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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

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

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

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

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 ...

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.