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We investigate the use of a novel two-spacecraft Liouville-mapping method using data from the Magnetospheric Multiscale mission to determine local magnetic-field-aligned (parallel) electric fields in the Earth's magnetotail. The method detects the presence of local acceleration potentials by mapping phase-space density between electron velocity distribution functions from two field-aligned spacecraft upstream and downstream of acceleration regions. Applying the method to a magnetic reconnection event, we find that local parallel electric fields near the current sheet (CS) center are, on average, directed away from the center, resulting from the need to maintain quasi-neutrality across the CS. Despite significant measurement uncertainties, we find that the local acceleration potentials are smaller than the total acceleration potential, typically 1%–2% on average and up to 9% for individual measurements. This indicates that many potential drops, over distances much larger than the spacecraft separations, contribute to the net work done on electrons by parallel electric fields.

The reconfiguration of the magnetotail current sheet during substorms often includes the formation of a thin current sheet (TCS) with a strong magnetic field line tension force. This force cannot be balanced by isotropic plasma pressure gradients, and force balance in such a TCS requires ion anisotropy and/or agyrotropy of plasma pressure. A statistical investigation of these plasma properties in the magnetotail is challenging because a significant contribution to ion anisotropy or agyrotropy stems from the suprathermal ion population, which often has energies beyond the upper limit of electrostatic analyzer measurements. In this study, we compare ion measurements from Cluster, MMS, and THEMIS to investigate the ion anisotropy in the magnetotail current sheet. We show that the central region of the magnetotail is characterized by transverse anisotropy of suprathermal ions, whereas subthermal field-aligned anisotropic ions populate the magnetotail current sheet boundaries. Our results indicate that different ion populations (subthermal, thermal, and suprathermal ions) exhibit distinct behaviors within the current sheet, showing that different combinations of these populations contribute differently to the stability of current sheets.

In collisionless plasmas, relaxation of the deviations of ion velocity distribution functions (VDFs) from local thermodynamic equilibrium occurs through particle interactions with electromagnetic fields. In particular, in the Earth's magnetotail, the deviations of the ion VDFs, typically consisting of multiple components, from the equilibrium must be limited to maintain stability of the current sheet. Curvature scattering is a leading candidate mechanism to limit such deviations, but its role remains insufficiently understood. We investigate the limits of ion temperature anisotropy in a magnetotail-like configuration by modeling a quasi-1D current sheet with a finite magnetic field curvature and three ion populations. We derive analytical thresholds for anisotropy based on current sheet stability and validate against spacecraft observations and numerical simulations. Our findings demonstrate that curvature scattering imposes limits on ion anisotropies, thereby maintaining the stability of the current sheet.

Earthward bursty bulk flows (BBFs) in the central plasma sheet are frequently accompanied by dipolarization fronts, and together they account for a large fraction of the energy and momentum transported from the magnetotail to the high-latitude ionosphere through field-aligned currents (FACs). On 7 December 2023, between 18:00 and 22:00 UT, six BBFs-intervals were detected by THEMIS, with clear dipolarization signatures in the last four intervals. These flows map to the pre-midnight magnetic local time sector over northern Fennoscandia, where the ionospheric response is examined using horizontal equivalent current density derived from the IMAGE magnetometer network and auroral emissions from all-sky cameras. At the beginning of the interval, the ionospheric current system exhibits a stable Harang discontinuity, which becomes progressively distorted after each BBF and ultimately evolves into a clear westward electrojet. The first two dipolarizing intervals are accompanied by auroral pseudo-breakups, whereas the final two display an auroral breakup and poleward expansion characteristic of substorms. A key interval occurs during the fourth BBF, whose mapped footpoint intersects the Swarm A/C orbit and reveals a strongly localized pair of upward and downward FACs spanning 35-50 km in latitude, with the upward current colocated with a discrete auroral arc. Throughout the BBF activity, the horizontal ionospheric current density reorganizes rapidly around the mapped footpoints, and the auroral arc intensifies simultaneously with THEMIS enhancements of 9-20 keV electron fluxes. These observations demonstrate that localized magnetotail flows can drive fast, structured, and cumulative modifications of the ionospheric electrodynamics, emphasizing the importance of multi-point measurements for resolving the temporal evolution of magnetosphere-ionosphere coupling during BBFs.

Non-Maxwellian electron velocity distributions (eVDFs) are ubiquitous in collisionless plasmas. For example, various types of non-Maxwellian eVDFs exist in magnetic reconnection jets in the Earth's magnetotail. At thermal energies, eVDF can be flat-topped due to electron trapping associated with magnetic reconnection. However, the occurrence of such eVDFs in magnetotail reconnection remains largely unconstrained. Here, we statistically investigate flat-top eVDFs in fast plasma jets in the magnetotail using a new method for classifying eVDFs. We show that at least 7% of the eVDFs in the jets are flat-tops. In addition, we find that most jets exhibit flat-top eVDFs, indicating that this signature of parallel acceleration and electron streaming is characteristic of the jets. We find that these flat-top eVDFs are localized within an ion-inertial-length-scale region near the edges of the current sheet and the ion diffusion region. Our results highlight the importance of flat-top eVDFs in non-local thermodynamic equilibrium collisionless plasmas.

