Electron Acceleration and Thermalization at Magnetotail Separatrices

Abstract

In this study we use the Magnetospheric Multiscale mission to investigate the electron acceleration and thermalization occurring along the magnetic reconnection separatrices in the magnetotail. We find that initially cold electron lobe populations are accelerated toward the X line forming beams with energies up to a few kiloelectron volts, corresponding to a substantial fraction of the electron thermal energy inside the exhaust. The accelerated electron populations are unstable to the formation of electrostatic waves which develop into nonlinear electrostatic solitary waves. The waves' amplitudes are large enough to interact efficiently with a large part of the electron population, including the electron beam. The wave-particle interaction gradually thermalizes the beam, transforming directed drift energy to thermal energy.

Multisatellite MMS Analysis of Electron Holes in the Earth's Magnetotail: Origin, Properties, Velocity Gap, and Transverse Instability

“…In particular, the MMS measurements have already allowed resolving the three‐dimensional structure of electron phase space holes (Holmes et al., 2018; Steinvall, Khotyaintsev, Graham, Vaivads, Lindqvist, et al, 2019; Tong et al., 2018), measuring the dearth of the phase space density of trapped electrons (Mozer et al., 2018), and resolving electromagnetic structure of subrelativistic electron phase space holes (Le Contel et al., 2017). In addition, the measurements of the MMS spacecraft allowed detecting whistler waves emitted by electron phase space holes via the Cherenkov resonance (Steinvall, Khotyaintsev, Graham, Vaivads, Le Contel, et al, 2019) and identifying electron acceleration and thermalization associated with electron phase space holes (Khotyaintsev et al., 2020; Mozer, Agapitov, Artemyev, et al, 2016; Mozer, Artemyev, Agapitov, et al, 2016; Norgren et al., 2020).…”

Collisionless Magnetic Reconnection and Waves: Progress Review

“…The wave-particle interaction can be seen as modulations of the accelerated population. Figure 5c shows corresponding observations from a magnetic reconnection separatrix in the magnetotail made by MMS (Norgren et al, 2019). The colormap shows the 1-D reduced electron VDF, similar to Figure 5b.…”

“…The electric field amplitudes range from < 1 mV/m to a few hundreds mV/m in more rare cases (Graham et al, 2016b;Khotyaintsev et al, 2016), and are generally larger in the magnetotail than at the dayside magnetopause. Normalized to the electron temperature, the potential amplitude can span several orders of magnitude, from eφ/k B T e = 10 −5 to above unity (Franz et al, 2005;Graham et al, 2016b;Norgren et al, 2019). At the magnetopause the potentials of EWs are in general lower than those of ESWs (Graham et al, 2016b).…”

“…In symmetric antiparallel reconnection, the strongest fieldaligned electron flows are formed at the four separatrices, by electrons streaming toward the X line (Fujimoto, 2014;Egedal et al, 2015;Norgren et al, 2019). Figure 5a shows the localized parallel bipolar electric fields developed as a consequence of these accelerated electron flows in a numerical simulation of symmetric magnetic reconnection (Fujimoto, 2014).…”

Wave Activity in a Dynamically Evolving Reconnection Separatrix

“…A recent study by Norgren et al. (2020) of acceleration and thermalization of electrons on the separatrix investigated the details of the heating process. They clearly identified low‐density electron acceleration channels where beam velocities gradually increase with the electrostatic potential traversed by the Magnetospheric Multiscale (MMS) spacecraft (Burch et al., 2016).…”