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Ann. Geophys., 24, 475-491, 2006
www.ann-geophys.net/24/475/2006/
© European Geosciences Union 2006
Infrasound - the cause of strong Polar Mesosphere Winter Echoes?
S. Kirkwood1, P. Chilson2, E. Belova1, P. Dalin1, I. Häggström3, M. Rietveld4, and W. Singer5
1Swedish Institute of Space Physics, Box 812, 98128 Kiruna, Sweden
2School of Meteorology, University of Oklahoma, Norman, OK, USA
3EISCAT Scientific Association, Kiruna, Sweden
4EISCAT Scientific Association, Ramfjordmoen, Norway
5Leibniz Institute of Atmospheric Physics, Kühlungsborn, Germany
Abstract. The ESRAD 52-MHz and the EISCAT 224-MHz radars in northern Scandinavia
observed thin layers of strongly enhanced radar echoes from the mesosphere
(Polar Mesosphere Winter Echoes - PMWE) during a solar proton event in
November 2004. Using the interferometric capabilities of ESRAD it was found
that the scatterers responsible for PMWE show very high horizontal travel
speeds, up to 500 ms-1 or more, and high aspect sensitivity, with echo
arrival angles spread over as little as 0.3°. ESRAD also detected, on
some occasions, discrete scattering regions moving across the field of view
with periodicities of a few seconds. The very narrow, vertically directed
beam of the more powerful EISCAT radar allowed measurements of the spectral
widths of the radar echoes both inside the PMWE and from the background
plasma above and below the PMWE. Spectral widths inside the PMWE were found
to be indistinguishable from those from the background plasma. We propose
that scatter from highly-damped ion-acoustic waves generated by partial
reflection of infrasonic waves provides a reasonable explanation of the
characteristics of the very strong PMWE reported here.
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