Using fully kinetic 3D simulations, the reconnection dynamics of asymmetric current sheets are examined at the Earth's magnetopause. The plasma parameters are selected to model MMS magnetopause diffusion region crossings with guide fields of 0.1, 0.4, and 1 of the reconnecting magnetosheath field. In each case, strong drift-wave fluctuations are observed in the lower-hybrid frequency range at the steep density gradient across the magnetospheric separatrix. These fluctuations give rise to cross-field electron particle transport. In addition, this turbulent mixing leads to significantly enhanced electron parallel heating in comparison to 2D simulations. We study three different methods of quantifying the anomalous dissipation produced by the drift fluctuations, based on spatial averaging, temporal averaging, and temporal averaging followed by integrating along magnetic field lines. Comparison of the different methods reveals complications in identifying and measuring the anomalous dissipation. Nevertheless, the anomalous dissipation from short wavelength drift fluctuations appears weak for each case, and the reconnection rates observed in 3D are nearly the same as in 2D models. The 3D simulations feature a number of interesting new features that are consistent with recent MMS observations, including cold beams of magnetosheath electrons that penetrate into the hotter magnetospheric inflow, the related observation of decreasing temperature in regions of increasing total density, and an effective turbulent diffusion coefficient that agrees with predictions from quasi-linear theory.

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. 3 I. INTRODUCTIONMagnetic reconnection is arguably the most important transport and energy conversion process in collisionless plasmas 1,2 . It enables transport over large distances by means of a highly localized diffusion region, where, within different layers, ions and electrons become decoupled from the magnetic field. Particularly the physics of the smallest sub-region, the electron diffusion region, has been enigmatic for many years. Recent observations 3,4 , however, have demonstrated that the laminar, thermal electron inertia-based (or, quasi-viscous) model 5,6 of its structure appears to be correct.The laminar nature of the diffusion region has been suggested to be the consequence of finite electron residence time 7 .The electron diffusion region is of crucial importance to the reconnection process, but, owing to its diminutive size, it cannot be the main actor in the overall energy conversion process. Energy conversion in magnetic reconnection occurs over macroscopic spatial scales, in the case of the Earth's magnetosphere over tens of Earth radii, whereas the typical dimensions of the electron diffusion region are a mere ten to 100km. Consequently, other processes have to come into play to facilitate large-scale energy conversion.In magnetohydrodynamic (MHD) models this energy conversion is facilitated by slow shocks 8 , or, a more general set of discontinuities in non-coplanar geometries 9 or in anisotropic plasmas 10 . In MHD, these discontinuities facilitate the conversion of incoming Poynting flux to enthalpy and kinetic energy flux 11 . Among other effects, the energy conversion process also provides for the pressure balance in the current layer by increasing plasma temperature and pressure to the level required to balance the magnetic pressure in the inflow region 12 .

A remote sensing technique to infer the local reconnection electric field based on in situ multipoint spacecraft observation at the reconnection separatrix is proposed. In this technique, the increment of the reconnected magnetic flux is estimated by integrating the in-plane magnetic field during the sequential observation of the separatrix boundary by multipoint measurements. We tested this technique by applying it to virtual observations in a two-dimensional fully kinetic particle-in-cell simulation of magnetic reconnection without a guide field and confirmed that the estimated reconnection electric field indeed agrees well with the exact value computed at the X-line. We then applied this technique to an event observed by the Magnetospheric Multiscale mission when crossing an energetic plasma sheet boundary layer during an intense substorm. The estimated reconnection electric field for this event is nearly 1 order of magnitude higher than a typical value of magnetotail reconnection.Plain Language Summary Magnetic reconnection is an important phenomenon in space plasmas that explosively releases the accumulated magnetic energy. In the Earth's magnetotail, it is known that the released energy by this process leads to aurora substorms. The rate of reconnection, which defines how efficiently the magnetic flux is transferred at the central reconnection region called the diffusion region, is a key parameter to explore such reconnection physics. However, it is very challenging to directly measure this parameter by observing the small-scale diffusion region in situ. Thus, in this paper, we propose a new remote sensing technique that infers the reconnection rate along the boundary of the whole reconnection region called the separatrix from in situ measurements. In this technique, by observing how rapidly the magnetic flux passes through the separatrix, the rate can be remotely inferred even outside the diffusion region. We confirmed the adequacy of this technique by testing it in a kinetic simulation, and then applied it to a latest observation event by the MMS spacecraft during an intense substorm. The result shows a remarkably high rate. This indicates the positive correlation between the reconnection rate and the substorm strength and strongly motivates future survey using the technique proposed in this paper.

Magnetic reconnection is a key charged particle transport and energy conversion process in environments ranging from astrophysical systems to laboratory plasmas 1 . Magnetic reconnection facilitates plasma transport by establishing new connections of magnetic flux tubes, and it converts, often explosively, energy stored in the magnetic field to kinetic energy of charged particles 2 . The intensity of the magnetic reconnection process is

We report Magnetospheric Multiscale four-spacecraft observations of a thin reconnecting current sheet with weakly asymmetric inflow conditions and a guide field of approximately twice the reconnecting magnetic field. The event was observed at the interface of interlinked magnetic field lines at the flank magnetopause when the maximum spacecraft separation was 370 km and the spacecraft covered ∼1.7 ion inertial lengths (di) in the reconnection outflow direction. The ion-scale spacecraft separation made it possible to observe the transition from electron-only super ion-Alfvénic outflow near the electron diffusion region (EDR) to the emergence of sub-Alfvénic ion outflow in the ion diffusion region (IDR). The EDR to IDR evolution over a distance less than 2 di also shows the transition from a near-linear reconnecting magnetic field reversal to a more bifurcated current sheet as well as significant decreases in the parallel electric field and dissipation. Both the ion and electron heating in this diffusion region event were similar to the previously reported heating in the far downstream exhausts. The dimensionless reconnection rate, obtained four different ways, was in the range of 0.13–0.27. This event reveals the rapid spatial evolution of the plasma and electromagnetic fields through the EDR to IDR transition region.