This project will conduct a modeling and data analysis effort to assess the extent to which acoustic waves and high frequency gravity waves influence the thermal, chemical, and dynamical states of the upper atmosphere. Specifically, the project will: (1) observe ionospheric perturbations using digital high-frequency Doppler sounders (TIDDBIT) from two North American locations; (2) use a numerical wave propagation model to infer the underlying atmospheric perturbations that cause the observed ionospheric variations and to characterize the energy and momentum flux into the ionosphere; and (3) use group velocity and raytracing models to examine sources of acoustic waves.
This project is focused on infrasonic acoustic waves and high-frequency gravity waves in the period range between 1 and 20 minutes and their effects on the thermosphere and ionosphere using data analysis and models. The observations are based primarily on data from an HF Doppler sounder known as the Traveling Ionospheric Disturbance Detector Built in Texas (TIDDBIT). During this project we use numerical models, ray tracing, and ionospheric data sets at two diverse locations (on in Virginia and the other in Texas) to answer the following science questions: 1. What are the spectral and propagation characteristics of acoustic and high frequency TIDs and gravity waves in the bottomside F-region near Wallops Island and San Antonio Texas? 2. What are the fluxes of heat and constituents carried by these waves into the thermosphere and F-region ionosphere, and what is their effect on the mean state of the atmosphere? 3. What are the likely sources of the waves? To address these objectives we have: 1. Performed Fourier and wavelet analysis of TIDDBIT data from both Texas and Virginia. The wavelet analysis reveals the spectral content of acoustic disturbances localized in time. 2. Developed an extended theory of ray tracing in dissipative media and examined dissipative effects on the vertical propagation of wave packets. 3. Examined the effect of acoustic waves on Jeans escape and acoustic wave heating. Jeans escape is especially topical for the Mars atmosphere and less complex and our first application was an examination of losses to space for the Mars exosphere. We have performed initial alculations of waves reaching the Earth's upper thermosphere for assessing wave-induced Jeans escape for the Earth’s atmosphere. 4. Developed a time-dependent nonlinear model of acoustic wave generation by thunderstorms. The TIDDBIT instrument gives a direct measurement of the vertical velocity of an ion density surface. To make optimal use of the data this must be converted to the vertical velocity of the underlying acoustic wave. The model includes a coupled ionosphere wherein the ion continuity equation is solved subject to wave-induced transport and recombination. 5. Examined a wave induced instability mechanism. Our data suggest that an enhancement of power occurs near the Brunt-Vaisala frequency and this suggests a parametric instability. Findings: 1. Spectral/wavelet analysis of TIDDBIT data: In all cases examined we find significant power at acoustic frequencies. Although there is an overall diminishment of power with frequency, we find no strong evidence of a gap (void) in power spectra density between the Brunt-Vaisala frequency and the acoustic cutoff frequency. We have found a number of instances of wave events in the acoustic range with high coherency and large wavelet amplitude. We find an enhancement in the power near the Brunt-Vaisala frequency in about half of the spectra examined and is evidence of a parametric subharmonic instability suggested by Klostermeyer. 2. Modeling: We have used the parameters of the waves inferred from the analysis to provide inputs to model calculations. These calculations indicate that the observed waves could propagate from the lower atmosphere to the altitudes where they are observed without undue attenuation by evanescence or dissipation. We find that acoustic wave generation by vigorous convection can cause significant perturbations of the bottom side F-region and are consistent with the waves seen by the TIDDBIT system. 3. Ray tracing theory: The vertical group velocity of wave packets is a function of the central wave number of the wavepacket. As wave packets dissipate because of scale dependent viscosity the central wave number diminishes. A theoretical development indicates that this effect can be large and we have accounted for in ray tracing calculations in the thermosphere. 4. Mean state effects: We found that wave-induced heating and Jeans escape are very important for Mars where strong gusty flow over steep terrain is apt to be a prolific source of acoustic waves. The wave-induced escape can locally far exceed the background escape and might be a significant loss mechanism. We have found that waves excited by flow over terrain in the Earth’s atmosphere can reach exospheric altitudes and might also induce a important loss to space via Jeans escape. Publications Walterscheid, R. L. (2013), The propagation of transient wave packets in highly dissipative media, J. Geophys. Res., 118, doi:10.1002/jgra.50097 Walterscheid, R. L., M. P. Hickey, G. Schubert,, Wave heating and jeans escape in the Martian upper atmosphere (2013), J. Geophys. Res., in press Walterscheid, R. L., et al. (2013), Instability structures during periods of largeRichardson number (Ri>1=4): Evidence of parametric instability, J. Geophys. Res. Atmos., 118, 6929–6939, doi:10.1002/jgrd.50514. Walterscheid, R. L et al. (2012), An intense traveling airglow front in the upper mesosphere-lower thermosphere with characteristic of a bore observed over Alice Springs, Australia, during a strong two-day wave episode, J. Geophys. Res., 117, D22105, doi:10.1029/2012JD017847.
