The geopause in relation to the plasma sheet and the low‐latitude boundary layer: Comparison between Wind observations and multifluid simulations

Wind observations during the May 10, 1996 perigee are used in conjunction with multifluid global simulations to investigate the mixing of magnetosheath‐like plasma and ionospheric plasma in the dawn current sheet. Magnetosheath‐like plasma can play important roles in the formation of the low‐latitude boundary layer (LLBL) and the cold, dense plasma sheet (CDPS) identified previously in Geotail studies where the magnetosheath component is seen as a low energy component. The Wind observations show that this cold component can be seen well inside the nominal position of the magnetopause and even the CDPS, and is most pronounced near the magnetic equator. This cold plasma provides an important tracer for the geopause where the dynamics of the region inside the boundary is controlled by ionospheric plasma and the region outside is dominated by plasma of solar wind origin. The geopause position in the present case appears be about halfway between the noon‐midnight meridian and the magnetopause. In addition, it is shown that the cold magnetosheath‐like plasma has significant asymmetries between the northern and southern sides of the current sheet with enhanced cold plasma being observed in the Northern Hemisphere relative to that seen in the Southern Hemisphere. This asymmetry which is also seen in the modeling and is associated with the high dipole (> 20°) tilt during the event which allows enhanced entry of magnetosheath plasma in the Northern (summer) Hemisphere and enhanced ionospheric plasma outflows in the Southern (winter) Hemisphere. Flow asymmetries in the particle distributions that have properties similar to the magnetosheath boundary layer are also observed. However, the characteristics of the distributions indicate that the flow is between the LLBL and the CDPS as opposed to the LLBL and the magnetosheath.