Momentum Sources in Multifluid MHD and Their Relation to the Geopauses

In Trung et al. (2019b, ), we compared the geopause, a surface where the ionospheric plasma and solar wind plasma has equal parameters (by mass density, number density, or pressure), to the magnetopause to show that there was a transition in physics governing the motion of the plasmas between the different boundaries. In this study, we use multifluid magnetohydrodynamics simulation data to examine the relation of the source terms in the momentum equation and to the geopauses. We find that the geopauses do represent a transition in the physics governing the motion of the plasmas. While friction does not play a dominant role in the motion of the heliospheric and ionospheric plasmas, we find that friction shows indirectly that the ionospheric plasma can travel faster than the heliospheric plasma. Further satellite data is needed to verify the possibility of ionospheric plasma reaching speeds exceeding the solar wind.