Mercury's magnetosphere is an extremely dynamic environment, where substorm-like activity, including dipolarization fronts and particle acceleration, periodically disrupts the stability and radial extent of the closed field line region. The common narrowband ∼1 Hz waves at Mercury, near the local proton gyrofrequency are preferentially observed on closed field lines during quieter intervals, but their relationship to magnetospheric dynamics remains poorly understood. Here, we combine MESSENGER data sets of ∼1 Hz wave activity and dipolarization fronts to investigate whether newly closed field lines can sustain wave growth. We find that ∼1 Hz waves on the nightside are systematically absent during orbits with dipolarization intervals and typically confined within 1.3 R_M, compared to greater downtail extents during quiet intervals. The results indicate that ∼1 Hz waves require long-lived, stable closed field lines to grow; therefore, when they are observed, they can serve as diagnostics of Mercury's magnetospheric state, establishing a clear connection between field-line history, magnetospheric dynamics, and wave occurrence in Mercury's magnetotail.

Magnetic reconnection jets in Earth's magnetotail are regions of electron heating and acceleration. They are often associated with dipolarization fronts (DFs), which are characterized by sharp increases in the northward magnetic field component B z . Electrons can be accelerated at DFs mainly through the betatron and Fermi mechanisms. We perform a statistical study of betatron and Fermi electron rate of energy change at Earthward-propagating DFs observed by the Magnetospheric Multiscale (MMS) spacecraft. Our results show that betatron acceleration is dominant ahead of DFs, where suprathermal electron fluxes perpendicular to the magnetic field are detected. Up to ∼ 20 ion inertial lengths inside the outflow, Fermi acceleration acts on the low-density electron population, potentially driving them to high energies (∼100 k e V ). Spatially in the magnetotail, betatron acceleration is statistically larger near the most probable X-line location, while Fermi-dominated events are observed closer to the Earth.

Magnetic reconnection is a fundamental plasma process responsible for the sometimes explosive release of magnetic energy in space and laboratory plasmas. Inside the diffusion regions of magnetic reconnection, the plasma becomes demagnetized and decouples from the magnetic field, enabling the change in magnetic topology necessary to power the energy release over larger scales. Since it was launched in 2015, the Magnetospheric MultiScale (MMS) mission has significantly advanced the understanding of the particle dynamics key to magnetic reconnection by providing high-resolution, in-situ measurements able to resolve ion and electron kinetic scales, i.e. a fraction of a gyroradius, that have confirmed theoretical predictions, revealed new phenomena, and refined existing models. These breakthroughs are critical for understanding not only space plasmas but also laboratory and astrophysical plasmas where magnetic reconnection occurs. In this work, we review the ion and electron dynamics occurring within the diffusion regions, in the inflow, along the separatrices, and downstream of the diffusion regions, in different reconnection configurations: symmetric, asymmetric, antiparallel, and guide field reconnection.

We investigate electron heating by magnetic-field-aligned electric fields (EII) during antiparallel magnetic reconnection in the Earth's magnetotail. Using a statistical sample of 140 reconnection outflows, we infer the acceleration potential associated with EII from the shape of the electron velocity distribution functions. We show that heating by EII in the reconnection outflow can reach up to 10 times the inflow electron temperature. We demonstrate that the magnitude of the acceleration potential scales with the inflow Alfve<acute accent>n and electron thermal speeds to maintain quasineutrality in the reconnection region. Our results suggest that, as the inflow plasma parameter /3e infinity increases, EII becomes increasingly important to the ion-to-electron energy partition associated with magnetic reconnection.

Electron energisation by magnetic reconnection has historically been studied in the Lagrangian guiding-centre framework. Insights from such studies include that Fermi acceleration in magnetic islands can accelerate electrons to high energies. An alternative Eulerian fluid formulation of electron energisation was recently used to study electron energisation during magnetic reconnection in the absence of magnetic islands. Here, we use particle-in-cell simulations to compare the Eulerian and Lagrangian models of electron energisation in a set-up where reconnection leads to magnetic island formation. We find the largest energisation at the edges of magnetic islands. There, energisation related to the diamagnetic drift dominates in the Eulerian model, while the Fermi related term dominates in the Lagrangian model. The models predict significantly different energisation rates locally. A better agreement is found after integrating over the simulation domain. We show that strong magnetic curvature can break the magnetic moment conservation assumed by the Lagrangian model, leading to erroneous results. The Eulerian fluid model is a complete fluid description and accurately models bulk energisation. However, local measurements of its constituent energisation terms need not reflect locations where plasma is heated or accelerated. The Lagrangian guiding centre model can accurately describe the energisation of particles, but it cannot describe the evolution of the fluid energy. We conclude that while both models can be valid, they describe two fundamentally different quantities, and care should be taken when choosing which model to use.