The THEMIS mission approach to addressing the substorm question

THEMIS is a 5 spacecraft (probe) mission to launch in Fall of 2006. Its near equatorial probes will align once per 4 days to provide measurements along the tail between 10 and 30 Earth Radii, in order to distinguish between competing substorm theories. Over 180hrs of alignments will be possible each year. A ground network of white-light imagers and magnetomers in Canada and Alaska will ensure that the relevant alignments (at onset meridian) will qualify as prime candidates for extensive analysis. Particular emphasis is placed in determining current disruption and reconnection onset times, in relation to ground onset times. Current disruption and reconnection locations, onset times and speeds are determined using a variety of techniques: Energetic ion sounding or remote sensing, flow speeds, ExB speeds and dipolarization-front time-delays from one probe to another. It is expected that about twenty substorms will measured at their onset meridian during the 2-year nominal lifetime. This number will permit studies of onset mechanism dependence on external factors such as solar wind triggers, precursor pseudobreakups or ionospheric conditions. Interpretational challenges and ancillary datasets augmenting this mission beyond what was originally envisioned will also be discussed.