physics.space-ph

Electron-only reconnection (EREC) is a magnetic reconnection regime occurring within subion-scale current sheets (CSs), exhibiting only electron jets, without any ion outflows. EREC has been first observed in the Earth's magnetosheath, where its occurrence is linked to the small correlation length of magnetic fluctuations, limiting the growth of CSs to very large scales. On the other hand, the development of EREC in open systems with large magnetic correlation lengths, such as the solar wind (SW), remains an open question. To address this problem, we employ a large-scale 2D hybrid simulation with finite electron inertia, investigating the development of EREC driven by turbulence. By injecting energy at very large scales, we allow EREC to develop spontaneously due to the turbulent cascade, without any external small-scale forcing or imposed constraints on the turbulence correlation length. We find that EREC develops in our simulation via two distinct turbulence-driven mechanisms: (1) secondary EREC induced by the interaction of plasmoids in the outflows of large-scale ion-coupled reconnection; (2) EREC directly driven at subion scales by the electron Kelvin-Helmholtz instability in small-scale velocity shears. Furthermore, we perform a statistical analysis of CSs using the machine-learning clustering algorithm HDBSCAN, showing that subion-scale CSs capable of hosting EREC are dominant in our simulation. Our results suggest that EREC could occur even in large-scale space and astrophysical systems, like the SW, driven by secondary turbulent processes, potentially playing a key role in dissipating energy at kinetic scales.

Cosmic magnetic fields are typically inhomogeneous and often highly tangled due to large-scale plasma flows, turbulence, and instabilities. If the variations in the magnetic field occur on scales that are large compared to the gyro-radius of the plasma electrons, the electrons are primarily confined to gyro-centre trajectories along the field lines. Therefore, in-situ electron measurements help us map out the connectivity of the magnetic field in space plasmas. Gyro-centre drifts, wave-particle interactions, trapping, and cross-field diffusion are processes related to field inhomogeneities and fluctuations; they have the potential to modify or even disrupt the transport of electrons along field lines. We introduce the basic principles of electron transport in tangled magnetic fields and review the creation of tangled fields through turbulence and instabilities as well as the modulation of parallel electron transport through kinetic instabilities. We then describe trapping and de-trapping effects in inhomogeneous magnetic fields, as well as electron diffusion and energisation across the magnetic field. The transport of electrons in tangled fields results from a complex interplay of plasma processes that occur on a broad range of scales. A combination of in-situ plasma measurements, remote-sensing plasma observations, and plasma theory and simulations is required to resolve this contemporary challenge to the fields of heliophysics and astrophysics.

Electromagnetic whistler-mode waves are a natural emission in the outer radiation belt and the Earth's magnetotail. The resonant interaction of these waves and energetic electrons are responsible for electron acceleration and losses, thus coupling the magnetosphere and ionosphere. Near-equatorial spacecraft use search-coil magnetometers for whistler-mode wave measurements, and one of the largest (covering the longest period of time) dataset of such waves has been collected by the THEMIS mission operating in the near-Earth magnetosphere within 2008-2025. However, after 2017, the search-coil magnetometers on two THEMIS spacecraft, THEMIS E and D, experienced problems with their signal along the spacecraft spin axis and were only able to detect the spin plane components of the wave vector. This significantly reduces our ability to detect the total wave amplitude wave magnitudes and limits our ability to incorporate the THEMIS E, D datasets into investigation of whistler-mode waves. In this technical report, we propose and validate a technique for reconstruction of magnetic field spectral density for Fast Fourier transform data product collected during Fast-Survey mode hereafter referred to as the fff dataset collected by THEMIS E and D. We use measurements of the electric field instrument and cold plasma dispersion relation to evaluate the whistler-mode magnetic field spectral density. Verification of this technique by comparison with THEMIS A measurements (which retained their 3D measurement capability intact) confirms that restored magnetic field spectral density is within a factor of ~1.5 of the actually measured magnitudes.

