On the Effects of Mesospheric and Lower Thermospheric Oxygen Chemistry on the Thermosphere and Ionosphere Semiannual Oscillation

This study quantifies mesosphere/lower thermosphere (MLT) oxygen chemical contributions to the global thermosphere-ionosphere (T-I) semiannual oscillation (SAO) using a series of numerical experiments from the National Center for Atmospheric Research (NCAR) thermosphere-ionosphere-mesosphere-electrodynamics general circulation model (TIME-GCM) that isolate essential chemical processes affecting O and O-2 in the MLT region. We track the vertical dynamical, diffusive, and chemical fluxes of O and O-2 in and out of two control volumes between similar to 80 and 130 km using a finite volume approach to the individual species continuity equation to investigate their relative importance on the global T-I SAO. TIME-GCM results indicate that the global T-I SAO amplitude and phase is fairly insensitive to significant changes in odd oxygen chemical reaction rates in the MLT. While chemistry has an appreciable effect on O in the MLT region, sensitivity to changes in odd oxygen and odd hydrogen chemical rates appear to be offset by a consequent adjustment in the vertical bulk wind and eddy diffusive transport of O locally, rendering their effects inconsequential to the global T-I SAO aloft. The implications of our findings for reproducing a self-consistent global T-I SAO in the NCAR thermosphere-ionosphere-electrodynamics general circulation model (TIE-GCM) with a lower boundary near similar to 100 km are discussed. Specifically, including latitude-season variations in O, O-2, and N-2 from NRLMSIS (R) 2.0 at the lower boundary of the TIE-GCM near similar to 100 km improves its representation of the climatological T-I SAO. However, reformulating the TIE-GCM temperature lower boundary condition could further improve its ability to simulate the T-I SAO from first-principles.