Measurements of mid-latitude E-region, sporadic-E, and TID-related drifts using HF Doppler-sorted interferometry

Modern HF digital ionosondes have been used for Doppler-sorted interferometry (DSI) to automatically measure F-region plasma drifts by detecting the Doppler shift and angle-of-arrival of echoes. We report on the use of a Digisonde 256 receiving on a seven-element antenna array located at the Australian mid-latitude station Beveridge (37.5 degrees S, 144.9 degrees E, -48.0 degrees CGM latitude) to extend the routine application of drift measurements to the E-region, and especially sporadic-E (Es). Obtaining good quality E-region drift measurements required many more soundings independent in the frequency and time domains than are usually made for F-region measurements. This is because the E-region isoionic surfaces were usually more horizontally stratified than those in the more disturbed F-region, and so did not return as many of the oblique echoes upon which the accuracy of the technique depends. Smoothness of the ionosphere was less of a problem when performing Es measurements because of the patchy, cloud-like property of the layers. When making F-region drift measurements the motions are usually thought of as being uniform throughout the volume of ionosphere sampled for echoes. Application of the technique to E-region measurements is interesting because of the enhanced irregular neutral winds associated with gravity waves which grow in amplitude with height. These irregular winds control the motion of the plasma via high collision frequencies, and lead to the formation of Es layers at mid-latitudes. Therefore, the possibility of sharp vertical gradients in plasma drifts must be considered when making and analysing measurements. During numerous campaigns conducted throughout 1994/95, horizontal drifts in the range of approximate to 30-250 m/s were measured for Es patches lasting up to 6h. The drift measurements sometimes indicated the presence of wave-like motions in the ionosphere with periods in the range 5-90 min. A TID was observed to propagate towards the station from the south, and clearly showed an association between long-period motions in the F-region (approximate to 90 min), and the arrival of a patch of slant-Es and slant-F irregularities. A predominant drift direction was towards the NW or W, and was sometimes seen to veer through S towards the SE during intervals of approximate to 1-5h. There is evidence of these veering events occurring diurnally and semidiurnally. Our results can be understood in terms of the formation of Es under the influence of gravity waves and tides. (C) 1998 Elsevier Science Ltd. All rights reserved.