Neutral wind and density perturbations in the thermosphere created by gravity waves observed by the TIDDBIT sounder

In this paper, we study the 10 traveling ionospheric disturbances (TIDs) observed at zobs∼283 km by the TIDDBIT ionospheric sounder on 30 October 2007 at 0400–0700 UT near Wallops Island, USA. These TIDs propagated northwest/northward and were previously found to be secondary gravity waves (GWs) from tropical storm Noel. An instrumented sounding rocket simultaneously measured a large neutral wind peak uH′ with a similar azimuth at z ∼ 325 km. Using the measured TID amplitudes and wave vectors from the TIDDBIT system, together with ion‐neutral theory, GW dissipative polarization relations and ray tracing, we determine the GW neutral horizontal wind and density perturbations as a function of altitude from 220 to 380 km. We find that there is a serious discrepancy between the GW dissipative theory and the observations unless the molecular viscosity, μ, decreases with altitude in the middle to upper thermosphere. Assuming that μ∝ρ̄q, where ρ̄ is the density, we find using GW dissipative theory that the GWs could have been observed at zobs and that one or more of the GWs could have caused the uH′ wind peak at z≃325 km if q ∼ 0.67 for z≥220 km. This implies that the kinematic viscosity, ν=μ/ρ̄, increases less rapidly with altitude for z≥220 km: ν∝1/ρ̄0.33. This dependence makes sense because as ρ̄→0, the distance between molecules goes to infinity, which implies no molecular collisions and therefore no molecular viscosity μ. Key Points TIDs used to determine neutral wind and density perturbations from gravity waves Serious discrepancy between gravity wave theory and observation can be resolved by a decreasing molecular viscosity mu in the thermosphere Mu decreasing in thermosphere allows gravity waves to propagate much higher than previously thought