Characteristics of dayside auroral displays in relation to magnetospheric processes

Abstract

The use of dayside aurorae as a ground based monitor of magnetopause activity is explored in this thesis. The origin of diffuse (OI) 630.0 nm emissions in the midday auroral oval is considered first. Analysis of low altitude satellite records of precipitating charged particles within the cusp show an unstructured electron component that will produce a 0.5-1 kR 630.0 nm emission throughout the cusp. Distribution of the electrons is controlled by the requirement of charge neutrality in the cusp, predicting a diffuse 630.0 nm background even if the magnetosheath plasma is introduced into the magnetosphere in discrete merging events. Cusp electron fluxes also contain a structured component characterized by enhancements in the electron energy and energy flux over background values in narrow regions a few 10's of kilometers in width. These structured features are identified as the source of the transient midday arcs. An auroral model is developed to study the morphology of (OI) 630.0 nm auroral emissions produced by the transient arcs. The model demonstrates that a diffuse 630.0 nm background emission is produced by transient arcs due to the long lifetime of the O$(\sp1D)$ state. Two sources of diffuse 630.0 nm background emissions exist in the cusp which may originate in discrete merging events. The conclusion is that persistent 630.0 nm emissions cannot be interpreted as prima facie evidence for continuous particle transport from the magnetosheath across the magnetopause boundary and into the polar cusp. The second subject that is considered is the analysis of temporal and spatial variations of the diffuse 557.7 nm pulsating aurora in relation to the 630.0 nm dominated transient aurora. Temporal variations at the poleward boundary of the diffuse 557.7 nm aurora correlate with the formation of the 630.0 nm transient aurorae suggesting that the two events are related. The character of the auroral variations is consistent with the behavior of particle populations reported during satellite observations of flux transfer events near the dayside magnetopause. An interpretation of the events in terms of impulsive magnetic reconnection yields a new observation that relates the poleward moving transient auroral arcs in the midday sector to the flux transfer events.