Although substorms have been studied for over 30 years and several models have been developed to describe the phenomenological features (Kan, 1990), there are still many remaining queries. For example, what process initiates a substorm, and where in the magnetosphere is it active (Samson et al., 1992)? What controls the extent of the substorm current wedge? Why is there a clearly defined nightside current wedge for only ~70% of substorms (Lester et al., 1983)? Is there quasi-periodicity in the occurrence of substorms (Williams et al., 1989; Farrugia et al., 1993), and is this periodicity related to external forcing by the solar wind, or by processes internal to the magnetosphere? Can the spatial and temporal distribution of energy deposited in the ionosphere and thermosphere as a result of the substorm be predicted accurately? New evidence both from space (Angelopoulos et al., 1992) and from ground-based observations (Williams et al., 1992) suggests that much of the energy involved in the growth and expansion phases of substorms may be carried in short-lived, spatially-confined plasma jets. What causes them? What role do they play in the substorm cycle? Again the combination of the GGS spacecraft (especially GEOTAIL and POLAR) data and SESAME data provides an exciting opportunity for making progress on these difficult and important issues.

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