Substorm expansion triggered by a sudden impulse front propagating from the dayside magnetopause

We examine a substorm expansion on 21 June 2007, which occurred <2 min after a solar wind discontinuity accompanied by a dynamic pressure (P-d) increase impinged on the magnetopause. To investigate how the perturbation due to such P-d increase propagates in the magnetosphere and what the timing analysis may imply about how this type of substorms is triggered, we utilize a large number of in situ magnetic field and plasma observations, a remote sensing of auroral brightening, and ground-based magnetic field observations. The timing analysis shows that the front of a compression-associated sudden impulse can reach the substorm expansion onset site (dawnside near-Earth plasma sheet) at the substorm onset time. The onset site and time are determined from aurora images displaying aurora expansion, energetic electron data at geosynchronous orbit showing dispersed injection, and geomagnetic field data at high latitudes showing negative bays in the H component. Our 2-D calculations of the fast-mode propagation also demonstrate the arrival of the wavefront at the onset site and time. We suggest that for the class of substorms triggered by solar wind P-d increases, the triggering could occur within as short as 2 min after a P-d increase impinges on the magnetopause, consistent with the time needed for the sudden impulse wavefront to propagate inside the magnetosphere from the dayside magnetopause to the magnetotail.