Characteristics of Energetic Electrons Near Active Magnetotail Reconnection Sites
Abstract
Magnetic reconnection is considered a fundamental universal process where energy is transferred from magnetic fields to particles. However, it remains unclear whether this transfer of energy includes the direct acceleration of particles to high energies (10s of keV and above). Recently, in-situ observations from NASA's Magnetospheric Multiscale (MMS) mission have revolutionized understanding of magnetic reconnection providing high-resolution, multi-point in-situ measurements of reconnection events. Though several electron-scale studies of reconnection have shown evidence of electron energization, at times even up to tens of keV energies, no studies have focused on the significance of high-energy particle energization. In particular, the study of potential particle energization resulting from magnetic reconnection was a critical science objective for the MMS Energetic Particle Detector (EPD) investigation. The studies presented here focus on high-resolution electron energy and pitch angle measurements from one of the EPD instruments, the Fly's Eye Energetic Particles Spectrometers (FEEPS). To-date, eight potential electron diffusion regions (EDRs) have been observed by MMS in the magnetotail. We focus on these events as they are assumed to occur in very close proximity to active reconnection X-lines. The studies detailed here investigate both the spectral shape and angular distributions of $>$50 keV electrons to study the energization and fine-scale structure of acceleration. First, to investigate the potential significance of electron energization at/near an X-line, the spectral indices of the energetic ($>$50 keV) electrons observed by FEEPS for the EDR candidate events were compared with those from "typical, quiet-time" plasma sheet crossings (PSXs) by MMS, where no local source of energetic electrons would be expected. These plasma sheet crossings were identified using a set of locational, magnetic field, plasma, and global index criteria that resulted in a dataset of 275 events. The results provide evidence that processes associated with magnetic reconnection can result in local direct energization of electrons up to $\sim$m$ _{e}$c$ ^{2}$ (i.e., 511 keV, relativistic energies). Second, the nearly-full-sky angular coverage from FEEPS provides unprecedented multi-point investigation of the topology and evolution of energetic electrons near an active reconnection site. Analysis of an exemplary case, the 11 July 2017 EDR event, shows variations in angular responses across energy channels on a single spacecraft and at the same energy across different spacecraft. Despite this, the signal seems to be coherent in gyrophase and suggestive of a limited extent of the X-line on the scale of the gyroradius of $\sim$100 keV electrons ($\sim$500 km).
- Publication:
-
43rd COSPAR Scientific Assembly. Held 28 January - 4 February
- Pub Date:
- January 2021
- Bibcode:
- 2021cosp...43E1141C