Applied Physics Laboratory, The Johns Hopkins University, Laurel, Maryland

One of the outstanding issues in magnetospheric physics is the trigger mechanism(s) of the substorm expansion phase onset. Previous studies have shown connections between substorms and sudden commencements/sudden impulses (SSCs/SIs) caused by discontinuities in the solar wind and suggested a mechanism triggered by magnetospheric compression. However, the reported triggering probabilities are usually small, ranging from 4% to 49% under various conditions. A number of recent studies have indicated that some of the most widely used substorm onset signatures such as high-latitude magnetic bays, low-latitude Pi2 pulsations, and energetic particle injections at geosynchronous orbits occur more frequently than auroral breakups. Hence, it is reasonable to cast doubt on the compression trigger mechanism because the actual probability for compression triggering of substorm should be lower when using a more reliable substorm onset identifier such as auroral breakups. By examining 17 interplanetary shock events that occurred in 1998 with simultaneous auroral images from the Polar ultraviolet imager we found that (1) interplanetary shocks can enhance auroral electrojets and auroral luminosities, (2) magnetic negatives bays associated with SSCs/SIs are not always associated with substorms, and (3) there is only one event can be considered as an SSC triggered substorm. These results strongly indicate interplanetary shocks can produce magnetic negative bays but not auroral breakups.

submitted to Proceedings of the ICST-6 Conference, 2002