DAYSIDE AURORAL ACTIVITY AS A POSSIBLE PRECURSOR OF SUBSTORM ONSETS -A SURVEY USING POLAR ULTRAVIOLET IMAGERY

We have analysed the dayside amoral oval, with particular emphasis on the postnoon aurorally active region, prior to the onset of isolated s ubstorms using images acquired from the ultraviolet imager (UVI) on bo ard the Polar spacecraft. The UVI data set used for this investigation covers a time of approximately 3 months, from March 30 to July 13 in 1996. It is found that dayside auroral ''hot spots'' were active in 70 out of 95 surveyed substorm events at least 15 min before the onset, while 25 cases did not involve the dayside bright spots at all. Of the 70 cases with dayside activity during substorms, 51 cases of the days ide events were found to be spatially confined and showed little disce rnible changes prior to an active substorm onset, while only six cases were found to be in association with apparent eastward propagation th rough the dusk sector to the nightside prior to a substorm onset. This statistical result indicates that most of the postnoon bright spots a re spatially confined in longitude and that only a few candidate cases are possibly associated with substorms triggering. It also suggests t hat dayside amoral bright spots are distinct features from the nightsi de auroral substorms. These results suggest that the physical processe s responsible for the dayside amoral bright spots are different from t hose responsible for the nightside substorm activity.