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Coulomb collisions provide plasma resistivity and diffusion but in many low-density astrophysical plasmas such collisions between particles are extremely rare. Scattering of particles by electromagnetic waves can lower the plasma conductivity. Such anomalous resistivity due to wave-particle interactions could be crucial to many processes, including magnetic reconnection. It has been suggested that waves provide both diffusion and resistivity, which can support the reconnection electric field, but this requires direct observation to confirm. Here, we directly quantify anomalous resistivity, viscosity, and cross-field electron diffusion associated with lower hybrid waves using measurements from the four Magnetospheric Multiscale (MMS) spacecraft. We show that anomalous resistivity is approximately balanced by anomalous viscosity, and thus the waves do not contribute to the reconnection electric field. However, the waves do produce an anomalous electron drift and diffusion across the current layer associated with magnetic reconnection. This leads to relaxation of density gradients at timescales of order the ion cyclotron period, and hence modifies the reconnection process. It is suggested that waves can provide both diffusion and resistivity that can potentially support the reconnection electric field in low-density astrophysical plasmas. Here, the authors show, using direct spacecraft measurements, that the waves contribute to anomalous diffusion but do not contribute to the reconnection electric field.

The Magnetospheric Multiscale (MMS) spacecraft encounter an electron diffusion region (EDR) of asymmetric magnetic reconnection at Earth’s magnetopause. The EDR is characterized by agyrotropic electron velocity distributions on both sides of the neutral line. Various types of plasma waves are produced by the magnetic reconnection in and near the EDR. Here we report large-amplitude electron Bernstein waves (EBWs) at the electron-scale boundary of the Hall current reversal. The finite gyroradius effect of the outflow electrons generates the crescent-shaped agyrotropic electron distributions, which drive the EBWs. The EBWs propagate toward the central EDR. The amplitude of the EBWs is sufficiently large to thermalize and diffuse electrons around the EDR. The EBWs contribute to the cross-field diffusion of the electron-scale boundary of the Hall current reversal near the EDR. Various types of plasma waves are generated around electron diffusion regions (EDRs). Here the authors show electron Bernstein waves (EBWs), at the electron-scale boundary of the Hall current reversal near EDR, are sufficiently strong to diffuse electrons and modify electron pressure tensor.

The relative contribution of adiabatic and non‐adiabatic processes to electron heating across collisionless shocks remains an open question. We analyze the evolution of suprathermal electrons across 310 quasi‐perpendicular shocks with Alfvénic Mach numbers in the normal‐incidence frame MA−NIF$\\left({M}_{A-NIF}\\right)$ranging from 1.7 to 48, using in situ measurements of Earth's bow shock by the Magnetospheric Multiscale (MMS) spacecraft. We introduce a novel non‐adiabaticity measure derived from the electron distribution function and based on Liouville's theorem. Our results reveal, for the first time, that the electron heating mechanism is governed by the Alfvénic Mach number in the de Hoffman‐Teller frame MA−HT$\\left({M}_{A-HT}\\right)$ , with a transition from predominantly adiabatic to non‐adiabatic heating occurring at MA−HT≳30${M}_{A-HT}\\gtrsim 30$ . Furthermore, by examining the spectral index of the suprathermal electron distribution, we find that for shocks exhibiting dominant non‐adiabatic electron dynamics, the observed electron heating is consistent with the predictions of the stochastic shock drift acceleration (SSDA) mechanism. Plain Language Summary Understanding how electrons get heated across shock waves in space is a challenging scientific question. These shocks can heat electrons through different processes: some involve smooth, gradual changes (adiabatic), while others involve more chaotic interactions (non‐adiabatic). In this study, we looked at data from 310 shock events near Earth using the Magnetospheric Multiscale (MMS) spacecraft, focusing on shocks with a normal vector almost perpendicular to the direction of the magnetic field. We developed a new way to measure how much of the heating is due to non‐adiabatic processes by studying the patterns in how the electrons are distributed in energy. Our findings show that the way electrons are heated is mainly controlled by a dimensionless parameter called the Alfvénic Mach number, which describes how fast the shock is moving compared to a specific speed in the plasma, in a particular frame of reference (the de Hoffman‐Teller frame). We discovered that when this Mach number is above about 30, the heating changes from being mostly adiabatic to mostly non‐adiabatic. Additionally, we found that when non‐adiabatic heating is dominant, it matches well with a known process called stochastic shock drift acceleration (SSDA). Key Points We analyze electron heating across 310 quasi‐perpendicular shocks observed by MMS We use a Liouville mapping technique to show the electron heating mechanism is controlled by the Mach number in the de Hoffmann‐Teller frame We find that electron heating at shocks with dominant nonadiabatic dynamics aligns with the stochastic shock drift acceleration mechanism

