Solar cycle variations of thermospheric composition at the solstices

We examine the solar cycle variability of thermospheric composition (O/N 2 ) at the solstices. Our observational and modeling studies show that the summer‐to‐winter latitudinal gradient of O/N 2 is small at solar minimum but large at solar maximum; O/N 2 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/N 2 , 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/N 2 in the summer hemisphere but increase it in the winter hemisphere; consequently, the vertical gradient of O/N 2 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/N 2 causes a seasonal asymmetry in the vertical advection of O/N 2 , 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/N 2 . 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/N 2 and their solar cycle variability.