Electron Acceleration in a Magnetotail Reconnection Outflow Region Using Magnetospheric MultiScale Data

Elin Eriksson,Daniel B Graham,Love Alm,Yuri V Khotyaintsev,Mats André,Andris Vaivads

doi:10.1029/2019gl085080

Abstract

AbstractWe study Magnetospheric MultiScale observations in the outflow region of magnetotail reconnection. We estimate the power density converted via the three fundamental electron acceleration mechanisms: Fermi, betatron, and parallel electric fields. The dominant mechanism, both on average and the peak values, is Fermi acceleration with a peak power density of about +200 pW/m3. The magnetic field curvature during the most intense Fermi acceleration is comparable to the electron gyroradius, consistent with efficient electron scattering. The peak power densities due to the betatron acceleration are a factor of 3 lower than that for the Fermi acceleration, the average betatron acceleration is close to zero and slightly negative. The contribution from parallel electric fields is significantly smaller than those from the Fermi and betatron acceleration. However, the observational uncertainties in the parallel electric field measurement prevent further conclusions. There is a strong variation in the power density on a characteristic ion time scale.

Highlights

Magnetic reconnection is a fundamental energy conversion process present in space (e.g. Paschmann et al, 2013) and laboratory environments (e.g. Yamada et al, 2010)
We focus on electron energization, which is important in astrophysical context, where electromagnetic radiation generated by accelerated electrons enables remote observations of reconnection regions, such as those in solar flares (Petrosian, 2016)
We show the first experimental estimates of the electron acceleration power densities in a reconnection outflow region due to the three mechanisms: Fermi acceleration, betatron acceleration, and acceleration due to E||

Summary

Introduction

Magnetic reconnection is a fundamental energy conversion process present in space (e.g. Paschmann et al, 2013) and laboratory environments (e.g. Yamada et al, 2010). The three terms correspond to the power density of electron acceleration due to the betatron acceleration, WBetatron, Fermi acceleration, WFermi, and acceleration by E||, WE|| These three terms have previously been estimated in numerical simulations of reconnection (Dahlin et al, 2014; Zhou et al, 2018). In Zhou et al (2018) a large-scale kinetic 3-D simulation based on an observed THEMIS magnetotail reconnection event is analyzed where the guide field is 10% They find that acceleration by E|| is the dominant acceleration mechanisms and contrary to Dahlin et al (2014) betatron acceleration is not negligible, instead WBetatron is much larger than WFermi. To observationally estimate the power density due to the three electron acceleration mechanisms requires high-resolution electron measurements of the MMS mission

Observations

Discussion

Conclusions