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).
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