Thermospheric wind effects on the distribution of helium and argon in the Earth's upper atmosphere

Abstract

The momentum and continuity equations for a minor gas are combined with the momentum equation for the major constituents to obtain the time-dependent continuity equation for the minor species reflecting a wind field in the background gas. This equation is used to study the distributions of helium and argon at times of low, medium, and high solar activity for a variety of latitudinal-seasonal wind cells. For helium the exospheric return flow at the higher thermospheric temperatures dominates the distribution to the extent that much larger latitudinal gradients can be maintained during periods of low solar activity than during periods of high activity. In comparison to the exospheric flow the smoothing effect of horizontal diffusion is almost negligible. The latitudinal variation of helium observed by satellite mass spectrometers can be reproduced by the effect of a wind system of air rising in the summer hemisphere, flowing across the equator with speeds of the order of 100–200 m/sec and descending in the winter hemisphere. Argon, being heavier than the mean mass in the lower thermosphere, reacts oppositely to helium in that it is enhanced in the summer hemisphere and depleted in the winter. By using winds that are effective in the lower thermosphere the anomalous vertical helium profiles observed from rockets can be reproduced. The time response of the helium density distribution following the initiation of a wind field implies the likelihood of a density enhancement of a factor of 2–4 at night over the daytime values.