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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 luminosity increased sharply. The ray structure implies precipitating electrons accelerated over a broad energy range at substorm onset. Two dimensional Fourier transform analysis of the auroral images showed the ray auroras have characteristic wavenumbers and harmonics that develop within a few seconds of the initial brightening. These results suggest that dispersive Alfvén waves containing turbulent parallel electric fields form in brightening regions at the beginning of auroral substorms.