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Neutral wind and density perturbations in the thermosphere created by gravity waves observed by the TIDDBIT sounder
Author(s) -
Vadas Sharon L.,
Crowley Geoff
Publication year - 2017
Publication title -
journal of geophysical research: space physics
Language(s) - English
Resource type - Journals
eISSN - 2169-9402
pISSN - 2169-9380
DOI - 10.1002/2016ja023828
Subject(s) - thermosphere , physics , sounding rocket , ionosphere , dissipative system , gravitational wave , depth sounding , altitude (triangle) , gravity wave , geophysics , jovian , amplitude , computational physics , atmospheric sciences , astrophysics , geology , astronomy , optics , geometry , planet , oceanography , mathematics , quantum mechanics , saturn
In this paper, we study the 10 traveling ionospheric disturbances (TIDs) observed at z obs ∼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 peaku H ′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 ∝ρ ̄q , whereρ ̄ is the density, we find using GW dissipative theory that the GWs could have been observed at z obs and that one or more of the GWs could have caused theu H ′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 rapidly with altitude for z ≥220 km: ν ∝ 1 /ρ ̄0 . 33 . This dependence makes sense because asρ ̄ → 0 , the distance between molecules goes to infinity, which implies no molecular collisions and therefore no molecular viscosity μ .