We have used density measurements made by the Freja satellite to analyze the dynamical behavior of mesoscale auroral cavities at an altitude of 1700 km. Freja's orbital period of 109 min enabled a very large data set of cavity 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 consistent with persistence times of 2 or more hours for plasma cavities at this altitude. The statistical distribution also showed a weak but demonstrable correlation between cavity occurrence times and substorm onsets; this was also seen in the case studies, which both happened prior to substorm onsets. In the case studies, the motions of the cavities could be tracked with the support of ground data. It was found that the cavity followed the oval closely. As the oval moved southward during the substorm growth phase, the cavity also moved southward. When the expansion phase began, the data were consistent with the fact that the cavity expanded together with the oval. This suggests that the region where the cavities are observed is embedded within the auroral oval and can follow the motions of the oval. ¿ 1999 American Geophysical Union