The propagation of solar energetic particles (SEPs) in interplanetary space is modulated by solar wind turbulence, which significantly influences particle diffusion and energy evolution through scattering processes. Traditional analyses based on absolute flux measurements face inherent difficulties in disentangling source acceleration from subsequent transport, while temporal features such as onset and peak times are less affected and better suited for studying SEP transport. This study establishes a statistical relationship between the rise time of SEP events at different energies using multi-satellite observations at Earth and Mars. We use data from SOHO/ERNE and Tianwen-1/MEPA between November 2020 and March 2025, selecting 75 SEP events at 1 AU and 58 near Mars. For each energy range, onset times are determined by linear fitting, and peak times are extracted via a sliding median filter combined with Savitzky-Golay smoothing; the difference gives the SEP rise time. Comparing with the pure diffusion equation prediction, we examine the statistical behavior of rise time at Earth and Mars. Despite event selection uncertainties, SEP rise time follows a clear power-law relation with energy. The flatter power-law at Mars indicates weaker energy dependence with increasing solar distance. Using these empirical relations, we constrain the rigidity dependence of the parallel mean free path within the parallel diffusion model. Our results show that turbulence scattering at Mars approaches a rigidity-independent regime, reflecting turbulence evolution toward a dissipation-dominated state from Earth to Mars.

During the propagation of interplanetary coronal mass ejections (ICMEs), evolution of the ICME-driven shock along with interactions with other solar wind structures, planetary bodies, and general changes to their morphology can alter particle acceleration efficiency and transport effects at their associated shocks. While the underlying mechanisms for these processes have been studied, the connection between the radial evolution of the ICME-driven shock during propagation and resulting gradual Solar Energetic Particle (SEP)and Energetic Storm Particle (ESP) intensities, composition, and acceleration has yet to be fully understood. The current distributed array of spacecraft at varying heliocentric distances provides a welcome opportunity to statistically analyze the radial dependency of particle populations and acceleration mechanisms present at ICME-driven shocks. We compile a database of 39 multipoint ICME events from 2016-2023, which are observed in situ by at least two of the following spacecraft: Parker Solar Probe (PSP), Solar Orbiter, ACE, Wind, and STEREO-A. Using the magnetic field, plasma, and ion compositional data provided by these spacecraft, we derive both local shock and ESP spectral shape parameters. By comparing the changes in these parameters at different stages of ICME propagation, we analyze the connection between the evolution of the local shock conditions and the spectral shape. We find evidence to suggest a consistent increase in shock acceleration efficiency with heliocentric distance while the parent ICME is within 0.7 au, followed by a reduction in shock efficiency at further distances.

For the first time, this paper presents three very-near-Earth reconnection (VNERX) events observed within the same 12-hour-long storm main phase. The THEMIS inner probes observed the hallmarks of three episodes of tailward retreating x-lines positioned between magnetic local time (MLT) 23-24 and radial distance 12-13 Earth radii (RE). The events occurred within a thin current sheet, < 1 RE thick. Simultaneously, dispersionless energetic particle injections above 10s of keV and magnetic field dipolarizations were observed near and earthward of geosynchronous altitude by the KOMPSAT and Arase satellites. Arase observed earthward flow bursts at or below geosynchronous altitude via ExB enhancements, suggesting VNERX ejecta proceed below geosynchronous orbit. These observations demonstrate that VNERX events, which predominantly occur during the storm main phase, can be frequent and essential for driving injections that can effectively power the ring current. However, they can be observed only at the pre-midnight sector, close to the neutral sheet.

