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 near the current sheet (CS) center are, on average, directed away from the center, resulting from the need to maintain quasi‐neutrality across the CS. Despite significant measurement uncertainties, 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.