Mapping and distortions of auroral structures in the quiet magnetosphere

The objective of this report is to identify those features of commonly observed dayside auroras that can be explained easily by either of two processes: mapping distortions or distortions caused by nearby Birkeland currents. Mapping distortions in a quiet closed magnetosphere model yield very elongated arclike ionospheric images in the dayside auroral zone for a wide range of circular or elliptical tubelike equatorial sources. The length of a dayside auroral arc provides a good measure of the equatorial source dimension normal to the magnetopause. The orientation of the dayside arc provides a crude estimate of the solar magnetospheric x location of the equatorial source. Dayside arcs and Birkeland currents are found to map to both the dayside and the nightside magnetopause current layer and low latitude boundary layer. Nightside auroral arcs and Birkeland currents map to the magnetotail plasma sheet and plasma sheet boundary layer. Localized Birkeland currents produce characteristic distortions that are dependent on the current density gradient. Single and multiple linear and hooked auroral forms are easy to understand in terms of mapping distortions in a quiet magnetosphere. The shapes of bright twisted or folded auroral forms are more easily explained as distortions produced by localized Birkeland currents. Measurement of the size of a spiral auroral structure provides an estimate of the current magnitude. Relationships between the motion of ionospheric objects and the motion of equatorial sources are summarized by two color plates. Such motion sometimes differs markedly from what one might intuitively expect. For example, some tailward moving sources near the magnetopause can map to sunward moving ionospheric images