial gradient and temperature gradient of magnetospheric plasma. When considering the Vlasov–Maxwell system of equations to describe the wave interaction process, we have evaluated the fluctuating parts of the particle distribution function due to the ion-acoustic wave field for the modulated field and the nonlinear fluctuating parts of the distribution function due to X-mode. The nonlinear dispersion relation for X-mode enables us to estimate the growth of X-mode at the expense of the ion-acoustic wave energy of the magnetospheric plasma. We have also demonstrated that how this growth process is influenced by gradient parameters associated with this system.

Electromagnetic ion cyclotron (EMIC) waves can drive radiation belt depletion and Low‐Earth Orbit satellites can detect the resulting electron and proton precipitation. The ELFIN (Electron Losses and Fields InvestigatioN) CubeSats provide an excellent opportunity to study the properties of EMIC‐driven electron precipitation with much higher energy and pitch‐angle resolution than previously allowed. We collect EMIC‐driven electron precipitation events from ELFIN observations and use POES (Polar Orbiting Environmental Satellites) to search for 10s–100s keV proton precipitation nearby as a proxy of EMIC wave activity. Electron precipitation mainly occurs on localized radial scales (∼0.3 L), over 15–24 MLT and 5–8 L shells, stronger at ∼MeV energies and weaker down to ∼100–200 keV. Additionally, the observed loss cone pitch‐angle distribution agrees with quasilinear predictions at ≳250 keV (more filled loss cone with increasing energy), while additional mechanisms are needed to explain the observed low‐energy precipitation. Plain Language Summary Electromagnetic ion cyclotron (EMIC) emissions are a type of plasma wave that can be excited in the near‐Earth environment and interact with energetic electrons in the Earth's radiation belts. Through these wave‐particle interactions, electrons can be pushed into the loss cone and lost into the Earth's atmosphere (electron precipitation), where they deposit their energy by interacting with neutral atoms and cold charged particles. EMIC‐driven electron precipitation still needs to be fully characterized and understood. In this work, we use data from the Electron Losses and Fields InvestigatioN (ELFIN) CubeSats, which provide electron fluxes at high energy and pitch‐angle (look direction) resolution at ∼450 km of altitude. Our analysis reveals that precipitation is most efficient for ∼MeV electrons and is accompanied by weaker low‐energy precipitation down to ∼100–200 keV. Given the ELFIN CubeSats spin, we can also study the distribution of the precipitating electrons along different look directions (pitch‐angles). We find that the loss cone shape is well‐reproduced by quasilinear predictions of EMIC‐electron interactions at higher energies (≳250 keV), while quasilinear calculations underestimate the observed low‐energy precipitation. Key Points Energetic electron precipitation is observed by Electron Losses and Fields InvestigatioN nearby proton precipitation (a proxy for Electromagnetic ion cyclotron waves) primarily over 15–24 MLT Precipitation efficiency increases as a function of energy: weak ∼100s keV precipitation is concurrent with intense ∼MeV precipitation The observed pitch‐angle distribution shows a loss cone filling up with energy, similar to the pitch‐angle profiles from quasilinear theory

