Finding Local Parallel Electric Fields in Magnetotail Reconnection Using a Two-Spacecraft Method

We investigate the use of a novel two-spacecraft Liouville-mapping method using data from the Magnetospheric Multiscale mission to determine local magnetic-field-aligned (parallel) electric fields in the Earth's magnetotail. The method detects the presence of local acceleration potentials by mapping phase-space density between electron velocity distribution functions from two field-aligned spacecraft upstream and downstream of acceleration regions. Applying the method to a magnetic reconnection event, we find that local parallel electric fields are, on average, directed away from the current sheet center, resulting from the need to maintain quasi-neutrality across the current sheet. We find that the local acceleration potentials are smaller than the total acceleration potential, typically 1-2% on average and up to 9% for individual measurements. This indicates that many potential drops, over distances much larger than the spacecraft separations, contribute to the net work done on electrons by parallel electric fields.