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We present a model of electromagnetic drift waves in the current sheet adjacent to magnetic reconnection at the subsolar magnetopause. These drift waves are potentially important in governing 3‐D structure of subsolar magnetic reconnection and in generating turbulence. The drift waves propagate nearly parallel to the X line and are confined to a thin current sheet. The scale size normal to the current sheet is significantly less than the ion gyroradius and can be less than or on the order of the wavelength. The waves also have a limited extent along the magnetic field (B), making them a three‐dimensional eigenmode structure. In the current sheet, the background magnitudes of Band plasma density change significantly, calling for a treatment that incorporates an inhomogeneous plasma environment. Using detailed examination of Magnetospheric Multiscale observations, we find that the waves are best represented by series of electron vortices, superimposed on a primary electron drift, that propagate along the current sheet (parallel to the X line). The waves displace or corrugate the current sheet, which also potentially displaces the electron diffusion region. The model is based on fluid behavior of electrons, but ion motion must be treated kinetically. The strong electron drift along the X line is likely responsible for wave growth, similar to a lower hybrid drift instability. Contrary to a classical lower hybrid drift instability, however, the strong changes in the background Band no, the normal confinement to the current sheet, and the confinement along Bare critical to the wave description. Drift waves are potentially important in governing 3D structure of subsolar magnetic reconnection and in generating turbulenceDrift waves displace or corrugate the current sheet and potentially displace the electron diffusion region of magnetic reconnectionParallel electric fields arise in the drift waves

The structure of dipolarization jets with finite width in the dawn‐dusk direction relevant to magnetic reconnection in the Earth's magnetotail is explored with particle‐in‐cell simulations. We carry out Riemann simulations of the evolution of the jet in the dawn‐dusk, north‐south plane to investigate the dependence of the jet structure on the jet width in the dawn‐dusk direction. We find that the magnetic field and Earth‐directed ion flow structure depend on the dawn‐dusk width. A reversal in the usual Hall magnetic field near the center of the current sheet on the duskside of larger jets is observed. For small widths, the maximum velocity of the earthward flow is significantly reduced below the theoretical limit of the upstream Alfvén speed. However, the ion flow speed approaches this limit once the width exceeds the ion Larmor radius based on the normal magnetic field, Bz. Magnetic reconnection is a phenomenon occurring in the Earth's magnetotail in which energy stored in the magnetic field is converted into kinetic energy. This process creates a high‐speed dipolarization jetof plasma. However, the physical dimension of these jets in the dawn‐dusk direction is not well constrained. We study how the structure of dipolarization jets depends on their width in the dawn‐dusk direction and find that both the shape of the magnetic field and the profile of the ion outflow speed across the jet vary as a function of the jet width. In particular, the maximum ion outflow can fall well short of the theoretically expected outflow speed for small widths. Our results can be used by satellites to determine their positions relative to observed dipolarization jets and place limits on the jet widths. The dependence of the structure of magnetotail dipolarization jets on cross‐tail width is studied through 2‐D Riemann simulationsFor small widths the jet velocity falls well below expectations based on predictions of conventional reconnection modelsIn wide jets the self‐generated Hall magnetic field reverses sign compared with traditional predictions on the dawn edge of the jet

Electron ring velocity space distributions have previously been seen in numerical simulations of magnetic reconnection exhausts and have been suggested to be caused by the magnetization of the electron outflow jet by the compressed reconnected magnetic fields (Shuster et al., 2014, https://doi.org/10.1002/2014GL060608). We present a theory of the dependence of the major and minor radii of the ring distributions solely in terms of upstream (lobe) plasma conditions, thereby allowing a prediction of the associated temperature and temperature anisotropy of the rings in terms of upstream parameters. We test the validity of the prediction using 2.5‐dimensional particle‐in‐cell (PIC) simulations with varying upstream plasma density and temperature, finding excellent agreement between the predicted and simulated values. We confirm the Shuster et al. suggestion for the cause of the ring distributions, and also find that the ring distributions are located in a region marked by a plateau, or shoulder, in the reconnected magnetic field profile. The predictions of the temperature are consistent with observed electron temperatures in dipolarization fronts, and may provide an explanation for the generation of plasma with temperatures in the 10s of MK in super‐hot solar flares. A possible extension of the model to dayside reconnection is discussed. Since ring distributions are known to excite whistler waves, the present results should be useful for quantifying the generation of whistler waves in reconnection exhausts. Solar flares and geomagnetic substorms are naturally occurring eruptions in space that can impact humans on Earth due to space weather. Both are caused by magnetic reconnection, during which magnetic field lines break and release energy into the surrounding ionized gas (plasma). From past research, we know that electrons near the reconnection site get magnetized in the strong magnetic fields that have already undergone reconnection, leading to a characteristic ring distribution of their velocities where all particles have similar speed in the plane perpendicular to the magnetic field. We predict the speed of the particles in terms of the ambient properties of the easily measured surrounding plasma, and we confirm the prediction with numerical simulations. We show that the rings are located in a region where there is a leveling off of the magnetic field strength, which is a signature that can be used to identify ring distributions in future satellite measurements. We then use the result to predict temperatures in geomagnetic substorms and solar flares, finding that there is reasonable agreement. This suggests that we can understand the observed temperatures in terms of the ambient plasma properties, which will make it easier to predict these temperatures going forward. We predict major and minor radii of ring distributions during reconnection in terms of upstream parameters and confirm with particle‐in‐cell simulationsWe find that ring distributions occur at a shoulder (plateau) in the reconnected magnetic field in the simulationsThe predicted temperatures are comparable to observed values in dipolarization fronts in Earth's magnetotail and in super‐hot solar flares We predict major and minor radii of ring distributions during reconnection in terms of upstream parameters and confirm with particle‐in‐cell simulations We find that ring distributions occur at a shoulder (plateau) in the reconnected magnetic field in the simulations The predicted temperatures are comparable to observed values in dipolarization fronts in Earth's magnetotail and in super‐hot solar flares