Electromagnetic ion cyclotron (EMIC) waves are transverse plasma waves generated by anisotropic proton distributions with Tperp > Tpara. They are believed to play an important role in the dynamics of the ring current and potentially, of the radiation belts. Therefore it is important to know their localization in the magnetosphere and the magnetospheric and solar wind conditions which lead to their generation. Our earlier observations from three Time History of Events and Macroscale Interactions during Substorms (THEMIS) probes demonstrated that strong magnetospheric compressions associated with high solar wind dynamic pressure (Pdyn) may drive EMIC waves in the inner dayside magnetosphere, just inside the plasmapause. Previously, magnetospheric compressions were found to generate EMIC waves mainly close to the magnetopause. In this work we use an automated detection algorithm of EMIC Pc1 waves observed by THEMIS between May 2007 to December 2011 and present the occurrence rate of those waves as a function of L‐shell, magnetic local time (MLT), Pdyn, AE, and SYMH. Consistent with earlier studies we find that the dayside (sunward of the terminator) outer magnetosphere is a preferential location for EMIC activity, with the occurrence rate in this region being strongly controlled by solar wind dynamic pressure. High EMIC occurrence, preferentially at 12–15 MLT, is also associated with high AE. Our analysis of 26 magnetic storms with Dst < −50 nT showed that the storm‐time EMIC occurrence rate in the inner magnetosphere remains low (<10%). This brings into question the importance of EMIC waves in influencing energetic particle dynamics in the inner magnetosphere during disturbed geomagnetic conditions. Key Points Dayside outer magnetosphere is a preferential location for EMIC waves Dayside EMIC occurrence rate is controlled by solar wind pressure Storm‐time EMIC occurrence in the inner magnetosphere remains low

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.

The modulation of chorus waves on several‐second timescales in Earth's magnetosphere plays a crucial role in modulating electron precipitation intensity, leading to the formation of pulsating aurora. However, the physical mechanism underlying chorus modulation remains not fully understood. In this study, we perform self‐consistent particle‐in‐cell simulations with typical magnetospheric plasma parameters to quantify chorus modulation driven by plasma density variations and compressional magnetic field fluctuations. It is demonstrated that chorus modulation is determined by nonlinear wave‐particle interactions, in which the condition for nonlinear wave growth is highly sensitive to background plasma parameters. The resulting electron precipitation in the ∼10–200 keV energy range exhibits modulation on comparable timescales, consistent with observations of pulsating aurora. This study enhances our understanding of how variations in magnetospheric plasma parameters influence chorus wave excitation and the associated particle dynamics.

Electromagnetic ion cyclotron (EMIC) waves are commonly observed electromagnetic emissions in Earth's magnetosphere and are widely considered to efficiently scatter relativistic electrons into bounce loss cones. However, their precise scattering effects remain highly debated due to limited energy coverage and coarse resolution of previous measurements. Here, we present high‐energy‐resolution measurements of EMIC‐induced relativistic electron precipitation from the Relativistic Electron and Proton Telescope integrated little experiment‐2 (REPTile‐2) onboard the Colorado Inner Radiation Belt Experiment (CIRBE) CubeSat. A long duration >1 MeV electron precipitation event was measured by CIRBE/REPTile‐2 in both the northern and southern hemispheres on 25 April 2023. The energy versus L dispersions of these >1 MeV precipitating electrons show good agreement with minimum resonance energies of electrons interacting with He+ band EMIC waves at specific frequencies. These novel observations unveil the detailed scattering effect of EMIC waves and provide important clues regarding wave‐particle interaction processes near the equator.

Plasma density gradients, such as those that occur on plasmaspheric plume boundaries, have been shown to increase the obliquity of lower band chorus. Here, for the first time, this relationship is investigated more generally by considering the wave normal angle, θk${\\theta }_{k}$ , as a function of the magnitude of all observed density gradients. Both case studies, and a statistical analysis, reveal a direct correspondence between the magnitude of density gradients and wave obliquity, with θk${\\theta }_{k}$increasing near stronger density gradients. Wave propagation is also investigated as a function of local plasma density, revealing that chorus is more often oblique when observed in lower density mediums, and more field‐aligned in higher density regions. This result highlights the importance of retaining the intrinsic physical coupling between wave propagation properties and the cold plasma density, since both parameters strongly influence the calculation of the diffusion coefficients used to quantify wave‐particle interactions in the inner‐magnetosphere. Plain Language Summary Much like a prism refracts light, variations in the density of magnetospheric plasma impact the propagation of electromagnetic waves. This study investigates the relationship between chorus wave propagation and both the local plasma density, and gradients in the plasma density, for the first time. The propagation direction of chorus waves is shown to directly correspond to the local plasma density, as well as to the magnitude of density gradients that occur in the vicinity. Chorus waves can drive rapid acceleration of the Van Allen radiation belts up to relativistic energies, with calculations to quantify these wave‐particle interactions strongly influenced by both the plasma density and the propagation direction of the wave. As such, this result demonstrates that retaining the fundamental physical coupling between wave properties and the plasma density may be an important and worthwhile step toward accurate modeling and forecasting of the radiation belts. Key Points Wave normal angle of chorus is investigated as a function of both the local plasma density, and plasma density gradients, for the first time Chorus wave obliquity is shown to directly correspond to both local plasma density, and the magnitude of density gradients in the vicinity Results highlight the importance of retaining the intrinsic physical coupling between wave propagation properties and the plasma density

