SOFT ION PRECIPITATION AT VERY HIGH-LATITUDES DURING NORTHWARD INTERPLANETARY MAGNETIC-FIELD

During extended periods of northward interplanetary magnetic field (IM F), soft ion precipitation was frequently observed at very high latitu des (>80-degrees ILAT) on EXOS D (Akebono). The precipitating protons typically had temperatures of a few hundreds of eV, and were accompani ed by precipitating electrons with temperatures of several tens of eV. The densities of the precipitating protons were of the order of 10(-1 ) cm-3, and were usually lower than those of the electrons. During the se periods, the ion convection at the highest latitudes was chaotic or weak, but often had an average antisunward component. During periods of strongly northward IMF, ion precipitation was observed at all magne tic local times (MLT). A flux minimum, sometimes below the level of de tection of the Akebono suprathermal mass spectrometer instrument, was often found just poleward of the dayside auroral oval. This region is believed to coincide with the open magnetic field lines in the tail lo be. The region of ion precipitation was observed at auroral latitudes on the dawnside when the IMF B(y) changed polarity from negative to po sitive; it was located at higher latitudes (>80-degrees ILAT) on the d uskside when the IMF B(y) was positive. These observations suggest a c onnection between the ion precipitation and the auroral oval.