Evidence suggests internal triggering of substorms



A basic question in substorm research is whether the initiation of substorm expansion is due to an internal process or merely a magnetospheric response to an external driver. The answer to this question is important for several reasons. It provides a useful criterion for evaluating proposed substorm models and an important clue for developing a substorm theory. If the external condition for substorm expansion onsets could be clearly identified, our community would be able to predict substorm expansion onsets for use in forecasting space weather. My opinion is that substorm expansion onsets arise from an internal instability. The state of the magnetosphere just before an onset is analogous to a ball on a hill top. The descent of the ball down the hill can be initiated by either the inherent small random fluctuation of the system or by a deliberate external jolt to the system.