Characteristics of Energetic Electrons Near Active Magnetotail Reconnection Sites: Statistical Evidence for Local Energization. Issue 1 (9th January 2021)

• Record Type: Journal Article
• Authors: Cohen, Ian J.; Turner, Drew L.; Mauk, Barry H.; Bingham, Sam T.; Blake, J. Bern; Fennell, Joseph F.; Burch, James L.
• Is Part Of: Geophysical research letters. Volume 48:Issue 1(2021)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2021-01-09
• Subjects: particle acceleration -- magnetic reconnection -- energetic particles -- electron diffusion region -- electron acceleration; Geophysics -- Periodicals; Planets -- Periodicals; Lunar geology -- Periodicals; 550
• DOI: 10.1029/2020GL090087
• Languages: English
• ISSNs: 0094-8276

Abstract

Abstract: Magnetic reconnection is a universal process where energy is transferred from magnetic fields to particles. However, it remains unclear whether this transfer of energy includes the direct energization of particles to high energies (i.e., >10s of keV). We compare the spectral indices of energetic (50–200 keV) electrons from six magnetotail electron diffusion region candidates encountered by MMS with those from "quiet‐time" plasma sheet crossings to test whether energetic electron enhancements result from direct energization at/near the reconnection site or simply focusing high‐energy populations that do not originate from the nearby X‐line and already exist in the plasma sheet along the newly reconnected field lines. The study suggests that active reconnection can be a source of localized electron energization. However, the EDR spectra are not outside the range of those from the quiet‐time plasma sheet, which surprisingly still often have very energetic (up to and beyond 200 keV) electrons present. Plain Language Summary: Magnetic reconnection is a fundamental process in plasma physics and is hypothesized to be an efficient process to accelerate particles up to very high energies by transferring energy held in the magnetic field to the particles. Similar to how a taught rubber band can snap, releasing its latent energy—so too do magnetic fields "snap" in reconnection; as the field lines relax to their normal state, they transfer their energy to particle. This paper addresses an outstanding question of whether magnetic reconnection in fact produces high energy electrons. While many computer simulations have predicted this, most in situ observations have focused on energization at much lower energies than we look at here. However, this paper compares a handful of events very near the region where reconnection is believed to occur to a "control group" of nonactive regions not near a reconnection site and finds that the events nearest the reconnection do tend to have more energetic electrons. This provides evidence that the energetic electrons are in fact coming from the reconnection site. Key Points: Six candidate EDRs identified by the MMS team are compared to a statistical database of quiet‐time plasmasheet crossings The spectral indices of energetic electrons near the EDRs are indeed harder than the plasmasheet crossings, suggesting local energization Even the quiet‐time plasmasheet crossings include cases with surprisingly hard spectra, perhaps sourced by distant tail reconnection