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High-speed electron flows play an important role in the energy dissipation and conversion in the terrestrial magnetosphere and are widely observed in regions related to magnetic reconnection, e.g., the vicinity of electron diffusion regions (EDRs), and separatrix layers. NASA’s Magnetospheric Multiscale mission was designed to resolve the electron-scale kinetic processes of Earth’s magnetosphere. Here, we perform a systematic survey of high-speed electron flows in the terrestrial magnetotail using the MMS observations from 2017 to 2021. The high-speed electron flows are characterized by electron bulk speeds larger than 5000 km/s. We identified 649 events. Those events demonstrate unambiguous dawn-dusk asymmetry, and 73% of them locate in the dusk magnetotail. The selected events are found in EDRs, the reconnection separatrix boundary layer, and the lobe region. More than 70% of the events are identified in the separatrix boundary layer and the lobe region and are aligned with the ambient magnetic field. 75 cases, with magnetic field magnitude smaller than 5 nT, locate near the plasma-sheet neutral line. Approximately 20 cases among them have EDR signatures, and those high-speed electron flows are directed arbitrarily with respect to the ambient magnetic field. We also show other statistical properties of the events, including electron bulk speed, electron number density, and temperature anisotropy.

We report an event of intermittent reconnection from the terrestrial magnetotail observed by the Magnetospheric MultiScale mission. First, magnetic reconnection is active, inferred from a field-aligned off-equatorial plasma jet. Over 40 seconds, this jet is replaced by a quiet time interval with dusk-ward diamagnetic ion flow carried by a hot population that persists for about two minutes. During this interval, we observe signs of current sheet thickening followed by thinning. The change in the dawn-dusk current associated with the inferred thickening is provided by changes in the electron flux, and we argue this is a result of momentum conservation. Thereafter, we observe an equatorial jet of hot plasma that gradually builds up before the spacecraft encounter a dipolarization front about 20 seconds later. This first dipolarization front is associated with a transition from a hot pre-existing plasma sheet, to colder plasma of lobe origin. This event showcases behavior during intermittent magnetic reconnection and may help us understand the spatiotemporal evolution of reconnecting regions.

Using a resistive MagnetoHydroDynamic (MHD) simulation, we study how the magnitude and shape of diffusion influence magnetic reconnection. Specifically, we investigate how and why the reconnection rate is influenced by variations in the diffusion distribution and magnitude. By running multiple MHD simulations where we vary the localized resistivity, we find that the properties of the diffusion region greatly influence the rate of reconnection. Increasing the magnitude of the imposed resistivity results in a higher reconnection rate, but the rate saturates at approximately 0.2. We show how a redistribution of the current density, leading to a bifurcated current sheet, play a major role in this limitation. In addition, we investigate the impact of different shapes of resistive region. The shape of the diffusion region also plays a major role in how efficient the reconnection energy conversion can operate. The highest reconnection rate, approximately 0.25, is achieved for an optimal opening angle. Our results imply that reconnection has a speed limit that may depend on properties outside the diffusion region. publishedVersion

We investigate the onset of magnetic reconnection, utilizing a fully kinetic Particle-In-Cell (PIC) simulation. Characteristic features of the electron phase-space distributions immediately before reconnection onset are identified. These include signatures of pressure non-gyrotropy in the velocity distributions, and lemon shaped distributions in the in-plane velocity directions. Further, we explain how these features form through particle energization by the out-of-plane electric field. Identification of these features in the distributions can aid in analysis of data where clear signatures of ongoing reconnection are not yet present. publishedVersion

