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 (3–20 days) entering the ionosphere‐thermosphere system near 100 km is responsible for producing ±40 m/s and ±10–15 K PW period oscillations between 110 and 150 km 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 ( ρ ) response at 325 km 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 ρ over short periods (∼< 9 days) correlates with temperature changes, indicating a response of hydrostatic origin. Over longer periods ρ 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.