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Ann. Geophys., 23, 3323-3337, 2005
www.ann-geophys.net/23/3323/2005/
© European Geosciences Union 2005
Is there a plasma density gradient role on the generation of short-scale Farley-Buneman waves?
C. Haldoupis1, T. Ogawa2, K. Schlegel3, J. A. Koehler4, and T. Ono5
1Physics Department, University of Crete, Iraklion, 71003 Crete, Greece
2Solar-Terrestrial Environment Laboratory, Nagoya University, Toyokawa, Aichi 442-8507, Japan
3Max-Planck Institut für Sonnensystemforschung, 37191 Katlenburg-Lindau, Germany
4Department of Physics and Engineering Physics, University of Saskatchewan, Saskatoon, SK, S7N-5E2, Canada
5Department of Geophysics, Tohoku University, Sendai, Japan
Abstract. The physics of the unstable E-region plasma is based on the
modified two stream, or Farley-Buneman, and the gradient drift
instabilities. The theory combines both mechanisms into a single
dispersion relation which applies for the directly generated
short-scale plasma waves, known as type 1 irregularities. In the absence
of a plasma gradient it is only the two stream mechanism acting
which favors wave excitation if E×B electron
drifts relative to the ions exceed a threshold slightly above the
ion acoustic speed. On the other hand, the theory also predicts
that a destabilizing (stabilizing) electron density gradient acts
to decrease (increase) the ion acoustic threshold, and hence the wave
phase velocities at threshold, depending on the
gradient strength and the wavelength. Given a destabilizing plasma
gradient, the threshold reduction is larger at longer than shorter
wavelengths and thus the best way to test the gradient role is by
simultaneous observations of type 1 waves at two or more radio
backscatter frequencies. The present paper relies on dual
frequency backscatter observations of 1.1 m and 3.1 m type 1
irregularities made simultaneously at 144 MHz and 50 MHz,
respectively, in mid-latitude sporadic E-layers. Using as typical
plasma gradient scale lengths for destabilized sporadic E-layers
those that are obtained from rocket electron density profiles, the radar
observations are compared with the predictions of kinetic theory.
The results suggest that the plasma density gradient effect on
meter scale Farley-Buneman waves is not important. This is
reinforced further by the analysis of backscatter from
destabilized meteor trail plasma when very steep gradients are
expected in electron density. The present findings, and more from
past studies, question the electron density gradient role in the
generation of short-scale plasma waves as predicted by the linear
instability theory. This deserves attention and more study.
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