The electron fluxes at energies $E>$100\,keV are shown to be vanishing in the quiet time plasma sheet at geocentric distance of 60 Earth's radii (R$_E$) where the Moon traverses the magnetotail. Fluxes of energetic electrons up to relativistic energies were, however, observed during disturbed space weather conditions. In this paper, we study the data collected by the two lunar-orbiting Acceleration, Reconnection, Turbulence and Electrodynamics of Moon's Interaction with the Sun (ARTEMIS) spacecraft during their magnetotail traverses at two magnetic storm events. These observations allow us to compare plasma and field properties obtained at prior to storm and during the storm, including the storm recovery phase. We found that on the storms' recovery phases the average electron temperature increased by a factor of 4 compare to the pre-storm electron temperature. The ion temperature gain, however, did not increase a factor of 2. That leads to a decrease of ion to electron temperature ration to $\langle{T_i}/{T_e}\rangle\approx$3, in contrast to the pre-storm value of 7 to 9. We also found an increase in integral power of electrostatic fluctuations up to $\approx$2\,|mV/m|. Our observations suggest that the electrons were energized to energies $E>$100\,keV in the magnetotail. Although the exact mechanism of this energization remains unclear, we suggest that energization via continuous sporadic electron-only reconnection associated with electrostatic turbulence may be responsible for the anomalous electron energization.

We present case studies of two impulsive solar energetic electron (SEE) events during which particles at energies from 1-600 keV were detected by THEMIS-ARTEMIS orbiting the Moon, Wind at Earth's first Lagrange point, and (for one event) STEREO-A located at 1 AU, off the Sun-Earth line. The SEEs were initially highly anisotropic, traveling outward along the magnetic field with distinct energy-time dispersion. For one event, the spectra contained inverse velocity dispersion (IVD) signatures, whereby electrons at intermediate energies arrived to the spacecraft before those at higher energies. Similar features were recently discovered within 1 AU for energetic protons; this represents the first IVD detection for energetic electrons at Earth's orbital distance. During both events, a second beam of counter-streaming electrons was detected after a short time. Based on the time-delay in the detections at various energies, the path traveled by these counter-streaming electrons was on the order of 1-2 AU. We show that an interplanetary coronal mass ejection (ICME) passed the spacecraft a few days prior to the onset of each event and was located beyond 1 AU when the SEEs were detected, suggesting that the electrons were part of the same population, but reflected off the shock front of these precursor ICMEs. In the context of solar system exploration, this represents an unidentified hazard for astronaut safety beyond low-Earth orbit: although the initial phase of impulsive SEE events typically stream anti-Sunward, ICMEs located beyond Earth provide a mechanism for hazardous particles to travel Sunward during extreme events.

Plasma sheet bubbles, defined as entropy-depleted flux tubes, are widely regarded as an efficient mechanism for transporting plasma into the inner magnetosphere during geomagnetic storms. Equilibrium simulations using the Rice Convection Model (RCM-E) predict that bubbles can account for up to 60% of storm-time ring current energy during intense storms. However, global simulations and observations suggest a more moderate net contribution. In this study, we quantify the contribution of plasma sheet bubbles to ring current buildup using a Lagrangian particle backtracking technique applied to three idealized storm simulations conducted with the inertialized Rice Convection Model (RCM-I). A stratified ensemble of about 100,000 test particles, weighted by local plasma pressure and entropy, was traced backward in time to determine whether their energy originated from bubble injections, non-bubble plasma sheet transport, or pre-existing trapped populations. Our results show that bubble contributions increase with storm intensity but saturate near 40% of the total ring current energy inside R<6.6Re, even for strong storms (Dst about -180nT). The trapped population remains comparably important (about 40%), while non-bubble transport contributes about 15%. This saturation is notably lower than the 61% predicted by RCM-E and is attributed to inertial braking, which generates oscillatory flows and tailward return streams that remove approximately 40% of the inward bubble energy flux. When only newly transported plasma is considered, bubbles account for about 73% of the inward transport, consistent with global MHD and flux-based studies. These results reconcile equilibrium modeling, global simulations, and spacecraft observations by demonstrating that bubbles dominate inward transport but do not fully replace the resident ring current population due to inertial limitations.

