Evidence of Electron Acceleration at a Reconnecting Magnetopause

Abstract

It is still unknown nowadays whether magnetic reconnection—a process occurring both in the magnetotail and at the magnetopause—can intrinsically accelerate energetic electrons. Observations in the Earth's magnetotail usually indicate strong electron acceleration during magnetic reconnection, while observations at the Earth's magnetopause rarely show such features. With the recently launched Magnetospheric Multiscale (MMS) mission, here we report the first evidence of energetic‐electron acceleration at a reconnecting magnetopause. We find that the acceleration of electrons, with energy up to 70 times their thermal energy, occurs in the magnetosheath side of the ion diffusion region and is associated with strong whistler waves. Such acceleration—not contaminated by the magnetospheric population—is attributed to nonadiabatic wave‐particle interactions, as supported by analyses of the resonance condition. It manifests that energetic‐electron acceleration can happen at the reconnecting magnetopause, like that in the tail.

Ion‐Beam‐Driven Intense Electrostatic Solitary Waves in Reconnection Jet

“…The ion distribution changes from nearly flattop distribution to clearly counterstreaming distribution (Figure g). The sharp changes are also clear in the ion and electron energy spectrogram (Figures h and i), in which we see that the electrons get heated and accelerated inside the flux pileup region (Birn et al, ; Fu et al, ; Fu et al, ; Liu & Fu, ; Xu et al, ).…”

Electron Pitch‐Angle Distribution in Earth's Magnetotail: Pancake, Cigar, Isotropy, Butterfly, and Rolling‐Pin

“…In addition to the impact of the global magnetic fields on the PAD occurrence, local magnetic fields have been suggested to be crucial to the evolution of electron PAD as well. In particular, electron pancake and rolling‐pin distributions are suggested to occur preferentially inside local strong‐magnetic‐field regions, such as flux pileup regions behind dipolarization fronts (DFs) (e.g., Fu et al, 2011; Fu, Peng, et al, ; Runov et al, 2013; Wu et al, 2013; Liu, Fu, Xu, Wang, et al, 2017; Liu, Fu, Cao, Xu, et al, 2017; Liu, Liu, et al, 2018), or inside local magnetic field dip regions, such as magnetic holes (e.g., Sun et al, 2012; Yao et al, 2018, 2019). However, such suggestion is inferred from several case studies.…”

MMS Observations of Plasma Heating Associated With FTE Growth

“…At the trailing edge of the FTEs, both ions and electrons are cooled perpendicular to B, indicating a region of relaxing field topology. MMS observations suggest that acceleration mechanisms are enhanced farther away from the FTEs' outer edge. This suggests the following: Farther away from the FTE edge, most likely nearer to the reconnection X‐line(s) (e.g., Dahlin et al, ; Fu, Peng, et al, ), parallel electric fields and magnetic field gradients are most effective in plasma acceleration (see Figure 8 in Zhou et al, ), and/or Force‐free FTEs may slow down the rate at which plasmas are accelerated in the reconnection exhaust (e.g., Taylor, ). As FTEs grow, they seek to reach a lower energy state by rearranging magnetic field lines.…”

“…a Farther away from the FTE edge, most likely nearer to the reconnection X-line(s) (e.g., Dahlin et al, 2015;Fu, Peng, et al, 2019), parallel electric fields and magnetic field gradients are most effective in plasma acceleration (see Figure 8 in Zhou et al, 2018), and/or b Force-free FTEs may slow down the rate at which plasmas are accelerated in the reconnection exhaust (e.g., Taylor, 1974). As FTEs grow, they seek to reach a lower energy state by rearranging magnetic field lines.…”