S. Toledo-Redondo and J. Fornieles acknowledge support of the Ministry of Economy and Competitiveness (MINECO) of Spain (grant FIS2017-90102-R) and of Ministry of Science and Innovation (grant PID2020-112805GA-I00). Research at IRAP was supported by CNRS, CNES, and the University of Toulouse. We acknowledge support of the ISSI teams MMS and Cluster observations of magnetic reconnection and Cold plasma of ionospheric in the Earth's magnetosphere, and of the ESAC Science faculty., In-situ spacecraft missions are powerful assets to study processes that occur in space plasmas. One of their main limitations, however, is extrapolating such local measurements to the global scales of the system. To overcome this problem at least partially, multi-point measurements can be used. There are several multi-spacecraft missions currently operating in the Earth's magnetosphere, and the simultaneous use of the data collected by them provides new insights into the large-scale properties and evolution of magnetospheric plasma processes. In this work, we focus on studying the Earth's magnetopause (MP) using a conjunction between the Magnetospheric Multiscale and Cluster fleets, when both missions skimmed the MP for several hours at distant locations during radial interplanetary magnetic field (IMF) conditions. The observed MP positions as a function of the evolving solar wind conditions are compared to model predictions of the MP. We observe an inflation of the magnetosphere (similar to 0.7 R-E), consistent with magnetosheath pressure decrease during radial IMF conditions, which is less pronounced on the flank (, Ministry of Economy and Competitiveness (MINECO) of Spain FIS2017-90102-R, Spanish Government PID2020-112805GA-I00, Centre National de la Recherche Scientifique (CNRS), European Commission, Centre National D'etudes Spatiales, University of Toulouse, ESAC Science faculty

Heavier plasma species such as oxygen ions can have a large impact on the magnetic reconnection process. It has been hypothesized that the acceleration of demagnetized oxygen ions by the Hall electric field will lead to the formation of an oxygen wave that expands into the exhaust. By comparing data from NASA's Magnetospheric Multiscale mission to a fully kinetic particle-in-cell simulation, we can for the first time provide observational evidence of such an expanding oxygen wave. The wave is characterized by an oxygen jet consisting of cold ions directed toward the neutral sheet associated with a density cavity. This density cavity forms as the O+ are subject to collective acceleration by the Hall electric field leaving behind a region of low-density oxygen ions. Our results are important for the understanding of the role and effect of oxygen ions in magnetic reconnection. publishedVersion

Using fully kinetic 2.5 dimensional particle-in-cell simulations of anti-parallel symmetric magnetic reconnection, we investigate how initially cold ions are captured by the reconnection process, and how they evolve and behave in the exhaust. We find that initially cold ions can remain cold deep inside the exhaust. Cold ions that enter the exhaust downstream of active separatrices, closer to the dipolarization front, appear as cold counter-streaming beams behind the front. In the off-equatorial region, these cold ions generate ion-acoustic waves that aid in the thermalization both of the incoming and outgoing populations. Closest to the front, due to the stronger magnetization, the ions can remain relatively cold during the neutral plane crossing. In the intermediate exhaust, the weaker magnetization leads to enhanced pitch angle scattering and reflection. Cold ions that enter the exhaust closer to the X line, at active separatrices, evolve into a thermalized exhaust. Here, the cold populations are heated through a combination of thermalization at the separatrices and pitch angle scattering in the curved magnetic field around the neutral plane. Depending on where the ions enter the exhaust, and how long time they have spent there, they are accelerated to different energies. The superposition of separately thermalized ion populations that have been accelerated to different energies form the hot exhaust population. publishedVersion

International audience

We investigate the role of a background oxygen population in magnetic reconnection, using particle‐in‐cell simulations. We run several simulations, with different initial oxygen temperatures and densities, to understand how the reconnection rate is influenced, as oxygen is captured by the reconnection process. The oxygen remains approximately demagnetized on the relevant time and spatial scales and therefore has little direct (i.e., immediate mass loading) effect on the reconnection process itself. The reconnection rate is independent of the initial oxygen temperature but clearly dependent on the density. The reduced reconnection rate is twice as fast as predicted by mass loading. We describe a mechanism where the oxygen population (and the associated electrons) acts as an energy sink on the system, altering the energy partitioning. Based on a scaling analysis, we derive an estimate of the reconnection electric field that scales as (1+no/np)−1, where no and np is the oxygen and proton densities, respectively. publishedVersion

