Pressure balance at the subsolar magnetopause: Statistical study

The magnetopause would be located at a point where the total pressure on the solar wind/magnetosheath side is equal to its magnetospheric counterpart. The upstream pressure is a sum of plasma thermal pressure and magnetic pressure whereas the magnetic pressure would strongly dominate on the magnetospheric side because the density of magnetospheric plasma is typically very low. Statistics of over ten years of THEMIS subsolar magnetopause (Y_GSM<5 R_E) observations reveals that about 2 % of magnetopause crossings exhibit notably higher magnetic field in the magnetosheath than within the magnetosphere and thus the pressure balance is apparently violated. On the other hand, long series of multiple crossings suggest that the magnetopause is close to its equilibrium position and upstream and downstream pressures should be about equal. Interestingly, such crossings were found under both polarities of IMF B_Z. In the paper, we study possible sources and mechanisms keeping the pressure balance. We discuss namely the upstream solar wind conditions and state of the outer magnetosphere. Comparison of these unusual magnetopause crossings with the rest of them shows that the cold plasma population coming to the magnetopause from the plasmasphere during periods of geomagnetic storms plays an important role in setting the pressure balance in the magnetopause region.