Transport of Mass and Energy in Mercury's Plasma Sheet

We examined the transport of mass and energy in Mercury's plasma sheet (PS) using MESSENGER magnetic field and plasma measurements obtained during 759 PS crossings. Regression analysis of proton density and plasma pressure shows a strong linear relationship. We calculated the polytropic index γ for Mercury's PS to be ~0.687, indicating that the plasma in the tail PS behaves nonadiabatically as it is transported sunward. Using the average magnetic field intensity of Mercury's tail lobe as a proxy for magnetotail activity level, we demonstrated that γ is lower during active time periods. A minimum in γ was observed at R ~ 1.4 RM, which coincides with previously observed location of Mercury's substorm current wedge. We suggest that the nonadiabatic behavior of plasma as it is transported into Mercury's near-tail region is primarily driven by particle precipitation and particle scattering due to large loss cone and particle acceleration effect, respectively.