A new look at the structure of the electron diffusion region in collision less magnetic reconnection is presented. The research is based on a particle-in-cell simulation of asymmetric magnetic reconnection, which includes a temperature gradient across the current layer in addition to density and magnetic field gradient. We find that none of X-point, flow stagnation point, and local current density peak coincide. Current and energy balance analyses around the flow stagnation point and current density peak show consistently that current dissipation is associated with the divergence of nongyrotropic electron pressure. Furthermore, the same pressure terms, when combined with shear-type gradients of the electron flow velocity, also serve to maintain local thermal energy against convective losses. These effects are similar to those found also in symmetric magnetic reconnection. In addition, we find here significant effects related to the convection of current, which we can relate to a generalized diamagnetic drift by the nongyrotropic pressure divergence. Therefore, only part of the pressure force serves to dissipate the current density. However, the prior conclusion that the role of the reconnection electric field is to maintain the current density, which was obtained for a symmetric system, applies here as well. Finally, we discuss related features of electron distribution function in the electron diffusion region (EDR). Specifically, we analyze both new crescent substructures as well as outer, higher energy crescents generated by accelerated magnetospheric particles. publishedVersion

We investigate how magnetic reconnection is influenced by an inflow of a dense cold ion population. We compare two 2.5D Particle-In-Cell simulations, one containing the cold population and one without. We find that the cold population influences the reconnection process on both global and kinetic scales, and that the dominant contribution can be explained through mass-loading. We provide an analysis of how these multiscale changes are related through kinetic processes in the ion diffusion region, the so-called micro-macro coupling of mass-loading. The inertia of the cold ion population is found to be the significant link that connects the changes on different scales. The cold and warm populations exhibit counter streaming behavior when and after the ion diffusion region reorganizes itself in response to the arrival of the cold population. This signature of the cold population should be observable by spacecraft observatories such as MMS. publishedVersion

Using 2.5-D Particle-In-Cell (PIC) simulations, we investigate how magnetotail reconnection is affected by a cold, streaming, oxygen plasma population, attributed to an ionospheric source, in the inflow region. As the tailward streaming oxygen reaches the current layer, a tailward motion of the reconnection site is induced. Due to the much longer cyclotron period of the oxygen ions, oxygen cannot couple as directly into the reconnection dynamics as protons. We find that the oxygen ions couple indirectly by means of impacting the electron dynamics. Therefore, a demagnetized species can, in fact, alter the dynamics of the reconnection site. We see further that the reconnection rate remains unchanged relative to a nonstreaming run. Our results may prove useful for understanding the development and dynamics of magnetospheric substorms and storms. publishedVersion

At some level, magnetic reconnection functions by means of a balance between current dissipation, and current maintenance due to the reconnection electric field. While this dissipation is well understood process in symmetric magnetic reconnection, the way nonideal electric fields interact with the current density in asymmetric reconnection is still unclear. In symmetric reconnection, the current density maximum, the X point location, and the nonideal electric field determined by the divergence of the electron pressure tensor usually coincide. In asymmetric reconnection, however, the electric field at the X point can be partly provided by bulk inertia terms, implying that the X point cannot be the dominant location of dissipation. On the other hand, we know that the nongyrotropic pressure-based electric field must dominate at the stagnation point of the in-plane electron flow, and that electron distributions here feature crescents. The further fact that the current density peak is shifted off the position of the X point indicates that there may be a relation between this current density enhancement, the location of the stagnation point, and the electron nongyrotropies. In this presentation we report on further progress investigating the physics of the electron diffusion region in asymmetric reconnection with a focus on how to explain the dissipation operating under these conditions.

We report electrostatic Debye-scale turbulence developing within the diffusion region of asymmetric magnetopause reconnection with moderate guide field using observations by the Magnetospheric Multiscale (MMS) mission. We show that Buneman waves and beam modes cause efficient and fast thermalization of the reconnection electron jet by irreversible phase mixing, during which the jet kinetic energy is transferred into thermal energy. Our results show that the reconnection diffusion region in the presence of a moderate guide field is highly turbulent, and that electrostatic turbulence plays an important role in electron heating., Comment: 5 pages, 4 figures

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.

