Statistics of energetic electrons in the magnetotail reconnection

Abstract

Magnetic reconnection has long been regarded as an important site for producing energetic electrons in solar terrestrial and astrophysical plasmas. The motivation of this paper is to provide the average properties of energetic electrons in reconnection region, which are crucial for understanding electron energization mechanism but are rarely known. We statistically analyzed the energetic electrons through 21 magnetotail reconnection events observed by Cluster spacecraft during the years of 2001–2005. Approximately 1200 data points with time resolution of 8 s have been collected for each spacecraft. Two parameters are examined: energetic electron rate (EER) and power law index. EER, which is defined as the ratio of the integrated energetic electron flux to the lower energy electron flux, is used to quantify the electron acceleration efficiency. We find that EER and energetic electron flux (EEF) are positively correlated with the power law index, i.e., the higher rate and flux generally corresponds to softer spectrum. This unexpected correlation is probably caused by some nonadiabatic heating/acceleration mechanisms that tend to soft the spectrum with high temperature. EER is much larger within the earthward flow than the tailward flow. It is positively correlated with the outflow speed Vx, while the correlation between EER and Bz is less clear. With the increment of earthward outflow speed, the occurrence rate of high EER also monotonically increases. We find that EER generally does not increase with the increment of perpendicular electric field |E⊥|, suggesting that adiabatic betatron and Fermi acceleration probably play minor roles in electron energization during magnetotail reconnection.

Contrasting the Mechanisms of Reconnection-driven Electron Acceleration with In Situ Observations from MMS in the Terrestrial Magnetotail

“…We find that first-order Fermi and betatron acceleration in the earthward reconnection outflow is larger than that in the tailward reconnection outflow. This is consistent with Zhou et al (2016), demonstrating that the energetic electron rate is larger earthward of the X line than tailward of it in the magnetotail. They explain that this is likely caused by the trapping of electrons in the closed field lines earthward of the X line; consequently, electrons may cross the acceleration region more times than those in the tailward flow of the X line.…”

“…The Earth's magnetotail, where reconnection and energetic electrons are frequently observed (Eastwood et al 2010;Zhou et al 2016;Fu et al 2019b), is an ideal laboratory for investigating electron acceleration via in situ observations. Previous studies have pointed out that different mechanisms active in different regions can energize electrons during magnetic reconnection (Øieroset et al 2002;Imada et al 2007;Wang et al 2010b;Zhou et al 2015Zhou et al , 2019Eriksson et al 2020;Norgren et al 2020).…”

Direct Evidence for Electron Acceleration Within Ion‐Scale Flux Rope

“…It has been shown that energetic electrons carry up to 50% of the released energy in solar flares (Holman et al, 2003;Lin et al, 2003). Magnetic reconnection, which is a fundamental process converting magnetic energy into plasma energy, is believed to be one major engine for energizing electrons in space and laboratory plasmas (Hurley et al, 2005;Imada et al, 2007;Øieroset et al, 2002;Zhou et al, 2015Zhou et al, , 2016. A long-standing question is how are electrons accelerated to high energy during magnetic reconnection.…”

Suprathermal Electron Acceleration in a Reconnecting Magnetotail: Large‐Scale Kinetic Simulation

“…Suprathermal electrons have been observed in Earth's magnetotail from the midtail to the inner edge of the plasma sheet (Meng et al, ; Øieroset et al, ; Zhou et al, ). Magnetic reconnection releases a large amount of energy that heats and accelerates plasma.…”