Freja (F3H, F4)

The Swedish satellite Freja was launched on 6 October 1992 from the Jiuquan Satellite Launch Center in China. Freja imaged the aurora and measured particles and fields in the upper ionosphere and the lower magnetosphere. 

Swedish, German, Canadian and American scientific instruments flew on the satellite manufactured by the Swedish Space Corporation. The instruments measured charged electrons and ions, as well as electric and magnetic fields. IRF in Kiruna was responsible for one instrument, F3H, with two sensors for measuring charged particles. One sensor, TICS (Three-Dimensional Ion Composition Spectrometer), measured positive ions (for example protons, helium ions and oxygen ions) in the energy range 1 eV to 10 keV. The second sensor, MATE (MAgnetic imaging two-dimensional electron spectrometer), measured electrons in the energy range 0.1-115 keV. The Principal Investigator responsible for F3H was Lars Eliasson.

IRF in Uppsala was responsible for another instrument, the wave and plasma density instrument F4. Bengt Holback was Principal Investigator and responsible for the instrument, . which was equipped with seven sensors that measured plasma density and electrical wave fields, three magnetic field sensors and another antenna for waves up to several megahertz.

Measurement series in the aurora zone

Freja flew in an orbit that meant that it travell for long stretches along the aurora oval, not across it in the way most satellites study the aurora. It provided very long series of measurements in the auroral zone, for example of how some of the Earth’s oxygen escapes into space from the upper layers of the atmosphere. This very question was studied by both of IRF’s two instruments, where the wave measurements from F4 could be used to understand what had accelerated the oxygen ions that F3 saw escaping. The satellite was controlled via the Swedish Space Corporation’s Esrange ground station in Kiruna and it was operational between October 1992 and October 1996. Freja was funded by the Swedish National Space Board (now the Swedish National Space Agency).