Solar cycle variations of thermospheric composition at the solstices

Abstract

We examine the solar cycle variability of thermospheric composition (O/N2) at the solstices. Our observational and modeling studies show that the summer-to-winter latitudinal gradient of O/N2 is small at solar minimum but large at solar maximum; O/N2 is larger at solar maximum than at solar minimum on a global-mean basis; there is a seasonal asymmetry in the solar cycle variability of O/N2, with large solar cycle variations in the winter hemisphere and small solar cycle variations in the summer hemisphere. Model analysis reveals that vertical winds decrease the temperature-driven solar cycle variability in the vertical gradient of O/N2 in the summer hemisphere but increase it in the winter hemisphere; consequently, the vertical gradient of O/N2 does not change much in the summer hemisphere over a solar cycle, but it increases greatly from solar minimum to solar maximum in the winter hemisphere; this seasonal asymmetry in the solar cycle variability in the vertical gradient of O/N2 causes a seasonal asymmetry in the vertical advection of O/N2, with small solar cycle variability in the summer hemisphere and large variability in the winter hemisphere, which in turn drives the observed seasonal asymmetry in the solar cycle variability of O/N2. Since the equatorial ionization anomaly suppresses upwelling in the summer hemisphere and strengthens downwelling in the winter hemisphere through plasma-neutral collisional heating and ion drag, locations and relative magnitudes of the equatorial ionization anomaly crests and their solar cycle variabilities can significantly impact the summer-to-winter gradients of O/N2 and their solar cycle variability.