A Experimental Study of the Propagation and Dispersion of Internal Atmospheric Gravity Waves.

Traveling ionospheric disturbances (TID's) appear as large-scale transverse waves in the F-region (150-1000 km altitude), with frequencies on the order of 0.005 to 0.05 cycles per minute, which propagate horizontally over hundreds or even thousands of kilometers. These disturbances have been observed by various radiowave techniques over the past thirty-five years and are now generally accepted as being the manifestation, in the ionized medium, of internal atmospheric gravity waves. A model describing the propagation of gravity waves in an isothermal atmosphere is presented here. The dispersion relation is derived from fundamental principles, and the relation between phase velocity and group velocity is examined. The effects of the Coriolis force and horizontally stratified winds on wave propagation are also analyzed. Conservation of energy in the gravity wave requires increasing amplitude with increasing altitude, inasmuch as the atmospheric density decreases with height. However, this is counteracted by dissipation of wave energy by ion drag, thermal conductivity, and viscous damping. The production of TID's (in the ionized medium) by gravity waves (in the neutral medium) is discussed in quantitative terms, and the "vertical predictive function" is derived. Dartmouth College has operated a three-station ionosonde network in northern New Hampshire and Vermont on an intermittent basis since 1968. Seven large TID's, found in the 1969 data, are re-examined here in an exhaustive and successful comparison with the gravity wave model. Iso -true-height contours of electron density are used to determine several pertinent TID wave parameters as a function of height. Fourier decomposition of iso-height contours at a single station yields the spectral content at the various altitudes, while cross-spectral analysis among the three stations yield cross-spectral power, coherence, and phase velocity of each spectral component in the TID. Assuming a horizontally stratified medium, we have employed a realistic atmospheric model to determine the dissipative effects and the resulting imaginary part of the phase propagation vector. With these values and the use of a numerical iterative technique, the phase propagation vector may be converted from that for the electronic component (TID) to that for the neutral gas which, in turn, facilitates a direct comparison with the theoretical gravity-wave model predictions. The validity of the vertical predictive function is demonstrated by comparing the perturbation in electron density, calculated from the model wave parameters, to the experimentally observed TID amplitude. The group velocity is obtained from the phase propagation vector and winds postulated by the model. Inasmuch as the wave energy travels with the group speed, we can extrapolate backwards to determine the probable source region. Geophysical data from the proposed source region are presented in support of our deductions.