Abstract
Abstract The Magnetospheric Multiscale spacecraft encountered an electron diffusion region (EDR) in a symmetric reconnection in the Earth’s magnetotail. The EDR contained a guide field of about 2 nT, which was 13% of the magnetic field in the inflow region, and its thickness was about 2 local electron inertial lengths. Intense energy dissipation, a super-Alfvénic electron jet, electron nongyrotropy, and crescent-shaped electron velocity distributions were observed in association with this EDR. These features are similar to those of the EDRs in asymmetric reconnection at the dayside magnetopause. Electrons gained about 50% of their energy from the immediate upstream to the EDR. Crescent electron distributions were seen at the boundary of the EDR, while highly curved magnetic field lines inside the EDR may have gyrotropized the electrons. The EDR was characterized by a parallel current that was carried by antiparallel drifting electrons that were probably accelerated by a parallel electric field along the guide field. These results reveal the essential electron physics of the EDR and provide a significant example of an EDR in symmetric reconnection with a weak guide field.
Highlights
Magnetic reconnection is a universal energy release process in space, astrophysical, and laboratory plasmas
The collisionless Hall-reconnection model points out that the diffusion region consists of an ion-scale ion diffusion region (IDR) and an embedded electron-scale electron diffusion region (EDR) due to the different masses of ions and electrons (Birn et al 2001)
The crescent was less visible (Figure 5(h)) and the degree of electron nongyrotropy was smaller inside the EDR than at the boundary of the EDR
Summary
Magnetic reconnection is a universal energy release process in space, astrophysical, and laboratory plasmas. Dozens of EDRs have been identified in association with asymmetric reconnection at the dayside magnetopause by MMS data (e.g., Burch et al 2016; Khotyaintsev et al 2016; Lavraud et al 2016; Webster et al 2018) These EDRs were associated with the ion bulk flow reversal, energy dissipation in the electron frame J · (E + Ve × B) > 0 (J · (E+ Ve × B) was occasionally negative in the EDR as reported in Genestreti et al 2017 and Wang et al 2017) and crescent-shaped electron velocity distribution, a novel feature uncovered by the unprecedented high-resolution particle data (e.g., Burch et al 2016). The following instruments on board MMS have been used in this study: the Fluxgate Magnetometer provides magnetic field observations (Russell et al 2016); the Electric field Double Probe provides three component electric field observations (Ergun et al 2016; Lindqvist et al 2016; Torbert et al 2016b); the Fast Plasma Instrument provides 3D particle velocity distributions and the corresponding integrated plasma moments (Pollock et al 2016); and the Fly’s Eye Energetic Particle Spectrometer provides all-sky snapshots of electron fluxes from 30 to 600 keV (Blake et al 2016)
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