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Ann. Geophys., 20, 1921-1934, 2002
www.ann-geophys.net/20/1921/2002/
© European Geosciences Union 2002
An inter-hemispheric, statistical study of nightside spectral width distributions from coherent HF scatter radars
E. E. Woodfield1, K. Hosokawa2, S. E. Milan1, N. Sato3, and M. Lester1
1Department of Physics and Astronomy, University of Leicester, UK
2Department of Geophysics, Graduate School of Science, Kyoto University, Japan
3National Institute of Polar Research, Tokyo, Japan
Correspondence to: E. E. Woodfield
(Emma.Woodfield@ion.le.ac.uk)
Abstract. 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.
Key words. Ionosphere (auroral
ionosphere; ionospheric irregularities)
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