LAUSR

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 found to be secondary gravity waves (GWs) from tropical storm Noel [Vadas and Crowley, 2010]. An instrumented sounding rocket simultaneously measured a large neutral wind peak uH′ with a similar azimuth at z ∼ 325 km [Earle et al., 2010]. Using the measured TID amplitudes and wavevectors 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 mid to upper thermosphere. Assuming that μ∝ρ¯q, where ρ¯ 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/ρ¯, increases less slowly with altitude for z≥220 km: ν∝1/ρ¯0.33. This dependence makes sense because as ρ¯→0, the distance between molecules goes to infinity, which implies no molecular collisions and therefore no molecular viscosity μ.