THE FREJA MISSION - INTRODUCTION

Freja was a joint Swedish and German scientific satellite project to s tudy the interaction between hot magnetospheric plasma and the topside atmosphere/ionosphere. Freja was launched on October 6, 1992, and it operated successfully during 4 years until October 1996 when the comma nd system ceased to work. Freja enabled high temporal/spatial resoluti on measurements of auroral plasma characteristics. With a high telemet ry rate (520 kbit/s) and similar to 15 Mbyte distributed onboard memor ies Freja could resolve mesoscale and microscale phenomena in the 100 m range for particles and the 1-10 m range for electric and magnetic f ields. Novel plasma instruments enabled Freja to increase the spatial/ temporal resolution orders of magnitudes above that achieved by its pr edecessors. The main scientific objective of Freja was to study the in teraction between the hot magnetospheric plasma with the topside atmos phere/ionosphere. This interaction leads to a strong energization of m agnetospheric and ionospheric plasma and an associated erosion, and lo ss, of matter from the Terrestrial exosphere. Freja orbited with an al titude of similar 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, leadi ng to the escape of ionospheric plasma and the formation of large dens ity cavities.