Dynamics of the substorm expansive phase

This thesis investigates the substorm expansive phase with particular emphasis on the dynamics associated with the initiation of expansive phase onset and the development of the substorm current wedge.

The first study in this thesis looks at the highly controversial question surrounding the sequence of events of the magnetospheric substorm. That is, does expansive phase onset occur prior to, or as a result of, near-Earth neutral line formation in the mid-tail? A new technique has been developed to analyze the CANOPUS meridian scanning photometer (MSP) data that fits a Gaussian distribution to the MSP data in order to determine the latitudinal location of the poleward and equatorward borders of the 557.7 nm and 630.0 nm emission regions, and the maximum peak of the 486.1 nm emissions. The new technique permits the study of the dynamics of the maximum peak, the poleward and the equatorward borders of the photometer emissions throughout the duration of the substorm. The relative motion of the borders of the emissions show that not only is the initial intensification of the expansive phase explosive, but the poleward border of the 557.7 nm emissions rapidly expands poleward past the maximum latitude of dipolarization (shown by the maximum of the Hβ emissions) to reach the poleward border of the 630.0 nm emissions in approximately 1–5 minutes. The rapid tailward expansion of the poleward border of the 557.7 nm emissions has been interpreted as the propagation of an instability anti-earthward down the tail, causing dipolarization of stretched field lines and perhaps initiating the reconnection of lobe flux. The results of this thesis show consistently that neutral line formation associated with the substorm expansive phase is a result, and not a cause, of expansive phase onset. That is, expansive phase onset happens prior to lobe flux reconnection. The above-mentioned sequence of events and rapid timescale carry severe consequences for the other substorm models and theories that are currently published.

The second study in this thesis takes an indepth look at one of the main signatures of the expansive phase—the substorm current wedge. The presently accepted current wedge model creates the wedge by ‘short-circuiting’ the cross-tail current to the ionosphere, producing a pair of anti-parallel field-aligned current sheets flowing into and out of the ionosphere. The current circuit is closed by an ionospheric westward electrojet that is located in the same latitudinal region as the auroral emissions observed by the meridian scanning photometers. This current wedge is an equivalent current however, proposed to explain the magnetic signatures observed by satellite based and ground-based instruments at mid and low latitudes. The current wedge model is thus non-unique such that other model current systems can produce similar magnetic signatures. Therefore, the second objective of this thesis was to prove that another current configuration, based on the directly driven current system and variations in ionospheric electric fields and conductivities, can also produce the magnetic signatures of the substorm current wedge.