Statistical study on electron and ion temperatures in the near-Earth reconnection and magnetic pile-up regions

It remains an unresolved problem what determines the energy partition between ions and electrons during magnetic reconnection. It is thus important to investigate ion-electron temperature ratio around the reconnection regions. Plasma in the Earth's magnetosphere are heated up to 1-10 keV and stored in the plasma sheet in the magnetotail. Magnetic reconnection is a major process that heats/accelerates plasma up to this energy range. Heated/Accelerated plasma are transported both earthward and tailward as fast flows with a speed of several hundred kilometers per second or faster. It has been reported that the properties of plasma sheet plasma such as ion temperature T_i, electron temperature T_e, and ion electron temperature ratio T_i/T_e depend on fast flow conditions and spatially vary [Kaufmann et al., 2005; Wang et al., 2012; Runov et al., 2018]. However, there have been few studies which focus on macroscopic profiles of ion and electron temperatures around reconnection and magnetic pile-up regions in the magnetotail.

In this study, we examine the average profiles of the temperatures and their ratio in the plasma sheet in the range of -25 R_E < X_GSM< -15 R_E and -10 R_E < Y_GSM< 10 R_E by using data obtained from MMS for a period from May 1st to September 30th in 2017. We reconstruct both inflow and outflow regions of the reconnection using the X and Z components of the magnetic field. We then present spatial characteristics of ion and electron temperatures and their ratio. We also examine the flow speed dependence of the spatial characteristics.

Our analysis shows that both T_i and T_e increase and only electron entropy is constant from the lobe to magnetic reconnection regions. T_i and T_e increase and entropies are almost constant from the inflow to outflow regions. In magnetic pile-up region, electrons are heated more effectively than ions. Both ion and electron entropies increase from the outflow to magnetic pile-up regions.

The results suggest that adiabatic heatings of electron and nonadiabatic heatings of ion from the lobe to magnetic reconnection regions. From the inflow to outflow regions, both ion and electron are heated adiabatically. From the outflow to magnetic pile-up regions, electron are selectively heated nonadiabatically.