High time-resolution imaging of auroral arc deformation at substorm onset

Abstract

Observations of auroral disturbances such as westward-travelling surges, omega bands and auroral folds, are by necessity usually made when the luminous features have acquired their characteristic geometry, i.e. after the actual formation process. We demonstrate an event in which the initial brightening of an arc developed into a series of auroral spirals and finally into a westward-travelling surge. Two intervals of spiral formation were observed, but only the second of these generated a surge-like feature. Since the surge formation occurred on a time-scale of a few seconds, this type of observation can only be made with high time-resolution imaging devices. The auroral developments took place in the zenith above Kiruna, where they were recorded by ground-based monochromatic imagers. The subsequent poleward expansion of the activity was observed by the EISCAT incoherent scatter radar 200 km to the North of the initial activation. These observations are discussed in the light of existing substorm theories, and it is proposed that the instability responsible for the final break-up of the arc is self-destructive.