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Annales Geophysicae An interactive open-access journal of the European Geosciences Union
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Volume 23, issue 10
Ann. Geophys., 23, 3323–3337, 2005
https://doi.org/10.5194/angeo-23-3323-2005
© Author(s) 2005. This work is distributed under
the Creative Commons Attribution 3.0 License.
Ann. Geophys., 23, 3323–3337, 2005
https://doi.org/10.5194/angeo-23-3323-2005
© Author(s) 2005. This work is distributed under
the Creative Commons Attribution 3.0 License.

  30 Nov 2005

30 Nov 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 C. Haldoupis et al.
  • 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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