Fine structures and dynamics in auroral initial brightening at substorm onsets

We show four auroral initial brightening events at substorm onsets focusing on fine structures and their longitudinal dynamics, which were observed by all-sky TV cameras (30-Hz sampling) on January 2008, in Canada. For two initial brightenings started in the field of views of the cameras, we found that they started at longitudinal segments with a size of less than similar to 30-60 km. One brightening expanded with wavy structures and the other expanded as a straight arc. Although the two events had different structures, both brightening auroras expanded with an average speed of similar to 20 km/s in the first 10 s, and similar to 10 km/s in the following 10 s. The other two events show that brightening auroras developed with periodic structures, with longitudinal wavelengths of similar to 100-200 km. Assuming that the brightening auroras are mapped to the physical processes occurring in the plasma sheet, we found that the scale size (30-60 km) and the expanding speed ( 20 km/s) of brightening auroras correspond to the order of ion gyro radii (similar to 500-1400 km) and Alfven speed or fast ion-flow speed (similar to 400 km/s), respectively, in the plasma sheet.