Ion‐acoustic waves are routinely observed at collisionless shocks and could be an important source of resistivity. The source of instability and the effects of the waves are not fully understood. We show, using Magnetospheric Multiscale mission observations and numerical modeling, that across low Mach number shocks a large relative drift between protons and alpha particles develops, which can be unstable to the proton‐alpha streaming instability. Linear analysis and a numerical simulation show that the resulting waves agree with the observed wave properties. The generated ion‐acoustic waves are predicted to become nonlinear and form ion holes, maintained by trapped protons and alphas. The instability reduces the relative drift between protons and alphas, and heats the ions, thus providing a source of resistivity at shocks. Plain Language Summary Shock waves in collisionless plasmas are a fundamental process, which can produce intense particle acceleration. These shocks require a source of resistivity, which can be produced by plasma waves, to be sustained. A wide variety of waves have been reported at collisionless shocks, although the sources of the waves and their effects on the plasma are still debated. Using observations from the Magnetospheric Multiscale spacecraft and numerical modeling, we show that a large drift between protons and alpha particles occurs across the shock, and leads to the generation of large‐amplitude electrostatic waves. These waves subsequently heat the protons and alpha particles, and reduce the relative drift between the species. The waves can provide an important source of the resistivity required to sustain shocks. Key Points The proton‐alpha streaming instability caused by the cross‐shock potential can excite ion‐acoustic waves Theory and a simulation of the instability yield results consistent with observations The instability results in ion heating and reduces the relative drift between ion species

Kinetic-scale electromagnetic fluctuations are frequently observed in the reconnection electron diffusion region. However, their potential to accelerate magnetic reconnection through anomalous effects remains a topic of debate, with a lack of direct in-situ observational evidence. Using the unprecedented high-resolution data from Magnetospheric Multiscale mission, we directly and systematically calculate the secular field-particle energy exchange rate and anomalous electric fields associated with electromagnetic fluctuations within 13 electron diffusion regions observed in Earth’s magnetotail. Our findings reveal that the electromagnetic anomalous viscosity is the primary contributor to the anomalous electric field induced by electromagnetic fluctuations within the electron diffusion region. The maximum contribution of the anomalous viscosity can account for up to ~ 20% of the fast reconnection electric field, though it is typically less than 5% in most cases. We further find that locally growing electromagnetic fluctuations tend to accelerate reconnection, while locally damping electromagnetic fluctuations inhibit it. These results offer insights into the coupling between kinetic-scale electromagnetic fluctuations and magnetic reconnection in collisionless plasmas. The study analyses data from NASA’s MMS mission to examine electromagnetic fluctuations in the electron diffusion region of Earth’s magnetotail offering insights into the link between reconnection and turbulence. It finds that electromagnetic anomalous viscosity supplies, at times, around 20% of the reconnection electric field.

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.

