MMS Observation of Asymmetric Reconnection Supported by 3‐D Electron Pressure Divergence

Abstract

We identify the electron diffusion region (EDR) of a guide field dayside reconnection site encountered by the Magnetospheric Multiscale (MMS) mission and estimate the terms in generalized Ohm's law that controlled energy conversion near the X‐point. MMS crossed the moderate‐shear (∼130°) magnetopause southward of the exact X‐point. MMS likely entered the magnetopause far from the X‐point, outside the EDR, as the size of the reconnection layer was less than but comparable to the magnetosheath proton gyroradius, and also as anisotropic gyrotropic “outflow” crescent electron distributions were observed. MMS then approached the X‐point, where all four spacecraft simultaneously observed signatures of the EDR, for example, an intense out‐of‐plane electron current, moderate electron agyrotropy, intense electron anisotropy, nonideal electric fields, and nonideal energy conversion. We find that the electric field associated with the nonideal energy conversion is (a) well described by the sum of the electron inertial and pressure divergence terms in generalized Ohms law though (b) the pressure divergence term dominates the inertial term by roughly a factor of 5:1, (c) both the gyrotropic and agyrotropic pressure forces contribute to energy conversion at the X‐point, and (d) both out‐of‐the‐reconnection‐plane gradients (∂/∂M) and in‐plane (∂/∂L,N) in the pressure tensor contribute to energy conversion near the X‐point. This indicates that this EDR had some electron‐scale structure in the out‐of‐plane direction during the time when (and at the location where) the reconnection site was observed.

MMS Measurements of the Vlasov Equation: Probing the Electron Pressure Divergence Within Thin Current Sheets

“…Detection of electron‐scale current sheets in Earth's magnetic environment has become possible with the high‐resolution instrumentation onboard the Magnetospheric Multiscale (MMS) spacecraft, which can resolve electron skin depth ( d e ) scale, filamentary currents narrower than the spacecraft separation (Phan et al, ; Phan et al, ; Yordanova et al, ; Wilder et al, ). Current sheets with thicknesses on the order of a few d e are often observed in association with the electron diffusion region (EDR) and separatrix regions of magnetic reconnection (Burch et al, ; Chen et al, ; Chen et al, ; Genestreti et al, ; Norgren et al, ; Torbert et al, ; Torbert et al, ; Wang et al, ; Webster et al, ). Recent studies have shown that thin current sheets and their associated electron crescent distributions can result purely as a manifestation of diamagnetic effects (Egedal et al, ; Rager et al, ), though these are typically found in regions where there is significant dissipation (Eriksson et al, ; Torbert et al, ; Genestreti et al, ; Wilder et al, ).…”

Effect of the Reconnection Electric Field on Electron Distribution Functions in the Diffusion Region of Magnetotail Reconnection

“…For example, in National Aeronautics and Space Administration's Magnetospheric Multiscale (MMS) mission (Burch et al, 2015), electron VDFs with a crescent shape have been detected as evidence of fast reconnection Chen, Hesse, Wang, Gershman, et al, 2016;Chen et al, 2017;Genestreti et al, 2017Genestreti et al, , 2018Norgren et al, 2016;Phan et al, 2016;Rager et al, 2018). For example, in National Aeronautics and Space Administration's Magnetospheric Multiscale (MMS) mission (Burch et al, 2015), electron VDFs with a crescent shape have been detected as evidence of fast reconnection Chen, Hesse, Wang, Gershman, et al, 2016;Chen et al, 2017;Genestreti et al, 2017Genestreti et al, , 2018Norgren et al, 2016;Phan et al, 2016;Rager et al, 2018).…”

“…In space observations such as in Earth's magnetosphere, velocity distribution functions (VDFs) for ions and electrons provide us useful information about the physics working on local plasmas. For example, in National Aeronautics and Space Administration's Magnetospheric Multiscale (MMS) mission (Burch et al, 2015), electron VDFs with a crescent shape have been detected as evidence of fast reconnection Chen, Hesse, Wang, Gershman, et al, 2016;Chen et al, 2017;Genestreti et al, 2017Genestreti et al, , 2018Norgren et al, 2016;Phan et al, 2016;Rager et al, 2018).…”

Energy Conversion at Kinetic Scales in the Turbulent Magnetosheath

“…The relative importance of the terms in Equation (1) and of the EED measures in Equations (2-4) have been studied both numerically and experimentally at reconnecting current sheets (Hesse et al, 2016;Nakamura et al, 2016;Shay et al, 2016;Torbert et al, 2016b;Genestreti et al, 2018). According to these event studies, E ′ (left hand side of Equation 1) dominates outside the reconnection diffusion region, where both electrons and ions are attached to the magnetic field, and the right hand side of the Equation (1) is negligible.…”

“…Inside the electron diffusion region, where both ions and electrons are demagnetized, the terms on the right hand side of Equation (1) balance E ′ and the first term on the right, (∇.P e ), is much larger than the second inertial term. However, near the reconnection X-line the inertial term can reach half of the pressure divergence term (Genestreti et al, 2018). Since the probability of crossing multiple reconnection X-lines in the magnetosheath is low, the inertial term can be neglected.…”