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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.

Context. Spectral anisotropy is a ubiquitous characteristic of turbulence in magnetized space plasma environments, as it occurs spontaneously due to the effect of a background magnetic field. This anisotropy significantly affects the turbulent energy cascade and modifies the scale-to-scale balance between different terms in the von K & aacute;rm & aacute;n-Howarth equation.

Aims. We investigate the effect of the induced lag-space anisotropy on the determination of the scale-dependent energy transfer rate.

Methods. We compared traditional single-spacecraft and novel multispacecraft approaches using hybrid particle-in-cell simulations and in situ observations from the Magnetospheric Multiscale (MMS) mission in the Earth's magnetosheath.

Results. We show that the isotropy assumption, which is required for single-spacecraft measurements, leads to inaccuracies in the estimation of the turbulent cascade rate. On the other hand, the novel lag-polyhedra derivative ensemble method, which is specifically designed for the next generation of multipoint multiscale missions such as HelioSwarm and Plasma Observatory, can capture the anisotropy of the turbulent cascade.

Conclusions. These findings are used to interpret observations from the MMS mission in the Earth's magnetosheath. They highlight the importance of accounting for anisotropy in space plasma diagnostics.

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.

We report multipoint observations of a merger between a Short Large-Amplitude Magnetic Structure (SLAMS) and 1-Hz whistler wave packets upstream of Earth's quasi-parallel bow shock. Using high-resolution data from the Magnetospheric Multiscale (MMS) mission in a string-of-pearls configuration, we track the SLAMS as it evolves, exhibiting enhanced magnetic field strength, increasing spatial scale, variable ion reflection at the ramp, and increasingly complex morphology. The whistler wave packets show distinct propagation properties, propagating at a different velocity from the SLAMS and approaching it from upstream. Eventually, the SLAMS merges with the wave packets, altering its structure and forming a new ramp characterized by an intensified magnetic field and plasma density. These observations demonstrate how SLAMS and wave packets can coexist and interact, leading to changes in local plasma conditions and revealing the complexity and dynamic nature of bow shock transitions.

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.

The electron diffusion region is central to NASA's Magnetospheric Multiscale (MMS) mission to understand collisionless magnetic reconnection, the plasma physics phenomenon crucial to triggering the explosive energy release of solar flares, powering auroras generated in planetary magnetospheres, and driving sawtooth crashes in laboratory fusion devices. Inside the diffusion region, electron velocity distributions exhibit highly-structured velocity-space signatures critical for elucidating the kinetic mechanisms fueling reconnection. Recent multi-spacecraft analysis techniques enabled observational study of the spatial gradient in the electron velocity distribution, which has been reported in electron-scale current layers to explain the kinetic origins of electron pressure gradients. However, electron gradient distributions have not yet been investigated inside the reconnection diffusion region. In this work, we discover that electron gradient distributions exhibit a smile-shaped velocity-space structure in the electron diffusion region of asymmetric magnetic reconnection at Earth's magnetopause. Characterizing the nature and prevalence of these smile-shaped electron gradient distributions offers a kinetic perspective into how electrons spatially evolve to provide the net electron pressure divergence that self-consistently supports non-ideal electric fields in the electron diffusion region of magnetopause reconnection. These results are relevant to space, astrophysical, and laboratory plasma communities working to understand the long-standing mystery of collisionless magnetic reconnection.

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.

Global numerical simulations are valuable tools for understanding the Sun-Earth interaction as they provide a more thorough picture of the system when compared to typically sparse observations. Yet, comparisons of these global models against observations is complicated by the inherent uncertainty that the observed phenomena occurred at the same time and location in the simulation domain. Therefore, aggregate measures of the Sun-Earth interaction, such as activity indices (e.g., AL, AU, Dst, cross polar-cap potential), are often used, with the downside of missing important underlying physical processes, especially at the mesoscales. Metrics or parameters that provide deeper insight into the physics but do not rely precisely on the location of the observations are therefore needed. Data assimilative models can be used to reconstruct high-latitude ionospheric electrodynamic quantities using sparse observations, providing a new avenue for data-model comparisons. We demonstrate this new approach to data-model comparisons by assimilating global simulation output from Multiscale Atmosphere-Geospace Environment (MAGE) into Local mapping of the polar ionospheric electrodynamics (Lompe). We directly compare and find good agreement between the simulation output and real-data-assimilation patterns of field-aligned currents and their constituent terms deriving from the divergence of the ionospheric Ohm's Law. We further calculate reconnection electric fields and voltages and find good agreement during the initial expansion phase, but different behavior later in the event as MAGE enters into steady-magnetospheric convection, in contrast to the observations, which indicated a classic substorm.

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.