MEASUREMENTS OF MIDLATITUDE E-REGION, SPORADIC-E, AND TID-RELATED DRIFTS USING HF DOPPLER-SORTED INTERFEROMETRY

Modern HF digital ionosondes have been used for Doppler-sorted interfe rometry (DSI) to automatically measure F-region plasma drifts by detec ting the Doppler shift and angle-of-arrival of echoes. We report on th e use of a Digisonde 256 receiving on a seven-element antenna array lo cated at the Australian mid-latitude station Beveridge (37.5 degrees S , 144.9 degrees E, -48.0 degrees CGM latitude) to extend the routine a pplication of drift measurements to the E-region, and especially spora dic-E (Es). Obtaining good quality E-region drift measurements require d many more soundings independent in the frequency and time domains th an are usually made for F-region measurements. This is because the E-r egion isoionic surfaces were usually more horizontally stratified than those in the more disturbed F-region, and so did not return as many o f 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 thou ght of as being uniform throughout the volume of ionosphere sampled fo r echoes. Application of the technique to E-region measurements is int eresting because of the enhanced irregular neutral winds associated wi th 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, t he possibility of sharp vertical gradients in plasma drifts must be co nsidered when making and analysing measurements. During numerous campa igns conducted throughout 1994/95, horizontal drifts in the range of a pproximate 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 w as observed to propagate towards the station from the south, and clear ly 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 sla nt-F irregularities. A predominant drift direction was towards the NW or W, and was sometimes seen to veer through S towards the SE during i ntervals of approximate to 1-5h. There is evidence of these veering ev ents occurring diurnally and semidiurnally. Our results can be underst ood in terms of the formation of Es under the influence of gravity wav es and tides. (C) 1998 Elsevier Science Ltd. All rights reserved.