Substorm onset and development: The crucial role of flow channels

Abstract

In this paper, we described the basic features and observations bases of the scenario for the substorm expansion phase that we have developed from our research over the past ~10 years. Onset occurs along magnetic field lines of the inner proton plasma sheet, and is first seen in the aurora as beading along the onset aurora arc, which lies near the equatorward boundary of the auroral oval. Growth of these onset waves and large amplitude electric field oscillations indicates that an abrupt transition from a stable to unstable state leads to onset. The transition to instability occurs as a result of an intrusion of a low-entropy flow burst/channel (i.e., a plasma bubble) to the inner plasma sheet, and such flow bursts are marked in the auroral oval by the initiation of a poleward boundary intensification (PBI) that evolves into an auroral streamer. Plasma flows associated with these and other PBIs are generally associated with enhanced reconnection at the distant tail neutral line, the reconnection being triggered by an incoming flow channel from the polar cap. It is reasonable that the onset instability is triggered by the intruding reduced entropy plasma abruptly changing the entropy distribution in the inner plasma sheet, though the specific onset instability has not been identified. The onset instability is azimuthally aligned and expands azimuthally, these occurring because, as a bubble moves earthward, lower energy ions tending to follow the electric field drift towards the dawn side while higher energy ions magnetic drift towards the duskside. Thus entropy is not conserved along center of mass drift trajectories, and the bubble spreads in longitude and deepens. The transition to non-linearity of the growing onset waves leads to streamers. How this occurs is another major outstanding question, but the first streamers could initiate within the plasma sheet ~20 RE downtail, the region where reconnection has been inferred to occur soon after onset. It is likely that streamers and their associated reconnection initiate at the distant tail neutral line once the expansion phase auroral activity reaches the auroral poleward boundary. The substorm current wedge builds up from a sequence of longitudinally localized flow burst regions (wedgelets) that are associated with the expansion-phase streamers, dipolarize the local magnetic field, and give rise to traditional ground onset signatures on the ground, namely auroral zone H bays and mid-latitude positive H bays and Pi2 pulsations.