We present a comprehensive statistical study of the radial evolution of solar wind turbulence near Mercury's orbit using long-term magnetic field measurements from the MESSENGER mission. Owing to Mercury's highly elliptical orbit and the spacecraft's repeated, extended residence in the upstream solar wind, the data set provides more than 17,000 hours of observations, enabling robust statistics across well-defined heliocentric distance intervals (0.31-0.47 au). We find that inertial-range spectral slopes remain close to -3/2 throughout Mercury's orbit, showing no significant radial evolution. Combined with low magnetic compressibility, this result indicates a stable, predominantly Alfvenic inertial-range cascade already established here. In contrast, kinetic-range spectral slopes exhibit clear radial evolution, becoming progressively shallower with increasing heliocentric distance, highlighting the greater sensitivity of kinetic-scale turbulence to heliocentric conditions. The ion-scale spectral break frequency decreases with distance in the spacecraft frame, while its normalized form increases relative to the local proton cyclotron frequency, demonstrating that the break is not tied to a single ion scale but reflects evolving local plasma conditions. Magnetic compressibility shows a similar frequency dependence at all distances, with a subtle radial enhancement of compressive fluctuations at kinetic scales. Autocorrelation analysis reveals strong anisotropy, with the correlation times of field-aligned magnetic fluctuations increasing with heliocentric distance, while those of perpendicular fluctuations remain shorter and nearly invariant. Together, these results demonstrate a clear scale-dependent radial evolution of solar wind turbulence near Mercury's orbit, providing new constraints on the development of kinetic processes in the inner heliosphere.

Measuring field-aligned currents (FACs) using magnetic field observations provides a powerful means to probe the multi-scale interactions between the magnetosphere, ionosphere and thermosphere. In this study, we apply the curlometer technique to Swarm spacecraft observations and to simulations of the coupled magnetosphere-ionosphere system. We begin by correlating current density curlometer estimates derived from Swarm tetrahedra with varying spatial scales and barycentre locations. This confirms an apparent departure from stationarity for FACs at spatio-temporal scales below 100 km where measurements appear highly uncorrelated. We then analyse simulated magnetic perturbations, where true four-point measurements are available. This shows how, even at meso-scales of hundreds of kilometres, time-shifted FAC estimates can diverge significantly from this ground truth. In both observational and simulated data we find poor tetrahedral configurations can produce spurious perpendicular currents due to numerical instability in the inversion process. This can be mitigated using appropriate quality metrics and high-quality FAC reconstructions still achieved with a tetrahedral face well-aligned to the local magnetic field. These results highlight the dynamic nature of FACs at large as well as small scales, and underscore the substantial advantages of true four-point observations for their accurate analysis.

Very-low-Earth orbit drag uncertainty quantification in the rarefied/transitional Knudsen-number regime requires estimating not only the mean drag coefficient but also higher-order moments under atmospheric variability, which becomes prohibitively expensive when high-fidelity kinetic solvers are required. This work develops a multi-fidelity Monte Carlo (MFMC) estimator for the drag coefficient using a DSMC solver (PICLas) as the high-fidelity model and two free-molecular panel-method variants (ADBSat with Sentman and Cercignani-Lampis-Lord (CLL) gas-surface interaction models) as low-fidelity control variates. We treat E[C_D] and E[C_D^2] as the primary estimation targets and form the physically induced variance only afterwards via Var(C_D)=E[C_D^2]-(E[C_D])^2. High-fidelity reference moments are obtained from long DSMC sequences using objective convergence criteria based on sliding-window stability and 95% confidence intervals. The MFMC implementation is first numerically verified on an analytic toy model with closed-form moments, then assessed on a canonical CubeSat geometry (validation) and on SOAR, GOCE, and CHAMP configurations (verification) under MSIS-derived thermospheric variability and angle-of-attack uncertainty. When low-fidelity correlations are high for both C_D and C_D^2, MFMC reduces the relative RMSE of E[C_D] and E[C_D^2] by factors of several at matched high-fidelity-equivalent cost; improvements for Var(C_D) remain more case-dependent due to cancellation sensitivity. Overall, the study identifies practical drivers (moment correlations, cost ratios, and weight stability) that govern when panel models serve as effective control variates for DSMC-based drag uncertainty quantification.

