Wave Activity in a Dynamically Evolving Reconnection Separatrix

Waves driven by electron beams are frequently observed within the narrow separatrices of magnetic reconnection. Strong plasma instabilities can grow from mixing between the reconnection inflow and outflow, resulting in electron heating which ultimately influences the reconnection process. Observations of a separatrix encounter by the Magnetospheric MultiScale mission on July 11, 2017 feature an anti-parallel electron beam accelerated in a sub-ion-scale layer containing high amplitude (>50 mV/m) electrostatic waves. In two consecutive separatrix crossings, one exhibits faster electron beams, greater magnetic shear, higher amplitude waves, and more electron heating than the other. The observations suggest that variable reconnection outflow pressure is associated with these changes. Coincident with lower hybrid waves are Buneman and beam-mode instabilities, likely responsible for parallel heating of electrons. Dissipation (J⋅E) within the mixing layer is found to be negative where wave activity is strongest, indicating a slowing of electrons and growth of electromagnetic fields likely related to local thinning of the separatrix. Calculation of momentum balance via the generalized Ohm's law indicates an overall imbalance of >40 mV/m in the more extreme separatrix crossing, requiring a significant dissipative term or error in the electron inertial term to make up the remainder. Finally, an analysis of quasi-viscous heating shows that active heating of electrons is more prevalent in the presence of a high speed electron inflow and increased magnetic shear.

Key Points
A rapidly evolving separatrix exhibits thinning, increased shear, and enhanced electron beams associated with changes in the ion outflow
Evaluation of force balance shows contributions from time variation, electron inertia, or the resistive term may be significant
Quasi-viscous effects qualitatively agree with predictions and are largest in the region of increased shear and enhanced inflow

Plain Language Summary
Magnetic reconnection is a process which converts magnetic energy into kinetic energy of charged particles. Particles flowing into a region of reconnection are separated from those in the outflow by a narrow region called the separatrix. We look at a reconnection event observed by the Magnetospheric Multiscale mission where the separatrix appears to be compressed over time. When in the compressed state, violent wave activity is observed which enables mixing between inflowing and outflowing plasma. These waves heat electrons in the inflow, partially counteracting the thinning of the separatrix layer. Detailed analysis reveals evidence for other small scale, non-ideal processes which may be responsible for acceleration of the inflow electrons as well. Since the inflow feeds in to the reconnection site, electron heating and acceleration by compression may act as a feedback mechanism for the reconnection process.