Yuxi Chen

We investigate the onset of magnetic reconnection in Earth's magnetotail using a coupled magnetohydrodynamic with embedded particle‐in‐cell (MHD‐EPIC) model, which self‐consistently links global magnetospheric dynamics to electron‐scale kinetic processes. Unlike standalone kinetic simulations with idealized boundary conditions, reconnection in this framework is driven by large‐scale magnetotail stretching and plasma convection. Simulations are performed in two spatial dimensions to enable a comprehensive study of grid convergence and the effects of various kinetic scaling factors and mass ratios. A systematic convergence study demonstrates that resolving the electron inertial length with approximately 1.5 grid cells is required to achieve grid‐independent reconnection onset timing and current sheet structure. The current sheet thins to a universal thickness of at onset, indicating that reconnection initiation is controlled by electron‐scale physics. In contrast, both ideal and Hall MHD simulations fail to converge, with reconnection properties remaining dependent on grid resolution. The results in this study establish quantitative criteria for resolving kinetic scales in global simulations and demonstrate that reconnection onset in the magnetotail is fundamentally governed by electron‐scale processes embedded within global dynamics.

The ion foreshock is the region upstream of Earth's bow shock where magnetic field lines connect to the quasi‐parallel shock surface. Within this region, there exist a variety of backstreaming ion populations from the shock ramp that can generate ultra‐low frequency (ULF) waves through wave‐particle interactions. In this work, we use data from the Magnetospheric Multiscale (MMS) mission to study such ULF waves and backstreaming ions during a prolonged interval of above‐average solar wind helium abundance, embedded in a multi‐day period of strong solar activity driven by a coronal mass ejection. When interplanetary magnetic field orientations positioned MMS within the ion foreshock, the spacecraft captured the complete evolutionary sequence of the backstreaming ion velocity distributions: the initial formation of a reflected ion beam, followed by phase bunching and generation of coherent ULF waves, and eventual thermalization and randomization in velocity space to form diffuse ions. Intervals with elevated energetic He++ flux exhibited broadened and frequency‐downshifted wave spectra, consistent with heavy‐ion cyclotron resonance effects. The unusually rapid beam‐to‐diffuse transitions observed near the foreshock boundary likely result from the combined effects of multi‐species wave‐particle interactions and higher backstreaming ion densities during this active interval. These findings underscore the need for simulations and modeling that incorporate multi‐species effects.

Magnetic reconnection plays a fundamental role in transporting energy, momentum, and plasma from the solar wind to the magnetosphere‐ionosphere system. During an interplanetary magnetic field (IMF) southward turning, reconnection forms at localized magnetopause X‐lines that subsequently expand azimuthally. However, the physical mechanisms governing the X‐line extent remain unclear. In this study, we investigate the evolution of the magnetopause X‐line and the associated dayside ionospheric convection during an IMF southward turning using a global magnetohydrodynamic (MHD) model with adaptively embedded particle‐in‐cell physics (MHD‐AEPIC), and compare the results with an ideal MHD simulation and Super Dual Auroral Radar Network (SuperDARN) observations. The ideal MHD simulation produces strong and narrow post‐noon flows associated with a smooth, continuous X‐line. The MHD‐AEPIC simulation presents multiple dynamic X‐lines, leading to broader and weaker ionospheric flows that peak in the pre‐noon region, which is consistent with the SuperDARN observation. Field‐line tracing demonstrates that ionospheric flows are more consistent with the magnetopause off‐equatorial flows, while the flows in the X‐line region exhibit azimuthally wider and more intermittent flow signatures. A force analysis shows that the poleward acceleration is dominated by the force, with additional modulation by upstream magnetosheath flows. The X‐line spreading is most consistent with the Doppler‐shifted Alfvén speed along the guide field in the inertial flow frame. Transient magnetic flux ropes introduce short‐lived distortion of the magnetopause off‐equatorial flows. These results demonstrate that kinetic‐scale physics is essential for capturing the meso‐scale structure, extent, and asymmetry of dayside reconnection‐driven ionospheric convection.

