Geotail Data Archive at NASA/GSFC

The GEOTAIL mission is a collaborative project undertaken by the Institute of
Space and Astronautical Science (ISAS) and the National Aeronautics and Space
Administration (NASA). Its primary objective is to study the dynamics of the
Earth's magnetotail over a wide range of distance, extending from the
near-Earth region (8 Earth radii (Re) from the Earth) to the distant tail
(about 200 Re). The GEOTAIL spacecraft was designed and built by ISAS and was
launched on July 24, 1992.

The Geotail mission measures global energy flow and transformation in the
magnetotail to increase understanding of fundamental magnetospheric processes.
This will include the physics of the magnetopause, the plasma sheet, and
reconnection and neutral line formation (i.e., the mechanisms of input,
transport, storage, release and conversion of energy in the magnetotail).

See:
http://pwg.gsfc.nasa.gov/istp/geotail/geotail_key_parameters.html
for a list of key parameters measured by the Geotail instruments.