From the Geotail website (listed below): "The Geotail satellite was launched on July 24, 1992, by a Delta II launch vehicle from Cape Canaveral Air Force Station, Florida, United States. The primary purpose of this mission is to study the structure and dynamics of the tail region of the magnetosphere with a comprehensive set of scientific instruments. For this purpose, the orbit has been designed to cover the magnetotail over a wide range of distances: 8 Re to 210 Re from the earth. This orbit also allows us to study the boundary region of the magnetosphere as it skims the magnetopause at perigees. In the first two years the double lunar swing-by technique was used to keep apogees in the distant magnetotail. The apogee was lowered down to 50 Re in mid November 1994 and then to 30 Re in February 1995 in order to study substorm processes in the near-Earth tail region. The present orbit is 9 Re x 30 Re with inclination of -7° to the ecliptic plane."
Geotail instruments studied electric fields, magnetic fields, plasmas, energetic particles, and plasma waves.