A survey of large-scale variations in thermospheric oxygen column density with magnetic activity as inferred from observations of the FUV dayglow

Abstract

Brightness of the terrestrial far-ultraviolet (FUV) dayglow is dominated by the 130.4nm emission of neutral atomic oxygen, O I, and variations in the brightness observed from altitudes high above the emitting region reflect variations in thermospheric oxygen density. This paper summarizes the results of an initial survey of the Dynamics Explorer 1 observations of the FUV dayglow through a presentation of 13 representative events selected to demonstrate the spatial extent and short-term temporal stability of the brightness perturbations. The emphasis here is on the morning sector of local time and the polar cap for observations obtained in the time interval from September 23, 1981, through January 19, 1982. An analytic expression is derived for the average response of the FUV photometer to the dayglow during periods of high-latitude magnetic quiescence. The remaining observations in this time interval are then analyzed for their deviations from the established quiet time values. Deviations of -40% to +30% are found following intervals of increased magnetic activity. The most significant decreases (-30% to -40%) are observed equatorward of the instantaneous auroral oval only after sustained periods (∼6 hours) of intense magnetic activity (average AE greater than ∼700 nT). Decreases extend equatorward from the aurora to geographic latitudes as low as ∼30° N. Decreases of lesser magnitude that do not extend as far equatorward are associated with sustained periods of more moderate activity in which the average value of AE is smaller (∼300-400 nT). Also, the spatial extent and magnitude of the decreases in the morning sector appear greater when the IMF By component is positive. In both cases, decreases are readily observed within the polar cap. Localized enhancements of +20% to +30% occur much less frequently and are detected at the middle latitudes, well equatorward of the auroral oval. Copyright 1997 by the American Geophysical Union.