Ground-based observations and plasma instabilities in auroral substorms

One of the "grand challenge" problems in space science today involves deter mining what mechanism triggers the intensification and expansive phase of t he magnetospheric substorm. Remote sensing of the ionosphere has provided s ome clues to the process, leading researchers to the conclusion that the ph ysics of the intensification is inherently fast and explosive. In particula r, ground-based observations show the brightening of the auroral arc and th e formation of large scale vortex structures in the ionosphere to take plac e on time scales of tens of seconds. High earthward pressure gradients, enh anced field line curvature, and strong convective flows with large-amplitud e westward components, point to the shear flow ballooning instability (SFBI ) as the mechanism leading to the intensification. New results from the mer idian scanning photometers and the all sky imager of the Canadian Auroral N etwork for the OPEN Program Unified Study array show the time delay between the formation of these vortex structures and the beginning of enhanced rec onnection 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 ex pansive phase onset. (C) 2001 American Institute of Physics.