Summary
The so-called ionosonde drift technique of determining ionospheric velocities from angle-of-arrival and Doppler shift measurements is now well established. In the standard digital ionosonde technique, commonly called the drift mode, the velocity of the F-region is usually determined without detail of its height variation. In this paper, we report the measurement of the height variation of the velocity profile through the E- and F-regions at Casey, Antarctica. In the standard analysis, velocities are interpreted as if they arose from mirror reflectors in free space. According to Dyson (1975), this gives valid results when the ionosphere moves with a constant bulk velocity. However, the velocity may vary through the ionosphere and the purpose of this paper is to discuss the affect of such variations on the derivation of the drift velocity. A simplified inversion routine is developed enabling average drift velocity profiles to be corrected for the velocity variation with height. The procedure has been applied to an average daytime velocity height profile obtained at Casey during a four day campaign in March 1996. The drift velocity component perpendicular to the magnetic field direction was found to increase rapidly through the bottom of the E-region from a value of 170 ms-1 at 95 km to a peak value of over 800 ms-1 at 110km. It then dropped rapidly to around 700 ms-1 at 115km and remained in the range 600–700 m s-1 up into the F-region. The corrected values were about 10% larger in the lower E-region but the difference is not important because of the very rapid change in velocity with height. In the F-region the correction was 3% or less. At 110km where a narrow, intense velocity peak occurred, the correction was 20%, dropping to 7.5% near 150km. Even in the region of largest correction, the change in the shape of the velocity profile was modest. The results support the interpretation of drift velocities as reflection point velocities (at least within ~5–10%)