MMS observation of cold electrons in the magnetotail reconnection separatrix region

Cold plasmas can modify dynamical processes during magnetic reconnection. There are many studies on the roles of cold ions in mod-ifying reconnection process. However, there have been a lack of studies on the roles of cold electrons. Using MMS high-cadence data, we present observation of cold electrons originated from the terrestrial lobe at the magnetotail reconnection separatrix region. The number density of these cold electrons relative to the total number density was 67%-95%. Current density carried by them can reach-50nA/m2, which is comparable to or even greater than that of current driven by other electron populations. Currents driven by cold and high-energy electrons generally canceled each other out. These findings indicate that cold electrons can greatly modify the current at the sep-aratrix. Currents driven by cold electrons are stronger in the inner part of the separatrix region, and have electron-scale structures, where current density changes by -20nA/m2 in -3 de. Our observations provide insights into roles of cold electrons during reconnection and electron dynamics in reconnection jet far downstream of the X-line. (c) 2023 COSPAR. Published by Elsevier B.V. All rights reserved.