Combined with the magnetic field, the distribution of charged particles in the inflow region is expected to control the rate of magnetic reconnection. This paper investigates how the reconnection process is altered by a cold, asymmetrically distributed, oxygen population, which is initially located away from the current layer in the inflow regions. A particle-in-cell simulation is used to gain further insight into the dynamics of the system. The time evolution of the reconnection process proceeds rapidly compared to the cyclotron period of O urn:x-wiley:grl:media:grl59941:grl59941-math-0001. Therefore, the oxygen remains, to a good approximation, demagnetized. Thus, Alfvén scaling is not an adequate description of the reconnection rate. A scaling relation for the reconnection rate for an asymmetrically distributed, demagnetized species has been developed. Additionally, we find that an asymmetric density configuration leads to a distinct motion of the reconnection site and generates an asymmetry of the diffusion region and the Hall electric field. publishedVersion

In this study we investigate the performance of the University of Michigan’s Space Weather Modeling Framework (SWMF) in prediction of ground magnetic perturbations (ΔB) and their rate of change with time (dB/dt), which is directly connected to geomagnetically induced currents (GICs). We use the SWMF set-up where the global magnetosphere provided by the Block Adaptive Tree Solar-wind Roe-type Upwind Scheme (BATS-R-US) MHD code, is coupled to the inner magnetosphere and the ionospheric electrodynamics. The validation is done for ΔB and dB/dt separately. The performance is evaluated via data-model comparison through a metrics-based approach. For ΔB, the normalized root mean square error (nRMS) and the correlation coefficient are used. For dB/dt, the probability of detection, the probability of false detection, the Heidke skill score, and the frequency bias are used for different dB/dt thresholds. The performance is evaluated for eleven ground magnetometer stations located between 59° and 85° magnetic latitude and spanning about five magnetic local times. Eight geomagnetic storms are studied. Our results show that the SWMF predicts the northward component of the perturbations better at lower latitudes (59°–67°) than at higher latitudes (>67°), whereas for the eastward component, the model performs better at high latitudes. Generally, the SWMF performs well in the prediction of dB/dt for a 0.3 nT/s threshold, with a high probability of detection ≈0.8, low probability of false detection (B/dt prediction performance generally decreases as the threshold is raised, except for the probability of false detection, which improves.

International audience; We have used a large number of magnetopause crossings by the Magnetospheric Multiscale (MMS) mission to investigate macroscopic properties of this current sheet, with emphasis on the flanks of the magnetopause. Macroscopic features such as thickness, location, and motion of the magnetopause were calculated as a function of local time sector. The results show that the flanks of the magnetopause are significantly thicker than the dayside magnetopause. Thicknesses vary from about 650 km near noon to over 1,000 km near the terminator. Current densities vary in a similar manner, with average current densities around noon almost twice as high as near the terminator. We also find a dawn-dusk asymmetry in many of the macroscopic parameters; the dawn magnetopause is thicker than at dusk, while the dusk flank is more dynamic, with a higher average normal velocity.

We employ a 2.5D particle-in-cell simulation to study a scenario where the reconnection process captures cold streaming protons. As soon as the tailward streaming protons become involved, they contribute to the overall momentum balance, altering the initially symmetric dynamics. Adding tailward-directed momentum to the reconnection process results in a tailward propagation of the reconnection site. We investigate how the reconnection process reorganizes itself due to the changing momentum conditions on the kinetic scale and how the reconnection rate is affected. We find that adding tailward momentum does not result in a significantly different reconnection rate compared to the case without cold streaming protons, when scaled to the total Alfvén velocity. This implies that the effect of changing inflow conditions due to the motion of the reconnection site appears to be minimal. The dynamics of the particles are, however, significantly different depending on whether they enter on the tailward or Earthward side of the reconnection site. On the Earthward side they are reflected and thermalized, while on the tailward side they are picked up and accelerated. The cold proton density and Ez on the Earthward side are turbulent, while the tailward side has laminar cold proton density striations and an embedded Ez layer. Also, since the initial plasma sheet population is swept up on one side and flushed out on the other, asymmetries in the densities and strength of Hall fields emerge. Our results are important for understanding the development and dynamics of magnetospheric substorms and storms. publishedVersion

