Latitudinal structure of thermospheric composition perturbations

Abstract

Abstract The N 2 O density ratio measured by the ESRO 4 satellite is used to describe the latitudinal structure of thermospheric composition changes. To isolate the geomagnetic activity effect, all N 2 O data are divided by the quiet-time ( Ap = 0) ESRO 4 model. Subsequently, the latitudinal profiles of the residuals are superimposed in such a way that the equatorward boundary of the composition disturbance serves as a common reference location. Finally, the medians of these superimposed profiles are determined for different geophysical conditions. This averaging procedure preserves the typical latitudinal structure of the disturbances. The results are summarized in a simple analytical model. It describes the latitudinal variation of the N 2 O perturbation at 280 km altitude as a function of local time, season, and level of magnetic activity. The N 2 O density ratio is also used to describe the perturbations of the individual gas densities. To establish the respective correlations, 22 storms observed during the ESRO 4 mission are analyzed. The magnetic activity effect is isolated by normalizing the data to prestorm conditions. It is found that at 280 km altitude and irrespective of season and local time, the Ar and N 2 densities increase and the O and He densities decrease with increasing N 2 O perturbation. Simple analytical functions are used to describe this behavior. Combined with the above model of the N 2 O perturbations, these algorithms successfully predict the magnetic activity effect on the Ar, N 2 , O, and He densities at 280 km altitude.