Responses of the Mean Thermosphere Circulation, O/N2 Ratio and Ne to Solar and Magnetospheric Forcing From Above and Tidal Forcing From Below

Abstract

AbstractThe day‐to‐day variability (“weather”) associated with the diurnal‐ and zonal‐mean (DZM) circulation, O/N2 ratio and electron density (Ne) of the I‐T system due to tidal “forcing from below” and solar flux and magnetosphere (SM) “forcing from above” during 2021 are delineated, diagnosed and quantitatively compared using a series of model simulations designed to separate these responses with respect to their origins. The external forcings are driven by actual tidal, solar wind, and solar flux observations. Both circulation systems occupy the full extent of the I‐T, and the SM‐forced DZM circulation is 2–3 times more vigorous in terms of vertical and meridional wind magnitudes. Tidal‐driven DZM Ne reductions of up to 30%–40% with respect to those of the fully forced I‐T system occur, mainly between ±30° latitude, compared to SM‐driven increases of up to 15%–20%. In terms of annual variances over this latitude range, tidal‐driven DZM Ne variances exceed or equal those of the SM‐driven variances. The former is mainly controlled by O/N2 ratio vis‐a‐vis tidal‐forced temperature variations above 150 km. While a similar cause‐effect relation exists for the latter, this is superseded by Ne variability associated with solar production. However, DZM I‐T system variability forced from below is underestimated in the simulations in two respects: the effects of gravity waves are omitted, and tidal forcing is represented by 45‐day running means, as compared with the more realistic actual daily variability of SM forcing. These shortcomings should be ameliorated once multi‐satellite missions planned for the future come to fruition.