Observation of Nongyrotropic Electron Distribution Across the Electron Diffusion Region in the Magnetotail Reconnection

Abstract

Using measurements by the Magnetospheric Multiscale spacecraft in the magnetotail, we studied electron distribution functions across an electron diffusion region. The dependence of the nongyrotropic distribution on the energy and vertical distance from the electron diffusion region midplane was revealed for the first time. The nongyrotropic distribution was observed everywhere except for an extremely narrow layer right at the electron diffusion region midplane. The energy of the nongyrotropic distribution increased with growth of the vertical distance from the midplane. For the electrons within certain energy range, they exhibited the nongyrotropic distribution at the distance further away from the midplane than that expected from the meandering motion. The correlation between the crescent-shaped distribution with multiple stripes and the large Hall electric field was established. It appears that the measured nongyrotropic distribution and the crescent-shaped distribution were caused by the meandering motion and the Hall electric field together. Plain Language SummaryThe Magnetospheric Multiscale mission is designed to study electron physics of magnetic reconnection, a key process for many explosive phenomena in solar flares and magnetosphere. Understanding electron motion is highly important in the study of magnetic reconnection, and the electron velocity distribution is an intuitive and effective way to study the electron dynamics. In this paper, we present a complete electron diffusion region crossing by Magnetospheric Multiscale during a nearly symmetric magnetic reconnection in the magnetotail and show the evolution of the electron velocity distributions across the electron diffusion region. In addition, the relationship between Hall electric field and crescent-shaped electron distribution is also shown in this paper. Key Points:• The electron nongyrotropic distribution depended on the energy and the normal distance from the midplane of the electron diffusion region • The electron crescent distributions were caused by the meandering motion and the Hall electric field together • The fine substructures were observed in the narrow electron diffusion region

Physical Implication of Two Types of Reconnection Electron Diffusion Regions With and Without Ion‐Coupling in the Magnetotail Current Sheet

“…These strong currents were verified to be carried by the electrons (Li et al, 2019;Huang et al, 2018;Wang et al, 2018;Zhou et al, 2019) and will be analyzed later in their individual local current coordinates. The local current system in Event I was derived from minimum variance analysis (Paschmann & Schwartz, 2000) The details on the choice of the local current system can be found in Li et al (2019). Because the four MMS satellites passed through the electron current layers one after another, the Timing method (Schwartz, 1998) was used to estimate the current layer velocity and the thickness.…”

“…, as an indication of in the EDR proximity (Burch et al, 2016;Egedal et al, 2016;Hesse et al, 2014;Li et al, 2019;Torbert et al, 2018), was observed. Thus, we conclude that the current layer in Event I was an EDR.…”

“…In both events, the spacecraft crossed the magnetotail plasma sheet from the south hemisphere to the north hemisphere and detected an extremely strong current enhancement, up to 200 nA/m 2 , at its center (Figures 1e and 1m). These strong currents were verified to be carried by the electrons (Li et al, 2019; Huang et al, 2018; Wang et al, 2018; Zhou et al, 2019) and will be analyzed later in their individual local current coordinates.…”

“…In this letter, by examining one reconnection event without ion‐coupling (Wang et al, 2018) and the other with ion‐coupling (Li et al, 2019; Zhou et al, 2019) in the magnetotail, we determine the differences between and similarities of the two events. Based on the comparison, we propose that the two forms of reconnection correspond to two different stages of reconnection in the macroscale current sheet of the magnetotail and examine the potential factors controlling the transition between the two stages.…”

Magnetic-field annihilation and island formation in electron-scale current sheet in Earth's magnetotail

“…Earlier studies of this current sheet 8,11 reported established signatures of EDRs 4,9 , including oppositely-directed electron jets with a speed exceeding -1 ( Fig. 2c), crescent-shaped electron velocity distributions and magnetic-to-particle energy conversion.…”