Charging of the Freja Satellite in the Auroral Zone

Event studies and results of a statistical investigation on charging events as detected by the Freja satellite in the auroral zone at an altitude of 1500-1700 km are presented. Charging up to kilovolts is sometimes observed, though lower values are more common. Comparing to Defense Meteorological Satellite Program (DMSP) at 840 km, Freja experiences fewer charging events, but some of them are found in sunlit conditions, which has not been observed on DMSP. No charging event occurred for a density above 2000 cm^-3, though correlation between density and charging was weak otherwise. All charging events show enhanced electron fluxes around or above 10 keV, including enhanced tails up to 100 keV, whereas electrons around a few hundreds of electronvolts instead counteract charging, as is expected from high secondary yields in this energy range. Simulations using the POLAR code reproduced moderate charging events, but not the events with kilovolt charging