THE FREJA SCIENCE MISSION

Freja(*), a joint Swedish and German scientific satellite launched on october 6 1992, is designed to give high temporal/spatial resolution measurements of auroral plasma characteristics. A high telemetry rate (520 kbits s(-1)) and approximate to 15 Mbyte distributed on board memories that give on the average 2 Mbits s(-1) for one minute enables Freja to resolve meso and micro scale phenomena in the 100 m range for particles and 1-10 m range for electric and magnetic fields. The on-board UV imager resolve auroral structures of kilometer size with a time resolution of one image per 6 s. Novel plasma instruments give Freja the capability to increase the spatial/temporal resolution orders of magnitudes above that achieved on satellites before. The scientific objective of Freja is to study the interaction between the hot magnetospheric plasma with the topside atmosphere/ionosphere. This interaction leads to a strong energization of magnetospheric and ionospheric plasma and an associated erosion, and loss, of matter from the Terrestrial exosphere. Freja orbits with an altitude of approximate to 600-1750 km, thus covering the lower part of the auroral acceleration region. This altitude range hosts processes that heat and energize the ionospheric plasma above the auroral zone, leading to the escape of ionospheric plasma and the formation of large density cavities.