Precipitation of solar wind like ions in the polar cap during northward interplanetary magnetic field

Abstract

At altitudes of 8000 to 10,000 km EXOS D (Akebono) has observed several cases of an almost monoenergetic band of ions that extends over spatial ranges of several degrees or more poleward of the cusp. The energy spectra of these ions vary little over large distances in each case, and are very similar to the high‐energy component of the ion spectrum in the cusp. The ion spectra are compatible with the field‐aligned beam where the energy of the bulk motion (0.7 ‐ 2 keV) is about 10 times the thermal energy (0.04 ‐ 0.2 keV). Number fluxes are of the order of 106 (cm² s)−1, and densities are of the order of 10−2 cm−3. These ion bands are observed during intervals of northward IMF, and are sometimes accompanied by electron precipitation of the polar rain type. These ions are likely to be the energetic component of the solar wind ions which are introduced into the magnetosphere together with MHD waves that are transmitted from the magnetosheath.