Waves around the lower hybrid frequency are frequently observed at Earth's magnetopause, and readily reach very large amplitudes. Determining the properties of lower hybrid waves is crucial because they are thought to contribute to electron and ion heating, cross-field particle diffusion, anomalous resistivity, and energy transfer between electrons and ions. All these processes could play an important role in magnetic reconnection at the magnetopause and the evolution of the boundary layer. In this paper, the properties of lower hybrid waves at Earth's magnetopause are investigated using the Magnetospheric Multiscale (MMS) mission. For the first time, the properties of the waves are investigated using fields and direct particle measurements. The highest-resolution electron moments resolve the velocity and density fluctuations of lower hybrid waves, confirming that electrons remain approximately frozen in at lower hybrid wave frequencies. Using fields and particle moments the dispersion relation is constructed and the wave-normal angle is estimated to be close to $90^{\circ}$ to the background magnetic field. The waves are shown to have a finite parallel wave vector, suggesting that they can interact with parallel propagating electrons. The observed wave properties are shown to agree with theoretical predictions, the previously used single-spacecraft method, and four-spacecraft timing analyses. These results show that single-spacecraft methods can accurately determine lower hybrid wave properties., Comment: 43 pages, 18 figures

Magnetic reconnection is a fundamental process whereby microscopic plasma processes cause macroscopic changes in magnetic field topology, leading to explosive energy release. Waves and turbulence generated during the reconnection process can produce particle diffusion and anomalous resistivity, as well as heat the plasma and accelerate plasma particles, all of which can impact the reconnection process. We review progress on waves related to reconnection achieved using high resolution multi-point in situ observations over the last decade, since early Cluster and THEMIS observations and ending with recent Magnetospheric Multiscale results. In particular, we focus on the waves most frequently observed in relation to reconnection, ranging from low-frequency kinetic Alfvén waves (KAW), to intermediate frequency lower hybrid and whistler-mode waves, electrostatic broadband and solitary waves, as well as the high-frequency upper hybrid, Langmuir, and electron Bernstein waves. Significant progress has been made in understanding localization of the different wave modes in the context of the reconnection picture, better quantification of generation mechanisms and wave-particle interactions, including anomalous resistivity. Examples include: temperature anisotropy driven whistlers in the flux pileup region, anomalous effects due to lower-hybrid waves, upper hybrid wave generation within the electron diffusion region, wave-particle interaction of electrostatic solitary waves. While being clearly identified in observations, some of the wave processes remain challenging for reconnection simulations (electron Bernstein, upper hybrid, Langmuir, whistler), as the instabilities (streaming, loss-cone, shell) which drive these waves require high resolution of distribution functions in phase space, and realistic ratio of Debye to electron inertia scales. We discuss how reconnection configuration, i.e., symmetric vs. asymmetric, guide-field vs. antiparallel, affect wave occurrence, generation, effect on particles, and feedback on the overall reconnection process. Finally, we outline some of the major open questions, such as generation of electromagnetic radiation by reconnection sites and role of waves in triggering/onset of reconnection.

We investigate the kinetic structure of electron-scale current sheets found in the vicinity of the magnetopause and embedded in the magnetosheath within the reconnection exhaust. A new technique for computing terms of the Vlasov equation using Magnetospheric Multiscale (MMS) measurements is presented and applied to study phase space density gradients and the kinetic origins of the electron pressure divergence found within these current sheets. Crescent-shaped structures in ∇(⟂2)f(e) give rise to bipolar and quadrupolar signatures in v · ∇f(e) measured near the maximum ∇ · P(e) inside the current layers. The current density perpendicular to the magnetic field is strong (J⟂ ∼2 μA/sq.m), and the thickness of the current layers ranges from 3 to 5 electron inertial lengths. The electron flows supporting the current layers mainly result from the combination of E × B and diamagnetic drifts. We find nonzero J · E′ within the current sheets even though they are observed apart from typical diffusion region signatures.

We present observations of asymmetric magnetic reconnection showing evidence of electron demagnetization in the electron outflow. The observations were made at the magnetopause by the four Magnetos ...

Observations by the four Magnetospheric Multiscale spacecraft are used to investigate the Hall physics of a magnetopause magnetic reconnection separatrix layer. Inside this layer of currents and st ...

A study of the role of microinstabilities at the reconnection separatrix can play in heating the electrons during the transition from inflow to outflow is being presented. We find that very strong flow shears at the separatrix layer lead to counterstreaming electron distributions in the region around the separatrix, which become unstable to a beam-type instability. Similar to what has been seen in earlier research, the ensuing instability leads to the formation of propagating electrostatic solitons. We show here that this region of strong electrostatic turbulence is the predominant electron heating site when transiting from inflow to outflow. The heating is the result of heating generated by electrostatic turbulence driven by overlapping beams, and its macroscopic effect is a quasi-viscous contribution to the overall electron energy balance. We suggest that instabilities at the separatrix can play a key role in the overall electron energy balance in magnetic reconnection.

