There are two theories for the substorm trigger. One possibility is magnetic reconnection, in which stressed magnetic field lines act like rubber bands that have been twisted too far and suddenly snap to a new shape. Another possibility is current disruption or implosion. Each possible trigger has a different location. Magnetic reconnection occurs at ~ 25 Earth radii, while current disruption is closer, around 10 Earth radii.
Because both substorm triggers start from a localized region of space (1-2 Earth-radii in linear dimension) and within 1-2 minutes their effects are felt over a vast volume of space single spacecraft missions are not well suited for tracking substorm onset and evolution.
Finding the trigger in this vast region, which reaches beyond the orbit of the moon, is like the hunt for the source of the Nile. Rather, a distributed array of identical satellites is necessary to track the flow of energy and pin-point the location of onset. The orbits of the THEMIS fleet are strategically placed to trace the flow of energy and pinpoint the trigger location. (Click on image to view animation.) Credit: NASA