Proceedings with Extended Abstracts (single PDF file) - Radio ...

Figure 2. Diurnal occurrence distribution forlacuna and SEC observed at Casey forDecember 1996 (top). Seasonal occurrencedistribution for lacuna observed at Caseybetween July 1996 and June 1997 (bottom).Following on from the Casey HFdigisonde study by Monselesan et al.(2003) we then conducted highresolutionE-region measurements usingthe MF radar located at Davis, to seewhether these spectral signatures wereevident during lacuna conditions (asscaled from the IPS Radio and SpaceServices 5D ionosonde ionograms). Remarkably we also observed the occurrence of Braggscatter spectral side-peak events at 1.94 MHz as shown in Figure 3 for 23 February 2001. Thisevent has similar structure and characteristics to the events detected using a digisondescanning in the 1.6 to 2.7 MHz range at Casey during 1996 as shown in Figure 1.Figure 3. MF radar Bragg spectralsignatures during a lacuna event on 23February 2001 at Davis.E-region drift velocities (see Monselesan et al., 2003).DiscussionIt is important to note thaterroneous E-region drift velocitiesresult when Doppler sortedinterferometry (DSI) techniques areapplied to DPS drift data duringintervals where Bragg backscatteror spectral bands occur in the highlatitudeionosphere. These eventsappear to be linked with SEC andlacuna conditions. Thus the velocitycontribution derived from the sidepeaksor Bragg backscatter echoesmust be removed from the datawhen determining the backgroundTsunoda et al. (1997) recorded similar spectra with the continuous-wave 12.3-MHz radar HF-OSCAR located at Søndre Strømfjord in Greenland. These authors concluded that the spectrawere produced by the modified two-stream instability, given their narrow spectral widths andpeak Doppler velocities comparable to the ion-acoustic speed at around 95-km altitude.Previous SEC/lacuna studies have been linked to E-region two-stream plasma instabilityprocesses (Haldoupis et al., 1993). Ionospheric absorption and associated E-region currentstudies on events similar to our observations as presented in Figure 1 were also linked to twostreamplasma wave instability processes (i.e. Schlegel and St-Maurice, 1981). We interpretthe velocities derived from the side-peaks on the Doppler spectra, as irregularities phasevelocities triggered primarily by two-stream instabilities, possibly influenced by gradient drift124