Inferring electric potentials from electron phase space density measurements in the lunar wake is complicated by two challenges: the asymmetry between the sunward and anti-sunward sides of the wake driven by the solar wind strahl, and the presence of ion acoustic shocks in the central wake. We develop the Hamiltonian inversion method, which infers the full spatial electric potential profile by exploiting the quasi-static Vlasov equilibrium condition $f = f(H)$, where $H$ is the electron Hamiltonian. The method addresses both challenges through a domain-decomposition strategy: on the two sides of the wake the potential is inferred independently by minimizing the misfit between the observed phase space density and a self-consistently reconstructed $f_\mathrm{interp}(\tilde{H})$, while in the central wake where flat-top trapped electron distributions are present the potential is inferred directly from the flat-top width. We validate the method against particle-in-cell simulation data at two evolutionary stages of the lunar wake: an early stage where strahl asymmetry is strong but no shocks have formed, and a later stage where ion acoustic shocks and flat-top distributions are present. We then apply the method to two ARTEMIS lunar wake crossings at the same evolutionary stages, inferring normalized potential drops of $e\Delta\varphi/T_e \sim 15$ and $\sim 5$ respectively and capturing shock-associated potential enhancements in the central wake. The method is broadly applicable to plasma environments where electrons are in quasi-static equilibrium with a field-aligned electric potential.

The plasma environment around Mars is highly variable because it is strongly influenced by the solar wind. Accurate identification of plasma regions around Mars is important for the community studying solar wind-Mars interactions, region-specific plasma processes, and atmospheric escape. In this study, we develop a machine-learning-based classifier to automatically identify three key plasma regions--solar wind, magnetosheath, and induced magnetosphere--using only ion omnidirectional energy spectra measured by the MAVEN Solar Wind Ion Analyzer (SWIA). Two neural network architectures are evaluated: a multilayer perceptron (MLP) and a convolutional neural network (CNN) that incorporates short temporal sequences. Our results show that the CNN can reliably distinguish the three plasma regions, whereas the MLP struggles to separate the solar wind and magnetosheath. Therefore, the CNN-based approach provides an efficient and accurate framework for large-scale plasma region identification at Mars and can be readily applied to future planetary missions.

We present observations from two consecutive TRACERS-2 orbits through the northern low-altitude cusp. During the first crossing, TRACERS-2 observed reversed cusp ion dispersion and sunward convection, consistent with magnetopause reconnection tailward of the cusp during this northward IMF interval. Simultaneous THEMIS-D observations at the equatorial magnetopause show heated magnetosheath plasma captured on closed field lines, with similar particle spectra as in in the low-altitude cusp, indicating that reconnection indeed occurred tailward of the cusp and in both hemispheres. When TRACERS-2 traversed the northern cusp again, 95 minutes later, the IMF was dominated by a negative BX component. Despite the different IMF conditions, TRACERS-2 recorded nearly the same cusp signatures as before, i.e., reversed ion dispersion and sunward convection. The observations indicate that tailward-of-cusp reconnection can occur for both northward and BX-dominated IMF and that these distinct IMF geometries can produce remarkably similar plasma and field signatures in the low-altitude cusp.

We report the properties of electron distributions and turbulence during a Coronal Mass Ejection (CME) in April 2023 observed by Magnetospheric Multiscale (MMS). The CME exhibits a clear sheath and magnetic cloud (MC), and within the MC, the solar wind becomes sub-Alfv\'enic for two hours. We investigate plasma and turbulence properties of the sub-Alfv\'enic CME wind and compare them with those in the super-Alfv\'enic solar wind in the MC and CME sheath. Electrons within the sub-Alfv\'enic MC show significantly higher temperatures than those in the CME sheath and the super-Alfv\'enic MC, with their one-dimensional distributions revealing super-thermal tail and a depletion in electron populations between 15-50 eV. Within the CME sheath, isolated regions of electron heating are observed, where parallel energy flux is enhanced up to ~1 keV. Magnetic field fluctuations within the sub-Alfv\'enic MC interval exhibit negligible cross helicity and steeper-than-Kolmogorov scaling in the inertial range, with no clear spectral break. These fluctuations also show reduced intermittency at ion and sub-ion scales, emerging intermittency at electron scales, and weak magnetic compressibility. Together, these observations point to the presence of weak magnetohydrodynamic (MHD) turbulence within the sub-Alfv\'enic MC, resembling conditions commonly observed in planetary magnetospheres such as Jupiter's.