Alongside magnetic reconnection, turbulence is another fundamental nonlinear plasma phenomenon that plays a key role in energy transport and conversion in space and astrophysical plasmas. From a numerical, theoretical, and observational point of view there is a long history of exploring the interplay between these two phenomena in space plasma environments; however, recent high-resolution, multi-spacecraft observations have ushered in a new era of understanding this complex topic. The interplay between reconnection and turbulence is both complex and multifaceted, and can be viewed through a number of different interrelated lenses - including turbulence acting to generate current sheets that undergo magnetic reconnection ( turbulence-driven reconnection ), magnetic reconnection driving turbulent dynamics in an environment ( reconnection-driven turbulence ) or acting as an intermediate step in the excitation of turbulence, and the random diffusive/dispersive nature of the magnetic field lines embedded in turbulent fluctuations enabling so-called stochastic reconnection . In this paper, we review the current state of knowledge on these different facets of the interplay between turbulence and reconnection in the context of collisionless plasmas, such as those found in many near-Earth astrophysical environments, from a theoretical, numerical, and observational perspective. Particular focus is given to several key regions in Earth’s magnetosphere – namely, Earth’s magnetosheath, magnetotail, and Kelvin-Helmholtz vortices on the magnetopause flanks – where NASA’s Magnetospheric Multiscale mission has been providing new insights into the topic.

Ion‐acoustic waves (IAWs) commonly occur near interplanetary (IP) shocks. These waves are important because of their potential role in the dissipation required for collisionless shocks to exist. We study IAW occurrence statistically at different heliocentric distances using Solar Orbiter to identify the processes responsible for IAW generation near IP shocks. We show that close to IP shocks the occurrence rate of IAW increases and peaks at the ramp. In the upstream region, the IAW activity is highly variable among different shocks and increases with decreasing distance from the Sun. We show that the observed currents near IP shocks are insufficient to reach the threshold for the current‐driven instability. We argue that two‐stream proton distributions and suprathermal electrons are likely sources of the waves. Plain Language Summary Ion‐acoustic waves (IAWs) are fluctuations in the electric field that occur at frequencies close to the ion plasma frequency. These waves are commonly found in the solar wind and often cluster around interplanetary (IP) shock waves. In this study, we investigate and quantify how common IAWs are in the vicinity of IP shocks. Our research revealed that IAW activity is enhanced before and after most IP shock passages. Furthermore, IAWs are more likely to be observed preceding IP shocks that are closer to the Sun. We find that the occurrence rate of IAWs shows no clear dependence on the IP shock parameters. We explore the possible mechanisms that could explain the presence of these IAWs. For instance, IAW modes can be excited by electric currents if the associated drift velocity between ions and electrons is above a certain threshold. However, the currents alone are not strong enough to generate the IAWs found near IP shocks. We discuss other potential generation mechanisms, such as velocity distributions of ions and electrons deviating from thermodynamic equilibrium. Key Points The occurrence of Ion‐acoustic waves (IAWs) is enhanced at interplanetary (IP) shocks, peaking at the shock ramp The occurrence rate of IAWs in the upstream region of IP shocks increases with decreasing radial distance from the Sun IAWs are observed upstream of an IP shock together with two‐stream protons and an electron strahl

There is ample evidence for magnetic reconnection in the solar system, but it is a nontrivial task to visualize, to determine the proper approaches and frames to study, and in turn to elucidate the physical processes at work in reconnection regions from in-situ measurements of plasma particles and electromagnetic fields. Here an overview is given of a variety of single- and multi-spacecraft data analysis techniques that are key to revealing the context of in-situ observations of magnetic reconnection in space and for detecting and analyzing the diffusion regions where ions and/or electrons are demagnetized. We focus on recent advances in the era of the Magnetospheric Multiscale mission, which has made electron-scale, multi-point measurements of magnetic reconnection in and around Earth’s magnetosphere.

This short article highlights unsolved problems of magnetic reconnection in collisionless plasma. Advanced in-situ plasma measurements and simulations have enabled scientists to gain a novel understanding of magnetic reconnection. Nevertheless, outstanding questions remain concerning the complex dynamics and structures in the diffusion region, cross-scale and regional couplings, the onset of magnetic reconnection, and the details of particle energization. We discuss future directions for magnetic reconnection research, including new observations, new simulations, and interdisciplinary approaches.