Electron flat‐top distributions around the magnetic reconnection region

Abstract

Cluster multisatellite observations provide snapshots of electron distributions around the magnetic neutral line. An isotropic flat‐top‐type electron distribution in phase space is frequently observed around the X line, together with large ion velocities and a Hall quadrupole‐like magnetic field inside the hot and tenuous plasma sheet in the magnetotail. The flat‐top distributions are also associated with a finite magnetic field in the direction normal to the neutral sheet, and the cross‐tail current density is sometimes very small. These results indicate that the flat‐top‐type distribution is mainly located near the outer boundary of the ion diffusion region in the plasma sheet outflow region, before reaching the pileup region with large normal component of the magnetic field. Simultaneously with the flat‐top distributions, strong field‐aligned electron beams mainly toward the X line are occasionally observed. This type of beam is mainly observed in the off‐equatorial plasma sheet and also appears well inside the plasma sheet. Typical energies of the beam are 4–10 keV, which is comparable to the upper edge of flat‐top energy. These highly accelerated electron distributions have a steep decrease in phase space density at the high‐energy end, and it is found that they are not correlated with the increase of the higher‐energy electrons related to suprathermal acceleration (>30 keV). This result indicates that the electron acceleration processes for the flat‐top‐type distributions are different from the suprathermal components, both of which are beyond the conventional MHD outflow acceleration and considered to be associated with some kinetic processes.