Interplanetary shock triggering of nightside geomagnetic activity: Substorms, pseudobreakups, and quiescent events

We use Wind solar wind data and Polar UV imaging data to study the nightside magnetospheric/magnetotail responses to interplanetary shocks/pressure pulses. Of 53 interplanetary shock/pressure pulse events that occurred in 1997 and 1998 at Wind, there are 18 cases where Polar near‐midnight UV images are available. All of these 18 events are used in this study. The nightside auroral responses can be classified into three types: substorm expansion phase (SS) (or substorm further intensification) events, pseudobreakup (PB) events, and quiescent (QE) events. It is found that the solar wind preconditions determine the causes of the different auroral responses. A ∼1.5‐hour interplanetary magnetic field (IMF) B s “precondition” (upstream of the interplanetary shock) gives good empirical results. The upstream IMF is strongly southward prior to substorm expansion phase triggerings (44% of all events), the IMF B z is ∼0 nT for PB triggerings (39% of all events), and the IMF is purely northward for quiescent events (17%). The evidence for IMF B s preconditioning is interpreted in terms of a plasma sheet loading mechanism. The interplanetary shock compression effects on the near‐Earth tail are discussed in light of existing substorm/PB triggering models.