Louis Richard

We investigate ion dynamics in the ion diffusion region of an antiparallel symmetric reconnection event observed by the Magnetospheric MultiScale mission in Earth's magnetotail. Through detailed investigation of the ion velocity distribution functions, we show that the ion dynamics are initially (upon first entering the current sheet) dominated by the acceleration from the Hall electric and magnetic fields. The Hall electric field ballistically accelerates the ions toward the neutral sheet. The subsequent magnetic forces associated with the reconnecting and Hall magnetic fields have two distinct effects. First, the magnetic force due to the reconnecting magnetic field upstream of the X line delays the Speiser acceleration by gyroturning the ions in the direction opposite to the reconnection electric field. Second, the magnetic force due to the Hall magnetic field gyroturns the ions toward the outflow, kick-starting the outflow jet over short distances. These dynamics are consistent with fast reconnection rates.

Magnetic reconnection occurs commonly at current sheets in magnetized plasmas, accelerating electrons to very high energies. Among the various acceleration processes, magnetic-field-aligned (parallel) electric fields can directly accelerate electrons during reconnection. In this study, we map the spatial distribution of parallel electric field structures and characterize their properties during a magnetic reconnection event in the Earth’s magnetotail, using 3D electric field measurements from the Magnetospheric Multiscale mission. The types of observed structures include electron and ion phase-space holes, electrostatic and electromagnetic waves, and double layer-like (DL-like) structures. We determine the properties and locations of the structures, and investigate in detail the effects of the DL-like structures on electron acceleration, trapping, and heating in the separatrix region (SR). Our observations reveal that 71% of all timeable structures propagate toward the X line, including 82% of electron-hole trains and 70% of electrostatic waves. Of the structures that are timeable with all four spacecraft (130 of 264), we find that only five, all in the SR, have a peak potential greater than a tenth of the local electron temperature. Five DL-like structures are found, all of which are located in the SR. One of these DL-like structures causes clear parallel acceleration of an electron beam toward the X line, which subsequently generates large-amplitude electron holes. An electron beam traveling from the X line is observed to reflect off the DL-like structure back toward the X line, contributing to electron trapping and regulating quasi-neutrality in the ion diffusion region.

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 occurs commonly at current sheets in magnetized plasmas, accelerating electrons to very high energies. Among the various acceleration processes, magnetic-field-aligned (parallel) electric fields can directly accelerate electrons during reconnection. In this study, we map the spatial distribution of parallel electric field structures and characterize their properties during a magnetic reconnection event in the Earth’s magnetotail, using 3D electric field measurements from the Magnetospheric Multiscale mission. The types of observed structures include electron and ion phase-space holes, electrostatic and electromagnetic waves, and double layer-like (DL-like) structures. We determine the properties and locations of the structures, and investigate in detail the effects of the DL-like structures on electron acceleration, trapping, and heating in the separatrix region (SR). Our observations reveal that 82% of electron-hole trains and 74% of electrostatic waves travel toward the X line, consistent with the direction of inflowing electron beams. We find that only 5 of the 264 identified structures, all in the SR, have a peak potential greater than a tenth of the local electron temperature. Five DL-like structures are found, all of which are located in the SR. One of these DL-like structures causes clear parallel acceleration of an electron beam toward the X line, which is subsequently thermalized by large-amplitude electron holes. An electron beam traveling from the X line is observed to reflect off the DL-like structure back toward the X line, contributing to electron trapping and regulating quasi-neutrality in the ion diffusion region.

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 . 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 ion inertial lengths inside the outflow, Fermi acceleration acts on the low‐density electron population, potentially driving them to high energies (∼100 ). 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.

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.

Collisions are nearly negligible in many space and astrophysical plasmas, allowing charged-particle velocity distribution functions (VDFs) to depart from local thermodynamic equilibrium (LTE). How collisionless plasmas relax these non-LTE distributions and convert turbulent energy into particle heating remains an open question. We investigate deviations from LTE in ion velocity distribution functions (iVDFs) within collisionless plasma turbulence using high-resolution measurements from the Magnetospheric Multiscale (MMS) mission. We find that the iVDFs' non-bi-Maxwellian features are widespread and can be significant. Their complexity increases with ion plasma beta and turbulence intensity, with pronounced high-order non-LTE features emerging during intervals of large-amplitude magnetic field fluctuations. In addition, we show that turbulence-induced magnetic curvature plays a significant role in ion scattering and contributes to the isotropization of the iVDF. These results highlight the complex interaction between turbulence and the velocity distribution of charged particles, providing new insight into the kinetic processes responsible for energy conversion in collisionless plasmas.

Bursty bulk flows (BBFs) are fast plasma flows in the central plasma sheet. These BBFs couple to the high-latitude ionosphere through field-aligned currents (FACs). A statistical study of FACs associated with BBF events at the BBF ionospheric footpoint in both hemispheres has been lacking. We analyzed single spacecraft FACs from Swarm A and B during a dataset of BBF intervals and compared to 200 different random datasets selected within the same years and months and geomagnetic conditions as the BBFs. Our results show that in the Southern Hemisphere, the probability of observing large FACS (80th percentile) is significantly greater during BBFs compared to random chance. This is valid around the typical BBF footpoints according to the mapping of thousands of BBFs. However, in the Northern Hemisphere, the larger FAC values do not coincide with the BBF-mapped region suggesting an inter-hemispheric asymmetry.

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.