Evolution of mesoscale auroral cavities before substorm onset

We have used density measurements made by the Freja satellite to analyze th e dynamical behavior of mesoscale auroral cavities at an altitude of 1700 k m. Freja's orbital period of 109 min enabled a very large data set of cavit y occurrences to be collected. The occurrence distribution over 1 year was analyzed statistically, and two case studies of individual events were used to provide additional information. It was found that the data are consiste nt with persistence times of 2 or more hours for plasma cavities at this al titude. The statistical distribution also showed a weak but demonstrable co rrelation between cavity occurrence times and substorm onsets; this was als o seen in the case studies, which both happened prior to substorm onsets. I n the case studies, the motions of the cavities could be tracked with the s upport of ground data. It was found that the cavity followed the oval close ly. As the oval moved southward during the substorm growth phase, the cavit y also moved southward. When the expansion phase began, the data were consi stent with the fact that the cavity expanded together with the oval. This s uggests that the region where the cavities are observed is embedded within the auroral oval and can follow the motions of the oval.