Large, abrupt pressure decreases as a substorm onset trigger

Sudden positive solar wind dynamic pressure increases have long been considered a substorm onset trigger, and evidence showing such a trigger exists. On the other hand, solar wind dynamic pressure decreases have not been considered as an onset trigger. In this letter we report evidence of substorm onsets associated with large, sudden solar wind pressure drops. We examine global auroral images acquired from the ultraviolet imager on‐board the Polar Satellite on 13 large solar wind pressure decreases associated with negative magnetic impulses ( SI − ). It is found that substorm triggering by pressure decreases is rare (3 of 13 events studied). A detailed analysis of the three substorm events indicates that they occurred within ∼10 min of an SI − when the sharp pressure decrease is followed by an increase in the open magnetic flux. Among the rest of SI − events, only one with such a solar wind condition did not lead to a substorm expansion phase onset. No systematic trend in the amplitude of an SI − was found for the substorm triggered events. Preconditioning of the magnetosphere may play a role but not crucial. While this finding may be in contradiction with current substorm theories, it may simply support the general idea that discontinuities in the solar wind can trigger substorms, though under certain conditions.