An inter-hemispheric, statistical study of nightside spectral width distributions from coherent HF scatter radars

A statistical investigation of the Doppler spectral width parameter routinely observed by HF coherent radars has been conducted between the Northern and Southern Hemispheres for the nightside ionosphere. Data from the SuperDARN radars at Thykkvibær, Iceland and Syowa East, Antarctica have been employed for this purpose. Both radars frequently observe regions of high (>200 ms-1) spectral width polewards of low (<200 ms-1) spectral width. Three years of data from both radars have been analysed both for the spectral width and line of sight velocity. The pointing direction of these two radars is such that the flow reversal boundary may be estimated from the velocity data, and therefore, we have an estimate of the open/closed field line boundary location for comparison with the high spectral widths. Five key observations regarding the behaviour of the spectral width on the nightside have been made. These are (i) the two radars observe similar characteristics on a statistical basis; (ii) a latitudinal dependence related to magnetic local time is found in both hemispheres; (iii) a seasonal dependence of the spectral width is observed by both radars, which shows a marked absence of latitudinal dependence during the summer months; (iv) in general, the Syowa East spectral width tends to be larger than that from Iceland East, and (v) the highest spectral widths seem to appear on both open and closed field lines. Points (i) and (ii) indicate that the cause of high spectral width is magnetospheric in origin. Point (iii) suggests that either the propagation of the HF radio waves to regions of high spectral width or the generating mechanism(s) for high spectral width is affected by solar illumination or other seasonal effects. Point (iv) suggests that the radar beams from each of the radars are subject either to different instrumental or propagation effects, or different geophysical conditions due to their locations, although we suggest that this result is more likely to be due to geophysical effects. Point (v) leads us to conclude that, in general, the boundary between low and high spectral width will not be a good proxy for the open/closed field line boundary.