The interaction between the solar wind and the terrestrial magnetosphere is a complex, multiscale process where global‐scale dynamics are fundamentally regulated by microscopic kinetic physics. We present a global two‐dimensional semi‐implicit particle‐in‐cell (PIC) simulation of the Earth's dayside magnetosphere using the FLEKS model, bridging the gap between fluid‐scale magnetohydrodynamics and kinetic‐scale plasma processes. The interaction of the solar wind with the terrestrial dipole is modeled with fully kinetic ions and electrons (mass ratio ), capturing the self‐consistent formation of the foreshock, bow shock, and magnetosheath. The system evolves from an initial MHD state to a complex kinetic equilibrium characterized by a reforming quasi‐parallel shock and a granular magnetosheath. We identify a causal link between foreshock‐generated ultra‐low frequency (ULF) waves, which propagate downstream, and the spatial organization of the magnetosheath, where anisotropy‐driven Alfvén ion cyclotron and mirror‐mode waves coexist with broadband magnetic turbulence. Particle tracking reveals that while protons are energized via a combination of Fermi acceleration and shock drift acceleration in the foreshock, backstreaming electrons are predominantly leakage particles from the thermalized magnetosheath. These results demonstrate that semi‐implicit PIC simulations can effectively resolve cross‐scale coupling mechanisms, such as modification of global shock geometry by kinetic instabilities, providing a comprehensive view of the collisionless dayside solar wind‐magnetosphere interaction.

The shape of the heliosphere, regarded as comet-like since the 1960s, has recently been the subject of intense debate in the last decade. There is disagreement as to whether the heliospheric tail extends to ∼10,000 au in a comet-like shape or if it is short (∼400 au) with a split. Energetic neutral atom (ENA) maps from Cassini/Ion and Neutral Camera (INCA) at energies from 5.2 to 13.5 keV revealed a global structure extending from the nose to the heliospheric tail, known as the Belt, whose origin has remained largely unexplored. Here, we use a state-of-the-art multi-ion magnetohydrodynamic model and a novel reconnection simulation to establish that the Belt structure is consistent with a split-tail heliosphere but not with a comet-like heliosphere. In a split-tail heliosphere, there is a region of low β (the ratio of thermal to magnetic pressure) in the downwind direction close to the heliopause. Direct simulations of this region reveal that magnetic reconnection is strong and drives the energetic particles that produce the >5.2 keV ENAs measured by INCA in the low-latitude portion of the Belt. Since the comet-like heliosphere does not produce this low- β region and the resultant reconnection-driven mechanism for the >5.2 keV ENAs, our results indicate that the INCA observations are inconsistent with a comet-like heliosphere. Further, these simulations and analysis establish for the first time that magnetic reconnection in the complex magnetic fields, expected in astrospheres across the Universe, is likely to be a source of energetic particles and radiation.

Mercury's small magnetosphere and strong solar wind driving result in the rapid formation of planetward and tailward‐moving flux ropes (FRs) and dipolarization fronts (DFs) in its magnetotail. These are characterized by large variations in the dipole‐aligned magnetic field component over s timescales and are often associated with fast planetward or tailward plasma flows. At present, the dynamic evolution of DFs and FRs and the relationship between them remains poorly understood at Mercury. To contextualize single‐point observations of these events by the MESSENGER spacecraft, we present coupled fluid‐kinetic simulations of Mercury's magnetosphere using the Magnetohydrodynamics with Adaptively Embedded Particle‐in‐Cell (MHD‐AEPIC) code. Driven by steady southward interplanetary magnetic field and nominal solar wind conditions about aphelion, the simulation produces recurrent DFs in the tail with peak occurrence rates of DFs per minute and jumps in magnetic field ranging from nT, mirroring those observed by MESSENGER. By tracking the 3D, time‐resolved propagation of the simulated DFs, we find that of events originate directly through reconnection at the nearest x‐line to the planet, while the remainder originate from FRs that undergo secondary reconnection or “re‐reconnection” closer to the planet to create DF‐like signatures. Our results suggest that this secondary reconnection process leads to localized heating of electrons along the reconnecting flux tube up to temperatures of 4 keV, which may help account for the suprathermal electrons observed in Mercury's low‐altitude current sheet.