We analyze the development and influence of turbulence in three‐dimensional particle‐in‐cell simulations of guide field magnetic reconnection at the magnetopause with parameters based on observations from the Magnetospheric Multiscale (MMS) mission. Along the separatrices the turbulence is a variant of the lower‐hybrid‐drift instability (LHDI) that produces electric field fluctuations with amplitudes much greater than the reconnection electric field. The turbulence controls the scale length of the density and current profiles while enabling significant transport across the magnetopause, despite the electrons remaining frozen‐in to the magnetic field. Transport of the out‐of‐plane current density exceeds that of the particle density. Near the X‐line the electrons are not frozen‐in and the turbulence, which differs from the LHDI, makes a significant net contribution to the generalized Ohm's law through an anomalous viscosity. The characteristics of the turbulence and associated particle transport are consistent with fluctuation amplitudes in the MMS observations. However, for this event the simulations suggest that the MMS spacecraft were not close enough to the core of the electron diffusion region to identify the region where anomalous viscosity is important. Simulations of magnetopause reconnection develop instabilities similar to observationsFor guide field reconnection, lower‐hybrid drift instability is stabilized at the X‐line but not the separatricesDespite the electrons being frozen‐in, significant transport occurs

We present detailed analysis of the turbulence observed in three‐dimensional particle‐in‐cell simulations of magnetic reconnection at the magnetopause. The parameters are representative of an electron diffusion region encounter of the Magnetospheric Multiscale (MMS) mission. The turbulence is found to develop around both the magnetic X line and separatrices, is electromagnetic in nature, is characterized by a wave vector kgiven by kρe∼(meTe/miTi)0.25with ρethe electron Larmor radius, and appears to have the ion pressure gradient as its source of free energy. Taken together, these results suggest the instability is a variant of the lower hybrid drift instability. The turbulence produces electric field fluctuations in the out‐of‐plane direction (the direction of the reconnection electric field) with an amplitude of around ±10 mV/m, which is much greater than the reconnection electric field of around 0.1 mV/m. Such large values of the out‐of‐plane electric field have been identified in the MMS data. The turbulence in the simulations controls the scale lengths of the density profile and current layers in asymmetric reconnection, driving them closer to ρeρithan the ρeor descalings seen in 2‐D reconnection simulations, and produces significant anomalous resistivity and viscosity in the electron diffusion region. Three‐dimensional particle‐in‐cell simulations of an MMS electron diffusion region encounter at the magnetopause were performedCharacteristics of the turbulence are identified and are consistent with an electromagnetic branch of the lower hybrid drift instabilityThe characteristic scales of the density and current are controlled by turbulence and are a hybrid of the electron and ion Larmor radii

We use theory and simulations to study how the out‐of‐plane (guide) magnetic field strength modifies the location where the energy conversion rate between the electric field and the plasma is appreciable during asymmetric magnetic reconnection, motivated by observations (Genestreti et al., 2017). For weak guide fields, energy conversion is maximum on the magnetospheric side of the X line, midway between the X line and electron stagnation point. As the guide field increases, the electron stagnation point gets closer to the X line, and energy conversion occurs closer to the electron stagnation point. We motivate one possible nonrigorous approach to extend the theory of the stagnation point location to include a guide field. The predictions are compared to two‐dimensional particle‐in‐cell (PIC) simulations with vastly different guide fields. The simulations have upstream parameters corresponding to three events observed with Magnetospheric Multiscale (MMS). The predictions agree reasonably well with the simulation results, capturing trends with the guide field. The theory correctly predicts that the X line and stagnation points approach each other as the guide field increases. The results are compared to MMS observations, Active Magnetosphere and Planetary Electrodynamics Response Experiment (AMPERE) observations of each event, and a global resistive‐magnetohydrodynamics simulation of the 16 October 2015 event. The PIC simulation results agree well with the global observations and simulation but differ in the strong electric fields and energy conversion rates found in MMS observations. The observational, theoretical, and numerical results suggest that the strong electric fields observed by MMS do not represent a steady global reconnection rate. We motivate a theory of the guide field dependence of the location within the diffusion region of X line, stagnation points, and nonzero J· E′Two‐dimensional PIC simulations of three MMS events confirm location of energy conversion moves toward electron stagnation point with increasing guide fieldReconnection rate in 2‐D PIC simulations agrees well with AMPERE observations and global simulations but are far lower than MMS observations