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

We use Wind solar wind data and Polar U-V imaging data to study the nightsi de magnetospheric/magnetotail responses to interplanetary shocks/pressure p ulses. Of 53 interplanetary shock/pressure pulse events that occurred in 19 97 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 nightsid e 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 precond itions determine the causes of the different auroral responses. A similar t o1.5-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% o f all events), the IMF B-Z is similar to0 nT for PB triggerings (39% of all events), and the IMF is purely northward for quiescent events (17%). The e vidence for IMF B-s preconditioning is interpreted in terms of a plasma she et loading mechanism. The interplanetary shock compression effects on the n ear-Earth tail are discussed in light of existing substorm/PB triggering mo dels.