Quiet Time Thermospheric Gravity Waves Observed by GOCE and CHAMP

The Gravity Field and Steady-State Ocean Circulation Explorer (GOCE) and CHAllenging Minisatellite Payload (CHAMP) satellites measure in-situ thermospheric density and cross-track wind. When propagating obliquely to the satellite track in a horizontal plane (i.e., not purely along-track or cross-track), gravity waves (GWs) can be observed both in the density and cross-track wind perturbations. We employ the Wavelet Analysis, red noise model, dissipative dispersion and polarization relations for thermospheric GWs, and specific criteria to determine whether a quiet-time (Kp < 3) thermospheric traveling atmospheric disturbances (TADs) event is a GW or not. The first global morphology of thermospheric GWs instead of TADs is reported. The fast intrinsic horizontal phase speed (c(IH) > 600 m/s) of most GWs suggests that they are not generated in the lower/middle atmosphere (where c(IH) < 300 m/s). A second population of GWs with slower speeds (c(IH) = 50-250 m/s) in GOCE are likely from the lower/middle atmosphere, but they occur much less frequently in CHAMP. GW hotspots occur during the high-latitude and the winter midlatitude regions. GW amplitudes exhibit semi-annual and annual variations. These findings suggest that most GOCE and CHAMP GWs are higher-order GWs from primary GW sources in the lower/middle atmosphere. Finally, the average propagation direction of the CHAMP GWs exhibits a clear diurnal cycle, with clockwise (counterclockwise) occurring in the northern (southern) hemisphere and equatorward propagation occurring at similar to 13 LST. This suggests that the predominant GW propagation direction is opposite to the background wind direction.