Mapping and Characterizing Parallel Electric Field Structures During a Magnetotail Reconnection Event

Magnetic reconnection occurs commonly at current sheets in magnetized plasmas, accelerating electrons to very high energies. Among the various acceleration processes, magnetic-field-aligned (parallel) electric fields can directly accelerate electrons during reconnection. In this study, we map the spatial distribution of parallel electric field structures and characterize their properties during a magnetic reconnection event in the Earth’s magnetotail, using 3D electric field measurements from the Magnetospheric Multiscale mission. The types of observed structures include electron and ion phase-space holes, electrostatic and electromagnetic waves, and double layer-like (DL-like) structures. We determine the properties and locations of the structures, and investigate in detail the effects of the DL-like structures on electron acceleration, trapping, and heating in the separatrix region (SR). Our observations reveal that 71% of all timeable structures propagate toward the X line, including 82% of electron-hole trains and 70% of electrostatic waves. Of the structures that are timeable with all four spacecraft (130 of 264), we find that only five, all in the SR, have a peak potential greater than a tenth of the local electron temperature. Five DL-like structures are found, all of which are located in the SR. One of these DL-like structures causes clear parallel acceleration of an electron beam toward the X line, which subsequently generates large-amplitude electron holes. An electron beam traveling from the X line is observed to reflect off the DL-like structure back toward the X line, contributing to electron trapping and regulating quasi-neutrality in the ion diffusion region.