Electric field structure and ion precipitation in the polar region associated with northward interplanetary magnetic field

We investigated electric field and charged particle data of the EXOS D (Akebono) satellite and studied magnetospheric plasma flow and its relation with particle precipitation in the dayside polar region during northward interplanetary magnetic field. Sunward flow characterizes the convection in this region and suggests that reconnection takes place at the high-latitude magnetopause. Sheath ion precipitation is often observed associated with the sunward flow. When it shows energy dispersion caused by the velocity filter effect, it has noteworthy characteristics: (1) the ion precipitation region has a long scale length (>200 km at the 120-km altitude) along the flow, and (2) the upstream edge of this region is located at latitudes of >80 degrees. We consider that ion precipitation with energy dispersion is a characteristic phenomenon when the Alfven Mach number Ma(A) is high at the reconnection site, and we propose a new interpretation for the transportation of sheath plasma into the magnetosphere during high Ma(A). In this model the injection of the sheath ion occurs on the field lines that have penetrated into the magnetosphere after the reconnection. When the sheath ions associated with the sunward flow are observed at latitudes of <80 degrees, on the other hand, the length of the ion precipitation region along the flow is generally short (100-150 km). The signature of these ions indicates that they have entered through the entry layer.