Unexpected features of the ion precipitation in the so‐called cleft/low‐latitude boundary layer region: Association with sunward convection and occurrence on open field lines

Ion precipitations along the boundary of the polar cap are studied using the Akebono (EXOS D) observations of plasma, electric field and magnetic field during the dawn‐dusk crossings across the dayside polar cap at the altitudes of 7500 km to 10,000 km. Only the cases where the convection streamlines consisted of two cells rather than four have been chosen for analysis, and hence the Kp index was moderate or high and the IMF polarity was southward in most of the intervals studied. It is found that the ion precipitations in morning and evening sectors along the polar cap boundary are associated with the sunward convection at the above heights, and judging from the presence of the polar rain the high‐latitude part of these precipitations occur in the region of open field lines. Association with the sunward convection and occurrence on open field lines contradict the view that these ions originate from the low‐latitude boundary layer (LLBL) on closed field lines. We propose to designate these ions as circumpolar ion precipitation (CPIP) in order to avoid questionable presumptions on their source region. The sunward convection in the morning and evening sectors of the CPIP is observed systematically, has speeds of about 1 km/s, and often extends over a few degrees or more in invariant latitude. The CPIP ions may have leaked from the boundary region of the plasma sheet, may have been generated in the distant neutral sheet, or may have entered from the magnetosheath. Only within a few hours of the noon meridian the CPIP ions are associated with the antisunward flow at the height range of the Akebono observations.