Electron Dispersion and Parallel Electron Beam Observed Near the Separatrix. Issue 9 (9th September 2019)

• Record Type: Journal Article
• Authors: Bai, Shi‐Chen; Shi, Quanqi; Zong, Qiu‐Gang; Wang, Xiaogang; Tian, Anmin; Degeling, Alexander W.; Yue, Chao; Rae, I. Jonathan; Pu, Zu‐Yin; Fu, Suiyan
• Is Part Of: Journal of geophysical research. Volume 124:Issue 9(2019)
• Page Start: 7494
• Page End: 7504
• Publication Date: 2019-09-09
• Subjects: magnetic reconnection -- energy dispersion -- separatrix -- electron edge -- ion edge; Magnetospheric physics -- Periodicals; Space environment -- Periodicals; Cosmic physics -- Periodicals; Planets -- Atmospheres -- Periodicals; Heliosphere (Astrophysics) -- Periodicals; Geophysics -- Periodicals; 523.01
• DOI: 10.1029/2019JA026836
• Languages: English
• ISSNs: 2169-9380

Abstract

Abstract: The separatrix region is the region between the separatrix and the reconnection jet. Due to the E × B drift and velocity filter effect in which high‐energy particles with high parallel speed can be seen prior to low‐energy particles along the field line, electrons are separated from ions. The electron dynamics in this region is of interest; however it has not been studied in detail, because of the insufficient resolution of plasma data. We present a slow separatrix crossing event observed by Magnetospheric Multiscale (MMS) satellite constellation on 1 January 2016, from the magnetosheath side with high‐resolution burst mode data. The electron edge and ion edge are clearly distinguished in the separatrix region. Two types of electron dispersion, one with a short duration (~0.3 s) and the other with a longer duration (~13 s) were detected between the electron and ion edges. The rapid dispersion (with small time scale) is mainly in the parallel direction, which might originate from a thin layer with non‐frozen‐in electrons close to the separatrix. The gradual (long time scale) dispersion is seen from parallel to perpendicular directions, which comes from the E × B drift of a curved D‐shape distribution of electrons. The width of the electron diffusion region on the magnetosheath side is estimated based on MMS observation. The observation also reveals an unexpected parallel electron beam outside of the electron edge. Wave‐particle interaction or parallel potential in the inflow region may be responsible for the generation of this electron population. Plain Language Summary: Magnetic reconnection is a process commonly occurred at the Sun, the dayside magnetopause, and in the magnetotail that changes magnetic topology, and converts electromagnetic energy into plasma kinetic and thermal energies. Due to the different parallel velocities of the particles and field line convection, electrons are separated from ions, and this creates the electron edge and ion edge in the separatrix region. These separations downstream of the X‐line widen the distance in the convection direction between the field lines involved in the reconnection. This, in turn, makes it easier for the satellite to observe the changes in the magnetic field topology and electronic dynamics during magnetic reconnection. However, high‐resolution satellite data are still required for the observation of the separatrix region in the downstream. The unprecedented high temporal and spatial resolution measurements of the Magnetospheric Multiscale (MMS) mission provide a unique opportunity to perform such studies. By analyzing the magnetic field and plasma data from MMS, this study has revealed the field line topology and electron dynamics between the electron and ion edges. Possible generation mechanisms based on the electron dynamics are also discussed in this paper. Key Points: Rapid dispersion is observed at the electron edge, indicating that electrons are decoupled from the magnetic field close to separatrix The width of the electron diffusion region on the magnetosheath side is estimated using in situ observations A parallel electron beam is observed outside of the separatrix in the magnetosheath for the first time