Electron Scale Interactions at the Reconnection Separatrix

The magnetic reconnection separatrix divides two distinct populations of plasma flowing toward and away from the X-line. Observations from the Magnetospheric Multiscale (MMS) Mission reveal a highly structured environment with waves and filamentary currents on electron dynamical scales. One of the more puzzling features of the separatrix is the presence of localized, high amplitude spikes in the parallel electric field (E ∥ ). They are likely signatures of strong parallel heating of inflowing electrons, and grow from streaming instabilities where the reconnection inflow and outflow mix across the separatrix boundary. It remains unclear how this mixing process begins and stops, or why it occurs in short bursts rather than continuously. To address this problem, we perform 2D particle-in-cell simulations of a thin separatrix model to investigate conditions for wave growth, particle acceleration, and heating. In the parallel direction, the inflow becomes Buneman unstable as outflowing ions drift across the boundary. In high shear cases, the Buneman mode alone is sufficient to reproduce MMS-observed electron heating. Lower-hybrid instabilities grow in the perpendicular plane, possibly enhancing the counter streaming populations. However, localized spikes in E ∥ are not found in the simulation, requiring either a 3D treatment or dynamics specific to a fully reconnecting current sheet such as the adiabatic expansion of inflowing electrons.