Azimuthal structures of ray auroras at the beginning of auroral substorms

The time evolution of optical structures in brightening auroras was examined for two substorm events that occurred on January 15, 2008 and February 27, 2009. The onsets of both auroral substorms were captured in the field of view of an all‐sky TV camera at Gillam, Canada. We found that in both cases, the brightening auroras formed vertical structures, or so‐called ‘ray auroras’, which lined up longitudinally for several tens of seconds before …