THE TRIGGER PHASE OF MAGNETOSPHERIC SUBSTORMS

The last few minutes of the substorm growth phase before the auroral breakup show features distinct both from the gradual growth phase development and from the rapidly evolving expansion phase. During this time the optical auroras fade, energetic particle precipitation into the auroral ionosphere decreases, and the auroral electrojets weaken. The 5-7 minutes before the auroral breakup when these effects are observed has earlier been termed ''the substorm trigger phase''. Here we report on initial results of a systematic statistical study of the trigger phase phenomena that includes both auroral and ground based observations as well as global magnetospheric and interplanetary observations. As an example, we present one substorm event selected from the collected database. In this event (5 October, 1986) the trigger phase was observed with a multitude of ground instrumentation, and for the first time also by the EISCAT radar.