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Acoustic Gravity Waves and Traveling Ionospheric Disturbances

Acoustic gravity waves (AGW) are generated by numerous lower atmospheric processes, such as storms (e.g. Walterscheid et al., 2001), and by auroral processes in the ionosphere (Fesen et al., 1989; Crowley, 1991; Immel et al., 2000). At ionospheric heights, the motion of the neutral gas in the AGW sets the ionospheric plasma into motion. The waves displace the isoionic contours, resulting in a traveling ionospheric disturbance (TID). TIDs can be thought of as traveling corrugations in the ionosphere, and they can seriously affect HF radio communications and surveillance systems. Consequently, their presence can be detected by HF radio systems, and their speed and direction can be determined (Georges 1967, Waldock and Jones 1986). One of the most sensitive methods for detecting transient changes in the ionosphere is the HF Doppler technique (Georges 1967).

Classification of Gravity Waves and TIDs

Gravity Waves and TIDs are generally placed into two classes: Medium Scale and Large Scale according to whether they propagate faster or slower than the sound speed in the lower atmosphere (300 m/s). The corresponding periods, horizontal propagation velocities, and horizontal wavelengths are summarized in Table 1. This classification is approximate, and all TIDs do not fit neatly into either category. For example, a TID can have medium scale TID (MSTID) periods, but propagate faster than 300 m/s. The aurorae in both hemispheres launch a spectrum of waves, including high speed, large scale waves. Slower waves generally originate from processes in the lower atmosphere (storms, weather fronts, winds blowing over topography, explosions) and can propagate into the ionosphere within several hours.

Table 1. Classification of Gravity Waves and TIDS

Determination Of Gravity Wave Sources
Jones and Waldock (1986) showed that gravity wave ray-tracing could be used to determine the likely sources of gravity waves in the ionosphere. They found that the large-scale waves propagated almost horizontally and were probably launched by the aurora. In contrast, most of their medium scale waves propagated almost vertically and probably had their origins in the lower atmosphere. Walterscheid et al. (2001) showed that waves from thunderstorms can penetrate into the ionosphere. Kelley (1997) measured ionospheric perturbations caused by waves over active thunderstorms. For the Chesapeake rocket campaign described on this website, there is some interest in trying to determine the source of waves that seed mid-latitude Spread-F. Therefore, the rocket launch criteria included the occurrence of Spread-F, and the presence of a large weather system with thunderstorms on the Atlantic Coast of the U.S.