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Ann. Geophys., 20, 1399-1413, 2002
www.ann-geophys.net/20/1399/2002/
© European Geosciences Union 2002
Nightside studies of coherent HF Radar spectral width behaviour
E. E. Woodfield1, J. A. Davies1, M. Lester1, T. K. Yeoman1, P. Eglitis2,4, and M. Lockwood3,5
1Department of Physics and Astronomy, University of Leicester, UK
2Formerly at Swedish Institute of Space Physics, Uppsala, Sweden and the Finnish Meteorological Institute, Helsinki, Finland
3Rutherford Appleton Laboratory, UK
4now at: Rhea System S.A., New Tech Center, Belgium
5also at: Department of Physics and Astronomy, University of Southampton, UK
Correspondence to: E. E. Woodfield
(Emma.Woodfield@ion.le.ac.uk)
Abstract. A previous case study
found a relationship between high spectral width measured by the CUTLASS
Finland HF radar and elevated electron temperatures observed by the EISCAT and
ESR incoherent scatter radars in the post-midnight sector of magnetic local
time. This paper expands that work by briefly re-examining that interval and
looking in depth at two further case studies. In all three cases a region of
high HF spectral width (>200 ms-1) exists poleward of a region of
low HF spectral width (<200 ms-1). Each case, however, occurs
under quite different geomagnetic conditions. The original case study occurred
during an interval with no observed electrojet activity, the second study
during a transition from quiet to active conditions with a clear band of ion
frictional heating indicating the location of the flow reversal boundary, and
the third during an isolated sub-storm. These case studies indicate that the
relationship between elevated electron temperature and high HF radar spectral
width appears on closed field lines after 03:00 magnetic local time (MLT) on
the nightside. It is not clear whether the same relationship would hold on open
field lines, since our analysis of this relationship is restricted in latitude.
We find two important properties of high spectral width data on the nightside.
Firstly the high spectral width values occur on both open and closed field
lines, and secondly that the power spectra which exhibit high widths are both
single-peak and multiple-peak. In general the regions of high spectral width
(>200 ms-1) have more multiple-peak spectra than the regions of
low spectral widths whilst still maintaining a majority of single-peak spectra.
We also find that the region of ion frictional heating is collocated with many
multiple-peak HF spectra. Several mechanisms for the generation of high
spectral width have been proposed which would produce multiple-peak spectra,
these are discussed in relation to the data presented here. Since the regions
of high spectral width are observed both on closed and open field lines the use
of the boundary between low and high spectral width as an ionospheric proxy for
the open/closed field line boundary is not a simple matter, if indeed it is
possible at all.
Key words. Ionosphere (auroral
ionosphere; ionospheric irregularities)
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