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. Plain Language Summary Subpackets within chorus waves are suggested to play a significant role in producing the substructures within pulsating aurora. How does the frequency change inside subpackets is still an open question. In this study, the subpackets are found with Van Allen Probe A observation to be excited upstream of the magnetic equator, and propagate toward downstream. The frequency of subpackets increases with time in the upstream region, while it keeps almost unchanged in the downstream region. Meanwhile, a particle‐in‐cell simulation has been performed to study the characteristics of subpackets, and the simulation results agree well with those in observations. The frequency variation of subpackets is influenced by both resonant electrons and wave amplitude. Our study provides a clue for better understanding the nonlinear wave‐particle interactions in the evolution of chorus subpackets. Key Points The source region of chorus subpackets has been observed by Van Allen Probe A The chorus subpackets have been investigated via the general curvilinear particle‐in‐cell simulation in the real‐size magnetosphere Nonlinear physics is a dominant process in the evolution of chorus subpackets

Electrostatic Cyclotron Harmonic (ECH) waves have been considered a potential cause of pitch angle scattering of electrons in the energy range from a few hundred eV to tens of keV. Theoretical studies have suggested that scattering by ECH waves is enhanced at lower pitch angles near the loss cone. Due to the insufficient angular resolution of particle detectors, it has been a great challenge to reveal ECH‐driven scattering based on electron measurements. This study reports on variations in electron pitch angle distributions associated with ECH wave activity observed by the Arase satellite. The variation is characterized by a decrease in fluxes near the loss cone, and energy and pitch angle dependence of the flux decrease is consistent with the region of enhanced pitch angle scattering rates predicted by the quasi‐linear diffusion theory. This study provides direct evidence for energy‐pitch angle dependence of pitch angle scattering driven by ECH waves. Plain Language Summary Near‐Earth space is surrounded by the ionized gas so‐called “plasma,” which is trapped by the Earth's intrinsic magnetic field, forming the magnetosphere of the Earth. A variety of waves are present in the plasma, and interaction between plasma and waves results in acceleration, transport, and loss of plasma in the Earth's magnetosphere. Electrostatic Cyclotron Harmonic (ECH) waves are one of the intense waves present in the magnetosphere. The waves are one candidate causing electron heating and loss of electrons in that region. Theoretical studies suggest that ECH waves contribute to a change in electron pitch angle, the angle between the velocity vector of electrons and the magnetic field, in the energy range from a few hundred electron volts to several tens of kilo‐electron‐volts. This process causes electron precipitation into the Earth's atmosphere, resulting in permanent loss of electrons from the magnetosphere. Here, based on the observation by the Arase satellite together with a theoretical framework, we show the observational evidence that ECH waves can cause significant pitch angle changes of electrons in the energy range of 7–20 keV. This study strongly suggests that ECH waves contribute to electron dynamics in the inner magnetosphere through wave‐particle interactions. Key Points A decrease in electron fluxes is observed near the loss cone associated with an enhanced Electrostatic Cyclotron Harmonic (ECH) wave activity Energy‐pitch angle dependent flux decrease is consistent with the prediction by the quasi‐linear diffusion theory The first direct observation showing that ECH waves are capable of energy‐dependent scattering of electrons near the loss cone