Dayside auroral activity as a possible precursor of substorm onsets: A survey using Polar ultraviolet imagery

We have analyzed the dayside auroral oval, with particular emphasis on the postnoon aurorally active region, prior to the onset of isolated substorms using images acquired from the ultraviolet imager (UVI) on board 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 dayside events were found to be spatially confined and showed little discernible changes prior to an active substorm onset, while only six cases were found to be in association with apparent eastward propagation through the dusk sector to the nightside prior to a substorm onset. This statistical result indicates that most of the postnoon bright spots are spatially confined in longitude and that only a few candidate cases are possibly associated with substorms triggering. It also suggests that dayside auroral bright spots are distinct features from the nightside auroral substorms. These results suggest that the physical processes responsible for the dayside auroral bright spots are different from those responsible for the nightside substorm activity.