Zonally-Symmetric Oscillations of the Thermosphere at Planetary-Wave Periods

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Friday, May 11, 2018

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 (NCAR) Thermosphere-Ionosphere- Electrodynamics General circulation Model (TIE-GCM).

responses at 120 km due to different lower boundary forcing of the TIEGCM image
Examples of responses at 120 km due to different lower boundary forcing of the TIEGCM. Top: Longitude vs. day of month depictions of diurnal-mean zonal winds over the equator. Bottom: The wavenumber vs. period spectra of the data shown above. (a) and (d): S0 forcing only. (b) and (e): S0 forcing plus tides. (c) and (f): tide contribution alone.

First, it is demonstrated that a tidal spectrum modulated at PW periods (3-20 days) entering the ionosphere-thermosphere (IT) system near 100 km is responsible for producing +/-40m/s and +/-10-15K PW-period oscillations between 110 and 150 km at low to middle latitudes. The dominant response is broadband and zonally-symmetric ("S0") over a range of periods, and is attributable to tidal dissipation; essentially the IT system "vacillates" in response to dissipation of the PW-modulated tidal spectrum. In addition, some specificc westward propagating PW such as the quasi-6-day wave are amplifyed by presence of the tidal spectrum; the underlying mechanism is hypothesized to be a second-stage nonlinear interaction. The S0 neutral total mass density 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 neutral mass density over short periods (approximately less than 9 days) correlates with temperature changes, indicating a response of hydrostatic origin. Over longer periods the neutral mass density 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.

Publication Name: JGR Space Physics

EOS highlight: https://eos.org/editor-highlights/tides-and-waves-interact-to-cause-hurricanes-in-near-space#.WwV6JjeVd08.email