Ion pickup at the outer planets' active moons is a fundamental plasma process in which newly ionized particles from moon exospheres interact with the ambient corotating plasma and are accelerated to match the background flow. Spacecraft observations have revealed intense electromagnetic wave activity commonly attributed to this pickup process. Here we investigate ion pickup using hybrid-kinetic simulations in which ions are treated kinetically while electrons are modeled as a massless fluid. In the moon's rest frame, ambient ions initially stream perpendicular to the background magnetic field at the corotation velocity, creating a nongyrotropic velocity distribution with two ion populations clustered at opposite gyrophases. Within a few ion gyroperiods, this configuration simultaneously excites transverse magnetic perturbations associated with electromagnetic ion cyclotron waves and compressional perturbations associated with mirror-mode and ion Bernstein waves, reaching amplitudes of several percent of the background field strength. Using field-particle correlation analysis, we quantify the energy transfer between waves and particles and demonstrate how these perturbations scatter ions in velocity space, efficiently incorporating newly created ions into the background plasma and leading to isotropization in both gyrophase and pitch angle. These results provide a kinetic framework for understanding pickup-driven wave-particle interactions and offer guidance for interpreting in situ measurements at active moons throughout the outer solar system.

Understanding solar wind variability throughout the heliosphere is essential for fundamental space physics and future exploration of the Moon and Mars. The Mars Atmosphere and Volatile EvolutioN (MAVEN) spacecraft has provided upstream solar wind measurements at Mars spanning Solar Cycles 24 and 25, enabling a statistical investigation of solar wind regimes at this heliocentric distance. In this work, we apply an unsupervised machine-learning framework combining Principal Component Analysis and K-Means clustering to a normalized, multi-dimensional solar wind dataset to identify recurrent solar wind regimes in a physically interpretable, data-driven manner. The resulting classification reveals distinct slow, fast, intermediate, and compressed solar wind regimes whose relative occurrence and temporal organization are strongly modulated by solar activity. This manuscript is part of the Heliophysics Summer School Machine Learning Special Collection.

Interplanetary (IP) shocks efficiently modify the proton temperature anisotropy of the solar wind. Analyzing ~800 IP shocks observed by the Wind spacecraft from 1997-2024, we present a statistical study of upstream and downstream proton temperature anisotropy and its dependence on shock geometry, compression, and distance from the shock. We find that (1) quasi-perpendicular shocks produce a pronounced enhancement of perpendicular temperature downstream (Tperp > Tpara), whereas parallel shocks remain near isotropic downstream due to typically stronger upstream Tpara; (2) comparisons with the Chew-Goldberger-Low (CGL) double-adiabatic model reveal geometry-dependent deviations. CGL overestimates downstream perpendicular heating and underestimates parallel heating at quasi-perpendicular shocks, with the opposite trend at quasi-parallel shocks, highlighting the importance of non-adiabatic processes beyond simple compression; (3) Shock-driven anisotropy is strongly localized near the shock and gradually relaxes toward typical solar wind conditions farther downstream as the shock's influence diminishes; and (4) downstream anisotropy is regulated by kinetic instabilities, with quasi-perpendicular shocks constrained by proton cyclotron and mirror instabilities and quasi-parallel shocks limited by the parallel firehose instability. Together, these results show that the evolution of temperature anisotropy at interplanetary shocks is controlled by shock geometry, localized processes, and instability driven regulation.