The spatial variation of auroral luminosity—I: The behavior of synthetic model auroras

Abstract

The determination of the spatial distribution of auroral luminosity using two-station triangulation techniques requires a knowledge of the geometrical effects of perspective and path length on the observations. In order to evaluate these effects model synthetic aurora with known vertical and horizontal luminosity profiles were constructed mathematically and were programmed into a high speed electronic computer. The appearance (intensity vs. zenith angle) of these model auroras positioned at various latitudes from each of the observing stations was then calculated. The analysis of this computed data has led to the formulation of a procedure for the determination of the spatial distribution of the volume emission rate of various auroral emissions within isolated discrete auroral forms. In addition, the analysis provides information pertinent to the interpretation of auroral observations obtained through commonly used optical techniques. It is shown that the observed intensity and appearance of an aurora as a function of zenith angle are highly dependent on the luminosity distribution within the auroral form and the distance to the aurora. The accuracy of the height and latitude of auroral forms determined by triangulation techniques depends on the position of the aurora with respect to the stations. The highest accuracy in location is obtained for auroras between stations but this is where the lowest accuracy in height occurs. Conversion of intensity vs. zenith angle observations into an intensity vs. height profile is a satisfactory first order approximation to the actual vertical volume emission rate profile if the height or location of the aurora is known. The most accurate profiles are determined for auroras positioned over the most poleward station when viewed by an equatorward station at least 200 km away. Observations of auroral intensities in a fixed direction are most easily interpreted when directed towards the magnetic zenith. In that direction path length effects are height dependent only. Therefore the intensities measured can be directly related to the energy deposit of the incident particles and the fraction of the energy distributed among the various excited auroral emissions can be determined.