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Ann. Geophys., 23, 767-772, 2005
www.ann-geophys.net/23/767/2005/
© European Geosciences Union 2005
A possible origin for large aspect angle "HAIR'' echoes seen by SuperDARN radars in the E region
J. Drexler1,2 and J.-P. St.-Maurice1
1Institute of Space and Atmospheric Studies, The University of Saskatchewan, Saskatoon, Saskatchewan, Canada
2now at: Department of Earth and Atmospheric Sciences, Cornell University, Ithaca, New York, USA
Abstract. Milan2004 have recently reported on close-range E region
decameter size echoes that seem to be relatively weak, have
apparently unusually large aspect angles, and possess Doppler
shifts that are slow and are clearly consistent with the ion drift
of the medium as opposed to, say, its electron drift or its
ion-acoustic speed. We argue that these irregularities are the
result of a nonlinear wave conversion process triggered by the
nonlocal evolution of decameter Farley-Buneman waves. According to
this picture, structures which have weak spontaneous growth rates
and are initially field-aligned undergo an evolution of their aspect
angle that results in a jump in the aspect
angle at some point in time and space. When this takes place, a
rapid mode conversion must follow, which takes energy away from a
standard two-stream signature and converts it either to a strongly
damped ion-acoustic mode or to a purely decaying mode, depending
on altitude.
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