We report on computer simulations and analytic theory to provide a self-consistent understanding of the role of the reconnection electric field, which extends substantially beyond the simple change of magnetic connections. Rather, we find that the reconnection electric field is essential to maintaining the current density in the diffusion region, which would otherwise be dissipated by a set of processes. Natural candidates for current dissipation are the average convection of current carriers away from the reconnection region by the outflow of accelerated particles, or the average rotation of the current density by the magnetic field reversal in the vicinity. Instead, we show here that the current dissipation is the result of thermal effects, underlying the statistical interaction of current-carrying particles with the adjacent magnetic field. We find that this interaction serves to redirect the directed acceleration of the reconnection electric field to thermal motion. This thermalization manifests itself in form of quasi-viscous terms in the thermal energy balance of the current layer. These quasi-viscous terms act to increase the average thermal energy. Our predictions regarding current and thermal energy balance are readily amenable to exploration in the laboratory or by satellite missions, in particular, by NASAs Magnetospheric Multiscale mission., Comment: 21 pages, 5 figures

Electrostatic solitary waves (ESWs), often interpreted as electron phase space holes, are commonly observed in plasmas and are manifestations of strongly nonlinear processes. Often slow ESWs are observed, suggesting generation by the Buneman instability. The instability criteria, however, are generally not satisfied. We show how slow electron holes can be generated by a modified Buneman instability in a plasma that includes a slow electron beam on top of a warm thermal electron background. This lowers the required current for marginal instability and allows for generation of slow electron holes for a wide range of beam parameters that covers expected plasma distributions in space, for example, in magnetic reconnection regions. At higher beam speeds, the electron-electron beam instability becomes dominant instead, producing faster electron holes. The range of phase speeds for this model is consistent with a statistical set of observations at the magnetopause made by Cluster.

We report multispacecraft observations of slow electrostatic solitary waves in the plasma sheet boundary layer. The electrostatic solitary waves are embedded in a region with field-aligned electron flows and are interpreted as electron phase space holes. We make unambiguous velocity and length estimates of the electron holes, vEH∼500 km/s and l||∼2–4λDe, where l|| is the parallel half width. We do not detect any magnetic signature of the holes. The electrostatic potentials of the holes are of the order eϕ/kBTe∼10%, indicating that they can affect electron motion and further couple the electron and ion dynamics.

International audience; Magnetosheath plasma usually determines properties of asymmetric magnetic reconnection at the subsolar region of Earth's magnetopause. However, cold plasma that originated from the ionosphere can also reach the magnetopause and modify the kinetic physics of asymmetric reconnection. We present a magnetopause crossing with high-density (10-60 cm-3) cold ions and ongoing reconnection from the observation of the Magnetospheric Multiscale (MMS) spacecraft. The magnetopause crossing is estimated to be 300 ion inertial lengths south of the X line. Two distinct ion populations are observed on the magnetosheath edge of the ion jet. One population with high parallel velocities (200-300 km/s) is identified to be cold ion beams, and the other population is the magnetosheath ions. In the deHoffman-Teller frame, the field-aligned magnetosheath ions are Alfvénic and move toward the jet region, while the field-aligned cold ion beams move toward the magnetosheath boundary layer, with much lower speeds. These cold ion beams are suggested to be from the cold ions entering the jet close to the X line. This is the first observation of the cold ionospheric ions in the reconnection outflow region, including the reconnection jet and the magnetosheath boundary layer.

During a magnetopause crossing the Magnetospheric Multiscale spacecraft encountered an electron diffusion region (EDR) of asymmetric reconnection. The EDR is characterized by agyrotropic beam and crescent electron distributions perpendicular to the magnetic field. Intense upper-hybrid (UH) waves are found at the boundary between the EDR and magnetosheath inflow region. The UH waves are generated by the agyrotropic electron beams. The UH waves are sufficiently large to contribute to electron diffusion and scattering, and are a potential source of radio emission near the EDR. These results provide observational evidence of wave-particle interactions at an EDR, and suggest that waves play an important role in determining the electron dynamics.

