Yu Lin

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.

The Tandem Reconnection And Cusp Electrodynamics Reconnaissance Satellites (TRACERS) small explorer mission was designed to: Determine whether magnetopause reconnection is primarily spatially or temporally variable for a range of solar wind conditions . Observations from the Analyzers for Cusp Ions (ACIs) on the two TRACERS spacecrafts are used to make this determination. Tandem cusp encounters from November 2025 through February 2026 are used to distinguish cusp ion structures resulting from spatial variability of magnetopause reconnection from those resulting from temporal variability. Preliminary results indicate that magnetopause reconnection is dominantly spatially variable for all interplanetary magnetic field (IMF) orientations and all solar wind dynamic pressures. Spatial variability dominates by at least a factor of 2 for all solar wind conditions. However, almost all encounters categorized as “spatial” display some temporal variability embedded in the larger spatial variability. More tandem cusp encounters are needed to verify these preliminary results and to complete the other science objectives of the mission.

Magnetic reconnection at the dayside magnetopause is the primary pathway for transferring mass, momentum, and energy from the solar wind into the terrestrial magnetosphere. Previous studies have shown that the spatiotemporal dynamics of dayside magnetic reconnection can be inferred remotely from properties of Earth's cusps, specifically their ion dispersion signatures, as well as structure and location within the magnetosphere. Despite the abundance of in situ cusp observations from multiple space‐based instruments, intermittent sampling spacecraft moving quickly along their trajectories precludes characterization of the cusp's global response to dynamic upstream drivers. To overcome this limitation, we leveraged existing data sets to develop a machine learning‐based model of three‐dimensional ion fluxes in the high‐altitude northern cusp that are modulated by solar‐wind forcing. This model is based on a residual neural network trained on solar wind parameters from NASA's OMNIWeb database, and ion flux measurements from the CIS/HIA instrument aboard ESA's Cluster mission. The model reproduces longitudinal and latitudinal displacements of the cusp under ‐ and ‐dominated interplanetary magnetic field regimes, in close agreement with reconnection theory. To quantify uncertainty and evaluate performance, we use an ensemble of 100 identical networks with randomized training and validation splits, reserving a fixed test set shared by all models. The statistics indicate consistent predictions within the central cusp region. Also, we used Shapley Additive exPlanations to visualize input feature contributions to the model's outputs. The analysis suggests that the magnitude of ion fluxes within the cusp is most strongly associated with , and solar wind parameters.

Electromagnetic ion cyclotron (EMIC) waves contribute to magnetospheric loss cone scattering, but the fundamental processes during EMIC wave formation and evolution have not been systematically studied. We examine the formation of EMIC waves, driven by temperature anisotropy, in a uniform plasma using one‐dimensional (1D) and two‐dimensional (2D) hybrid simulations in proton‐only and hydrogen‐helium mixed plasmas. In a 1D plasma, a hot proton population with sufficient temperature anisotropy leads to the growth of parallel‐propagating EMIC waves in line with the predictions of the linear‐fit dispersion relation. The changing plasma temperatures and wave structures during EMIC wave evolution are illustrated. Additionally, a standing structure with parallel wave‐vector was found to act as a mechanism for parallel heating of the cold background plasma. This electrostatic structure emerges with half the EMIC wavelength, and is generated by nonlinear EMIC self‐interaction. In the 2D case, we observe the formation of structures possessing perpendicular wavenumbers and a broad‐spectrum overlap between the EMIC and electrostatic waves leading to locally enhanced amplitudes. In simulations with a cold helium population, we note specific contributions of each ion species to the thermal evolution of the system; hot protons carry cross‐field thermal energy and cold protons and helium respond to the electrostatic mode and its harmonics respectively. Harmonics of the electrostatic mode are nontrivial in the mixed plasma and contribute substantially to heating in the cold plasma background. Overall, results show evidence of nonlinear processes contributing to particle heating and modulation of excited EMIC waves.

Whistler-mode waves play an important role in electron dynamics within planetary magnetospheres and are ubiquitously observed across a wide range of plasma environments. In Earth's inner magnetosphere, they are commonly classified into incoherent whistler, chorus, and hiss based on their spectral characteristics. This study presents a computational investigation under typical magnetospheric conditions to examine how plasma parameters determine the excitation of different whistler-mode waves. Under low-density conditions outside the plasmasphere, the parameter regimes leading to incoherent whistler, discrete-element chorus, and hiss-like chorus are identified. Peak wave amplitude is quantified for each simulation case, and an empirical relation is established to describe its dependence on hot electron number density and temperature anisotropy. Incoherent whistler waves are driven by linear instabilities of hot electrons, whereas chorus waves arise through nonlinear wave–particle interactions. The growth rates of chorus waves are quantified along the wave packet evolution and are consistent with theoretical predictions. Under high-density conditions within the plasmasphere or plume regions, hiss waves are preferentially generated. This study delineates the parameter regimes for simulating various whistler-mode waves and contributes to a deeper understanding of wave–particle interactions.

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

We investigate how the wave normal angle (WNA) and polarization of proton‐band electromagnetic ion cyclotron (EMIC) waves change as they travel from their source to Earth. This paper marks a significant milestone as the first full‐wave simulation of proton‐band EMIC waves reflecting from the ionosphere. Our findings show that the WNA can change rapidly during propagation, primarily due to plasma inhomogeneities, such as variations in the Alfvén speed. The wave polarization is strongly related to the WNA, consistent with theory. Newly generated EMIC waves near the equator propagate with a WNA of 0, then the WNA gradually shifts to 90 as they move toward Earth. In contrast, reflecting waves having 90 of WNA at Earth maintain a relatively larger WNA even near the magnetic equator. As a result, only the newly generated waves close to the source, where the magnetic latitude is less than approximately 20, show left‐handed polarization, while linear polarization remains dominant throughout the rest of the propagation.

Previous studies indicate that small magnetic flux ropes occupy ∼one third of solar‐wind intervals, yet their appearance in the magnetosheath remains unclear. We use the AuburN Global hybrid CodE in three Dimension (ANGIE3D) to investigate their evolution as they cross Earth's bow shock. We find that the ropes are compressed in the ram direction but retain their transverse dimensions. During shock crossing, backstreaming ions from the trailing edge of the flux rope produce foreshock signatures. In the magnetosheath, significant parallel temperature enhancement occurs within a thin layer on the flux rope's surface. From these results we derive observational signatures to aid identification of interplanetary SFRs in the magnetosheath. These results advance our understanding of coherent‐structure evolution and identification in magnetosheath.

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.

The precipitation of tens to hundreds of keV electrons from Earth's magnetosphere plays a crucial role in magnetosphere‐ionosphere coupling, primarily driven by chorus wave scattering. Most existing simulations of electron precipitation rely on test particle models that neglect particle feedback on waves. However, both theoretical and observational studies indicate that the feedback from energetic electrons significantly influences chorus wave excitation and evolution. In this study, we quantify electron precipitation driven by chorus waves using self‐consistent simulations at L = 6 with typical magnetospheric plasma parameters. Electrons in the ∼10–200 keV range are precipitated, exhibiting energy‐dispersive characteristics. The precipitation intensity reaches ∼ , consistent with the typical values in observations. As a comparison, test particle simulations underestimate the precipitation intensity by nearly an order of magnitude. These results highlight the importance of self‐consistent simulations in quantifying electron precipitation and investigating wave‐particle interactions that modulate magnetospheric dynamics.