Spacecraft measurements of Mercury indicate that it has a core dynamo with a surface field of 200–800 nT. These data also indicate that the northern hemisphere crust contains remanent magnetization likely produced by an ancient magnetic field. The inferred magnetization intensity is consistent with a wide range of paleofield strengths (0.2–50 μT), possibly indicating that Mercury once had a dynamo field much stronger than today. Recent modeling of ancient impacts on the Moon has demonstrated that plasma generated during basin‐formation can transiently amplify a planetary dynamo field near the surface. Simultaneously, impact‐induced pressure waves can then record these fields as a crustal shock remanent magnetization (SRM). Here, we present impact hydrocode and magnetohydrodynamic simulations of a Caloris‐sized basin (∼1,550 km diameter) formation event. Our results demonstrate that the ancient magnetospheric field (∼0.5–0.9 μT) created by the interaction of the ancient interplanetary magnetic field and Mercury's dynamo field can be amplified by the plasma up to ∼13 μT and, via impact pressure waves, recorded as SRM at the basin antipode. Such magnetization could produce ∼5 nT crustal fields at 20‐km altitude antipodal to Caloris detectable by future spacecraft like BepiColombo. Furthermore, impacts in the southern hemisphere that formed ∼1,000 km diameter basins (e.g., Andal‐Coleridge, Matisse‐Repin, Eitkou‐Milton, and Sadi‐Scopus) could impart crustal magnetization in the northern hemisphere, contributing to the overall remanent field measured by MESSENGER. Overall, the impact plasma amplification process can contribute to crustal magnetization on airless bodies and should be considered when reconstructing dynamo history from crustal anomaly measurements.

The interaction between the solar wind and the terrestrial magnetosphere is a complex, multiscale process where global-scale dynamics are fundamentally regulated by microscopic kinetic physics. We present a global two-dimensional semi-implicit particle-in-cell (PIC) simulation of the Earth’s dayside magnetosphere using the FLEKS model, bridging the gap between fluid-scale magnetohydrodynamics and kinetic-scale plasma processes. The interaction of the solar wind with the terrestrial dipole is modeled with fully kinetic ions and electrons (mass ratio $m_i/m_e = 25$), capturing the self-consistent formation of the foreshock, bow shock, and magnetosheath. The system evolves from an initial MHD state to a complex kinetic equilibrium characterized by a reforming quasi-parallel shock and a granular magnetosheath. We identify a causal link between foreshock-generated ultra-low frequency (ULF) waves, which propagate downstream, and the spatial organization of the magnetosheath, where anisotropy-driven Alfvén ion cyclotron and mirror-mode waves coexist with broadband magnetic turbulence. Particle tracking reveals that while protons are energized via a combination of Fermi acceleration and shock drift acceleration in the foreshock, backstreaming electrons are predominantly leakage particles from the thermalized magnetosheath. These results demonstrate that semi-implicit PIC simulations can effectively resolve cross-scale coupling mechanisms, such as modification of global shock geometry by kinetic instabilities, providing a comprehensive view of the collisionless dayside solar wind-magnetosphere interaction.

This study investigates the applicability of the semi-implicit particle-in-cell code FLexible Exascale Kinetic Simulator ( FLEKS ) to heliospheric shock simulations. We examine one- and two-dimensional local planar shock simulations, initialized using MHD states with upstream conditions representative of plasmas in the hypersonic, β ∼ 1 regime, for both quasi-perpendicular and quasi-parallel configurations. The refined algorithm in FLEKS proves robust, enabling accurate shock simulations with a grid resolution on the order of the electron inertial length d _e . Our simulations successfully capture key shock features, including shock structures (foot, ramp, overshoot, and undershoot), upstream and downstream waves (fast magnetosonic, whistler, Alfvén ion-cyclotron, and mirror modes), and non-Maxwellian particle distributions. Crucially, we find that at least two spatial dimensions are critical for accurately reproducing downstream-wave physics in quasi-perpendicular shocks and capturing the complex dynamics of quasi-parallel shocks, including surface rippling, shocklets, short, large-amplitude magnetic structures, magnetic reconnection, and jets. Furthermore, our parameter studies demonstrate the impact of mass ratio and grid resolution on shock physics. This work provides valuable guidance for selecting appropriate physical and numerical parameters for shock simulations using a semi-implicit PIC method, paving the way for incorporating kinetic shock processes into large-scale collisionless plasma simulations with the MHD-AEPIC model.