In a recent paper1 about electron heating at the reconnection separatrix, two figures depicting the contributions to the electron energy balance and the contribution to the total, quasi-viscous heating are incorrectly displayed. The correct figures are as follows: [Table Presented].

Dipolarization front (DF)—a sharp boundary separating hot tenuous plasmas from cold dense plasmas—is a key structure responsible for particle acceleration and energy transport in the magnetotail. How such a structure is formed has been unclear so far. Two possible mechanisms suggested in previous studies are magnetic reconnection and spontaneous formation. Both of them require current sheet thinning as a prerequisite. However, observational evidence of the DF formation associated with current sheet thinning has not been reported. In this study, we present such an observation, showing the DF formation after current sheet thinning. We estimate the half thickness of the current sheet to be ∼1000 km and the rate of current sheet thinning as ∼38 km/s. We find that the DF is likely formed at XGSM ≈ −20 RE. During the current sheet thinning, the plasma becomes cold and dense; during DF formation, many magnetic islands are produced. Although current sheet thinning and DF formation have been individually analyzed in the previous studies, this study, for the first time, links the two transient processes in the magnetotail.Dipolarization front (DF)—a sharp boundary separating hot tenuous plasmas from cold dense plasmas—is a key structure responsible for particle acceleration and energy transport in the magnetotail. How such a structure is formed has been unclear so far. Two possible mechanisms suggested in previous studies are magnetic reconnection and spontaneous formation. Both of them require current sheet thinning as a prerequisite. However, observational evidence of the DF formation associated with current sheet thinning has not been reported. In this study, we present such an observation, showing the DF formation after current sheet thinning. We estimate the half thickness of the current sheet to be ∼1000 km and the rate of current sheet thinning as ∼38 km/s. We find that the DF is likely formed at XGSM ≈ −20 RE. During the current sheet thinning, the plasma becomes cold and dense; during DF formation, many magnetic islands are produced. Although current sheet thinning and DF formation have been individually analyzed ...

© 2016 Author(s). The newly developed fully kinetic, semi-implicit, adaptive multi-level multi-domain (MLMD) method is used to simulate, at realistic mass ratio, the development of the lower hybrid drift instability (LHDI) in the terrestrial magnetotail over a large wavenumber range and at a low computational cost. The power spectra of the perpendicular electric field and of the fluctuations of the parallel magnetic field are studied at wavenumbers and times that allow to appreciate the onset of the electrostatic and electromagnetic LHDI branches and of the kink instability. The coupling between electric and magnetic field fluctuations observed by Norgren et al. ["Lower hybrid drift waves: Space observations," Phys. Rev. Lett. 109, 055001 (2012)] for high wavenumber LHDI waves in the terrestrial magnetotail is verified. In the MLMD simulations presented, a domain ("coarse grid") is simulated with low resolution. A small fraction of the entire domain is then simulated with higher resolution also ("refined grid") to capture smaller scale, higher frequency processes. Initially, the MLMD method is validated for LHDI simulations. MLMD simulations with different levels of grid refinement are validated against the standard semi-implicit particle in cell simulations of domains corresponding to both the coarse and the refined grid. Precious information regarding the applicability of the MLMD method to turbulence simulations is derived. The power spectra of MLMD simulations done with different levels of refinements are then compared. They consistently show a break in the magnetic field spectra at k⊥di∼30, with di the ion skin depth and k⊥ the perpendicular wavenumber. The break is observed at early simulated times, Ωcit

Lower hybrid drift waves (LHDWs) are commonly observed at plasma boundaries in space and laboratory, often having the strongest measured electric fields within these regions. We use data from two of the Cluster satellites (C3 and C4) located in Earth's magnetotail and separated by a distance of the order of the electron gyroscale. These conditions allow us, for the first time, to make cross-spacecraft correlations of the LHDWs and to determine the phase velocity and wavelength of the LHDWs. Our results are in good agreement with the theoretical prediction. We show that the electrostatic potential of LHDWs is linearly related to fluctuations in the magnetic field magnitude, which allows us to determine the velocity vector through the relation ∫δEdt·v = ϕ(δB)(∥). The electrostatic potential fluctuations correspond to ∼10% of the electron temperature, which suggests that the waves can strongly affect the electron dynamics.