Introduction to special section: The Freja Mission

Freja was a joint Swedish and German scientific satellite project to study 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 command system ceased to work. Freja enabled high temporal/spatial resolution measurements of auroral plasma characteristics. With a high telemetry rate (520 kbit/s) and ∼15 Mbyte distributed onboard memories Freja could resolve mesoscale and microscale phenomena in the 100 m range for particles and the 1–10 m range for electric and magnetic fields. Novel plasma instruments enabled Freja to increase the spatial/temporal resolution orders of magnitudes above that achieved by its predecessors. The main scientific objective of Freja was 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 orbited with an altitude of ∼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.