Spatial distributions of the ion to electron temperature ratio in the magnetosheath and plasma sheet

Abstract

We have used THEMIS measurements to determine how the ion and electron temperatures and their ratio (T i /T e ) change spatially in the magnetosheath and plasma sheet and to identify the processes responsible for the variations. Magnetosheath T i /T e varies from ∼4–12 with higher ratios observed during larger solar wind speed and at locations closer to the magnetopause. T i /T e remains almost unchanged as particles flow downstream and cool adiabatically. Across the flank magnetopause from the magnetosheath to a plasma sheet that is cool with abundant cold plasma, temperature and specific entropy for ions and electrons increase significantly while T i /T e remains similar, indicating that the magnetosheath ions and electrons are non‐adiabatically energized with similar proportion while entering the magnetosphere. Within the tail plasma sheet, T i /T e varies from ∼6 to 10 when plasma is relatively cool to ∼2 to 5 when relatively warm. With this correlation, T i /T e is higher closer to the flanks and during northward interplanetary magnetic field (IMF), while lower T i /T e is more often seen during higher AE around midnight. The distinguishably lower T i /T e for warmer plasma in the near‐Earth plasma sheet is likely due to additional non‐adiabatic heating of electrons more than ions as particles move earthward and are adiabatically energized. As particles move into the near‐Earth magnetosphere, strengthening magnetic drift brings more hotter ions toward dusk and more hotter electrons toward dawn, resulting in a strong T i /T e dawn‐dusk asymmetry with very high T i /T e (∼15 to 100) near dusk and very low T i /T e (∼1) near dawn.