The Substorm Onset Arc is Azimuthally Structured

An auroral substorm is the manifestation of a rapid global reconfiguration of the magnetotail which is marked by a sudden brightening and poleward expansion of the most equatorward auroral arc. Clear signatures of small-scale azimuthal structures, or "auroral beads", have been observed along some substorm onset arcs in the minutes leading up to auroral breakup. The question whether these fluctuations are ubiquitous to all onset arcs, or are rather a special case phenomenon, is pertinent to determining whether all substorms are initiated by a common process operating in the magnetotail. We present the most comprehensive statistical study to date, to characterize the spatial structuring of auroral onset arcs. We use 195 independently identified auroral onset arcs observed by the THEMIS mission's ground-based all sky imagers. We find that over 90% of arcs studied have auroral beads that can be resolved by the imagers. This confirms that they are a critical component of the substorm onset process. In some cases beads are present for several minutes prior to the explosive auroral brightening. By characterizing the amplitudes and spatial scales of auroral beads embedded within the arc we are able to provide detailed insight into their generation mechanism. Statistically we find that the beads evolve from small to larger spatial scales and the majority of auroral beads have low amplitudes compared to the onset arc intensity.