Ground-based observations and plasma instabilities in auroral substorms

One of the “grand challenge” problems in space science today involves determining what mechanism triggers the intensification and expansive phase of the magnetospheric substorm. Remote sensing of the ionosphere has provided some clues to the process, leading researchers to the conclusion that the physics of the intensification is inherently fast and explosive. In particular, ground-based observations show the brightening of the auroral arc and the formation of large scale vortex structures in the ionosphere to take place on time scales of tens of seconds. High earthward pressure gradients, enhanced field line curvature, and strong convective flows with large-amplitude westward components, point to the shear flow ballooning instability (SFBI) as the mechanism leading to the intensification. New results from the meridian scanning photometers and the all sky imager of the Canadian Auroral Network for the OPEN Program Unified Study array show the time delay between the formation of these vortex structures and the beginning of enhanced reconnection in the midtail region of the magnetotail to be of the order of 1–5 min. This timing presents a serious constraint for many substorm models. In this paper we shall discuss the observational support for the SFBI model and highlight some of the other plasma instabilities thought to lead to expansive phase onset.