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Ann. Geophys., 22, 1843-1849, 2004
www.ann-geophys.net/22/1843/2004/
© European Geosciences Union 2004
The effects of high-frequency ULF wave activity on the spectral characteristics of coherent HF radar returns: a case study
D. M. Wright1, T. K. Yeoman1, and E. E. Woodfield2
1Department of Physics and Astronomy, University of Leicester, University Road, Leicester, LE1 7RH, UK
2National Center for Atmospheric Research/High Altitude Observatory, P.O. Box 3000, Boulder, Colorado, 80307-3000, USA
Abstract. It is now a common practice to employ ground-based radars in an attempt to
distinguish between those regions of the Earth's upper atmosphere which are
magnetically conjugate to open and closed magnetic field lines. Radar
returns from ionospheric irregularities inside the polar cap and cusp
regions generally exhibit large spectral widths in contrast to those which
exist on closed field lines at lower latitudes. It has been suggested that
the so-called Spectral Width Boundary (SWB) might act as a proxy for the
open-closed field line boundary (OCFLB), which would then be an invaluable
tool for investigating reconnection rates in the magnetosphere. The exact
cause of the increased spectral widths observed at very high latitudes is
still subject to considerable debate. Several mechanisms have been proposed.
This paper compares a dusk-sector interval of coherent HF radar data with
measurements made by an induction coil magnetometer located at Tromsø,
Norway (66° N geomagnetic). On this occasion, a number of
equatorward excursions of the SWB in the radar backscatter are accompanied
by increases in spectral power of ULF waves in the Pc1-2 frequency band as
the SWB passes overhead. Thus, these observations support the possibility
that high-frequency magnetospheric wave activity at least contribute to the
observed spectral characteristics and that such wave activity might play a
significant role in the nightside ionosphere.
Key words. Ionosphere (auroral ionosphere) – Magnetospheric
physics (MHD waves and instabilities) – Radio science
(ionospheric physics)
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