Statistical properties of particle precipitation in the polar cap during intervals of northward interplanetary magnetic field

The characteristics of particle precipitation in the polar cap during intervals of northward interplanetary magnetic field (IMF) are investigated. When the IMF is directed northward, localized and structured electron fluxes, called polar showers, appear in the polar cap and interrupt uniform and soft electron precipitation. We classify polar showers into two types, taking into account the association of ion precipitation: (1) polar showers accompanied by ion fluxes (type A) and (2) polar showers without ion fluxes (type B). In order to investigate the characteristics of each type, we examine the DMSP F8 plasma data obtained for one-year period. We found that type A and B polar showers respond to the direction of the IMF in quite different ways. Type A polar showers do not significantly depend on either the IMF B-x or B-y component. Most type A ion fluxes are more intense and energetic than those of the solar wind, and their intensity is similar to that in the boundary plasma sheet. This intensity implies that type A polar showers occur on closed field lines. On the other hand, the statistical properties of the type B polar shower are similar to those of polar rain, which is believed to originate in the solar wind. We therefore suggest that the plasma in type B polar showers still originates in the solar wind and that some magnetic field lines in the polar cap are open even when the IMF is directed northward.