HF Doppler Sounder Principle
When a HF radio wave is reflected from the ionosphere, movement of the reflection point during passage of a traveling ionospheric disturbance (TID) produces a change in phase path and a Doppler shift proportional to the time rate of change of the phase path. If three or more spatially separated propagation paths are monitored, the time difference between the wave signatures from the three reflection points yields the speed and direction of the TID by triangulation. The separation between the receiver and each transmitter is selected to ensure the time-delays are measurable. Typically this means the separation has to be 50 – 300 km. Each sounding frequency is reflected at a particular height, and the use of multiple frequencies enables ionospheric motion to be monitored at different altitudes. Typically we use two frequencies to enable vertical TID velocities to be determined.
Relationship between Measured TID Properties and Underlying Gravity Waves
The TIDDBIT radar provides the TID parameters listed in Table 1 below. There is sometimes confusion about how the measured TID parameters relate to the properties of the underlying gravity wave (AGW) that is propagating through the neutral gas. Therefore the second column in Table 1 indicates which TID parameters accurately correspond to gravity wave properties, as explained below.
The Doppler system is sensitive to motions of the ionospheric reflection point, and it therefore provides an accurate measure of both the TID and acoustic gravity wave periods. Similarly, because the TID velocity is determined simply from triangulation using the time-delays between perturbations at different reflection points, the TID velocities are also an accurate estimate of the underlying gravity wave horizontal and vertical trace velocities. In contrast, the amplitude of the ionospheric disturbance does depend on the direction of propagation relative to the magnetic field (Hooke, 1968), therefore the shape of the TID amplitude spectrum does not directly provide the amplitude spectrum of the underlying gravity wave. We plan to address this issue by using a computer model to calibrate the TID amplitude spectrum. Thus, a complete specification of the underlying AGW characteristics can be determined from the HF Doppler measurements of TIDs.
Advantages of HF Doppler Systems over Other Techniques
HF Doppler systems have advantages over all other techniques for the measurement of TID characteristics. They are more amenable to analysis than data from ionosonde chains (Tedd and Morgan, 1985), and their time resolution (30 sec) is much higher than that of ionosondes (5-15 min). Unlike total electron content (TEC) methods, which respond to height-integrated TID effects, the HF Doppler radar responds to TIDs at the altitude of the radio reflection point. Finally, HF Doppler systems have low power consumption, so that both spatial and temporal resolution can be maintained for many days without the costs that would be associated with an incoherent-scatter radar. For example, the Antarctic HF Doppler system (Crowley, 1985; Dudeney et al, 1980; Crowley and Dudeney, 1984; Crowley et al., 1987) operated continuously for 5 years.