Average Electron Precipitation in the Polar Cusps, Cleft and Cap

Abstract

Results are given of the statistical properties of electron precipitation measured at low altitudes by polar orbiting satellites. The morphology of high latitude electron precipitation is determined in the polar cap, the polar cusp, and the polar cleft as a function of magnetic latitude, local time, and activity, Kp. Results show that a polar cusp region highly confined in latitude and local time can be Identified by a minimum in the precipitating electron average energy. This region lies very close to local noon, but is not spatially coincident with the maximum in precipitating electron flux which occurs several hours earlier. Surrounding the cusp is a well-defined region of low energy precipitation whose contours of constant integral flux have crescent shapes centered about the flux maxima. This is best seen in cases of low magnetic activity when boundary plasma sheet electron precipitation is not strong. We refer to this region as the cleft. The average energy of the cleft electrons increases steadily as one moves away from the cusp in local time and in latitude. The polar cap appears to have two states: an active state, when the IMF is northward; and a quiet state, characterized by polar rain precipitation and occurring when the oval is active. Two-dimensional maps of integral flux and average energy for polar rain occurrence show that the basic vatiation in this precipitation is from dayside to nightside. The axis ot symmetry for the variation is pre-noon to pre-midnight with the integral flux (average energy) increasing (decreasing) from day to night. These variations in number flux and average energy exhibit a symmetry that is an extension of that seen in the cleft.