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

Abstract

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∝trueρ̄q, where trueρ̄ 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/trueρ̄, increases less rapidly with altitude for z≥220 km: ν∝1/trueρ̄0.33. This dependence makes sense because as trueρ̄→0, the distance between molecules goes to infinity, which implies no molecular collisions and therefore no molecular viscosity μ.

Dynamical Coupling Between Hurricane Matthew and the Middle to Upper Atmosphere via Gravity Waves

“…The buoyancy frequency is N , and the kinematic viscosity is represented as , in which is from MSISE‐00 and μ is the molecular viscosity. According to equation 53 in Vadas and Crowley (), and with best fit values z μ = 220 km and β = 2, where z μ is the altitude where μ begins to decrease. Although the Prandtl number is Pr ~0.6 in the thermosphere, we assume Pr = 1 in order to simplify the dispersion relation.…”

“…The buoyancy frequency is N, and the kinematic viscosity is represented as ν ¼ μ=ρ, in which ρ is from MSISE-00 and μ is the molecular viscosity. According to equation 53 in Vadas and Crowley (2017)…”

Secondary Gravity Waves in the Winter Mesosphere: Results From a High‐Resolution Global Circulation Model

“…Model simulations show that primary GWs excited by deep convection can propagate well into the thermosphere, where they break or dissipate (Vadas & Liu, , ; Vadas & Crowley, ). This process creates intermittent, localized body forces which excite secondary GWs having λ H ∼100–1,000 s km (Vadas & Crowley, ); these secondary GWs then propagate to much higher altitudes and create significant variability in the neutral wind (Vadas & Crowley, ). Additionally, these secondary GWs enhance the variability of the ionosphere by creating medium to large‐scale traveling ionospheric disturbances via ion drag (Azeem et al, ; Nicolls et al, ; Vadas & Crowley, ; Vadas & Nicolls, ).…”

Compatibility Conditions, Complex Frequency, and Complex Vertical Wave Number for Models of Gravity Waves in the Thermosphere

“…This has been for the purpose of describing the altitude dependence of an individual Fourier component of fixed ω , k , l . A different approach was taken by Vadas and Fritts (2005), Vadas (2007), Vadas and Nicolls (2012), and Vadas and Crowley (2017), who assumed complex ω and real k =( k , l , m ). This was for the purpose of spatial ray tracing.…”