Dawn-dusk asymmetry in bursty hot electron enhancements in the midtail magnetosheath

Bursty (a few minutes) enhancements of hot electrons (1-10keV) in the tail magnetosheath, which we name hot electron enhancements (HEEs), are sometimes observed. To understand the processes leading to HEEs, we have used 4years of measurements from Acceleration Reconnection Turbulence and Electrodynamics of Moon's Interaction with the Sun mission to statistically investigate the dawn-dusk asymmetry of HEEs in the midtail (x from -30 to -70 R-E) magnetosheath and their correlations with the solar wind/interplanetary magnetic field (IMF) conditions. We find two strong dawn-dusk asymmetries associated with HEEs: (1) they occur about 3 to 4 times more frequently on the dawnside. (2) Their fluxes on the dawnside are about twice as large as those on the duskside. The magnitudes of HEE fluxes are similar to those of the magnetosphere fluxes near the magnetopause, which are also a factor of 2 higher on the dawnside, indicating that the magnetosphere electrons are likely the source for HEEs and the cause for the HEE flux asymmetry. HEEs occur preferentially during higher solar wind speed, and the majority of HEEs are associated with sharp IMF direction changes and are accompanied by large and transient magnetosheath density changes. These correlations are stronger on the dawnside, suggesting that perturbations created near the quasi-parallel bow shock, which is most of the time on the dawnside, associated with IMF discontinuities is a possible process leading to HEEs and could account for the higher HEE occurrence on the dawnside.