Zonally symmetric oscillations of the thermosphere at planetary wave periods

New mechanisms for imposing planetary wave (PW) variability on the ionosphere-thermosphere system are discovered in numerical experiments conducted with the National Center for Atmospheric Research thermosphere-ionosphere-electrodynamics general circulation model. First, it is demonstrated that a tidal spectrum modulated at PW periods (3-20days) entering the ionosphere-thermosphere system near 100km is responsible for producing +/- 40m/s and +/- 10-15K PW period oscillations between 110 and 150km at low to middle latitudes. The dominant response is broadband and zonally symmetric (i.e.,"S0") over a range of periods and is attributable to tidal dissipation; essentially, the ionosphere-thermosphere system vacillates in response to dissipation of the PW-modulated tidal spectrum. In addition, some specific westward propagating PWs such as the quasi-6-day wave are amplified by the presence of the tidal spectrum; the underlying mechanism is hypothesized to be a second-stage nonlinear interaction. The S0 total neutral mass density (rho) response at 325km consists of PW period fluctuations of order +/- 3-4%, roughly equivalent to the day-to-day variability associated with low-level geomagnetic activity. The variability in rho over short periods (similar to< 9days) correlates with temperature changes, indicating a response of hydrostatic origin. Over longer periods is also controlled by composition and mean molecular mass. While the upper-thermosphere impacts are modest, they do translate to more significant changes in the F region ionosphere.