Hemispheric Asymmetry and Seasonal Variation of Thermospheric Composition

The thermosphere exhibits pronounced annual and semi-annual cycles in neutral density, with the equinoxes higher than the solstices, and the December solstice higher than June. The electron density of the F-region ionosphere shows the same pattern, and it has long been presumed that these are linked. The underlying cause of this pattern is likely the variation of atomic/molecular neutral composition. This composition variation has long been observed through space-based in-situ and remote-sensing measurements, from Atmosphere Explorer to TIMED, and is captured by empirical models such as the MSIS series. However, the cause of the composition variation is not known. Various mechanisms have been proposed, including changes the general circulation of the thermosphere, the varying Sun-Earth distance, the inclination of the geomagnetic field, and atmospheric tidal and wave forcing. Hemispherical differences in atmospheric circulation, particularly at the solstices, and their interaction with the thermosphere through changes mesopause dynamics and turbopause mixing, are a prime candidate. In this paper, we present observational evidence demonstrating the linkage between seasonal/hemispheric changes in neutral density, ion density, and composition, and model simulations describing the interaction of seasonal variation in the lower atmosphere with the thermosphere/ionosphere system.