The precipitating electron signature of the polar cusp and boundary layer

Abstract

The low-altitude polar cusp region observations of precipitating electrons sometimes show the signature of two different plasma regimes; i.e. the electron average energy and total flux assume different values, and at times, the transition is very distinct. We systematically surveyed three months of DMSP satellite precipitating electron data to determine the existence and frequency of occurrence of such distinct structured magnetospheric cusp regions; the relationship of such observations to the orientation of the interplanetary magnetic field to solar wind parameters, and to geomagnetic activity was studied. Over 2000 cusp region observations were analyzed. Only 2% of the observations show two distinct regions with a sharp demarcation; they may be observed in either hemisphere. The frequency of occurrence of the events seems not to depend on the solar wind parameters; such occurrence has a dependence on the IMF orientation and reaches a maximum during moderately disturbed conditions, as indicated by the AE index. In 85% of the cases, the more energetic population is located equatorward of the other population; 62% of these cases show more intense flux in the equatorward region. The other 15% show the more energetic population located poleward of the lower energy population without any relation to the electron flux in the cusp. This study reveals that the polar cusp and boundary layer are sometimes distinguishable at low altitude based on the precipitating electron characteristics.