Comparisons of electron acceleration efficiency among different structures during magnetic reconnection: a Cluster multicase study

Deng

et al. 2016

JGR Space Physics

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.

Section: Data Setmentioning

confidence: 99%

Statistics of energetic electrons in the magnetotail reconnection

Deng

et al. 2016

JGR Space Physics

“…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.…”

Section: Introductionmentioning

confidence: 99%

Direct Evidence for Electron Acceleration Within Ion‐Scale Flux Rope

Zhong

Geophysical Research Letters

Tang

et al. 2020

Self Cite

Energetic electrons have frequently been observed in small‐scale flux ropes. However, whether these energetic electrons were energized directly within the flux rope or not is unknown. In this paper, we present concrete evidence provided by the Magnetospheric Multiscale mission that a secondary flux rope provided strong acceleration for electrons expelled by the reconnection X line. We find that the energetic electron fluxes inside the ion‐scale flux rope were larger than those outside the flux rope. Electrons were adiabatically accelerated by betatron and Fermi mechanisms inside the flux rope. The highest energy electrons (>100 keV) were produced by betatron acceleration, whereas Fermi acceleration was unable to accelerate the electrons to high energy probably due to the finite distance of the acceleration region along the field‐aligned direction. These results confirm the essential role of ion‐scale flux ropes in producing energetic electrons.

“…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). For example, Drake et al (2006) suggest that electrons can be accelerated within the contracting magnetic islands produced by reconnection, through the Fermi-like process originally proposed by Fermi (1949).…”

Section: Introductionmentioning

confidence: 99%

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

Zhong

et al. 2022

ApJ

The question of how magnetic reconnection accelerates particles is a long-standing problem in space physics and astrophysics. Earth’s magnetosphere is an ideal laboratory for investigating this issue via in situ satellite observations. This article presents a statistical study of the electron acceleration produced by different mechanisms in the near-Earth magnetotail using the unique measurement capabilities of the Magnetospheric Multiscale mission. We find that the average acceleration rates and occurrence rates of large acceleration tend to be higher in outflows with greater speeds. Betatron and first-order Fermi accelerations are intensified near the neutral sheet, while the acceleration from E ∣∣ is not only intensified in the neutral sheet but also significant far away from it, likely in the separatrix region. In contrast to previous studies suggesting that the acceleration and energy conversion predominantly occur in the outflow region, we find that the acceleration rate near the X line is comparable to that in the outflow.