Structure of Electron-Scale Plasma Mixing Along the Dayside Reconnection Separatrix

Interactions between magnetic reconnection inflows and outflows can result in a violent mixing process. In Magnetospheric MultiScale observations of asymmetric, low guide-field reconnection, highly sheared electron flow paired with sharp density and temperature gradients have been found in association with bursts of strong (≥100 mV/m) electric fields parallel to the ambient magnetic field. It is likely that large spikes in E‖ are part of a dynamic, small-scale structure which results from mixing between plasmas. In this study, a 1-D Vlasov simulation with parameters directly comparable to the observed plasma environment and interaction timescale is used to demonstrate that mixing at a sharp boundary between magnetospheric and magnetosheath electrons is qualitatively consistent with measured particle distributions and signatures in E‖. Properties of mixing structures such as net electric potential are estimated and found capable of accelerating electron beams toward the electron diffusion region but are not necessarily sufficient to generate the strongest observed jets. Obliquely propagating lower hybrid drift waves are also present and likely provide most of the energy for acceleration. Drift waves may be responsible for cross-field transport required to begin the mixing process. We conclude that parallel mixing primarily acts to mediate plasma boundaries, thermalizing electron beams contributing to the high anisotropy (Te‖>Te⊥) electron distributions found in the dayside reconnection magnetospheric inflow region.

Key Points
Violent mixing between plasmas is observed within an electron-scale boundary along the dayside magnetosphere reconnection separatrix
Consecutive negative spikes in parallel electric field can be explained by a single oscillating double-layer-like structure
Electrostatic mixing signatures likely contribute primarily to thermalization and heating rather than linear acceleration toward the X line