Abstract
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.
Highlights
Numerous radar studies on equatorial, auroral, and midlatitude E-region plasma irregularities have led to the conviction that the operating mechanisms that destabilize the plasma are the modified two stream, or Farley-Buneman (FB), and the gradient-drift (GD) instabilities
The aim of the present study was to investigate if there exists a plasma density gradient (GD) effect on short-scale FarleyBuneman (FB) plasma waves, in accord with the linear theory of E-region plasma instabilities
Since the theory predicts that GD effects are more severe at longer than shorter wavelengths, we tested the GD role by using simultaneous VHF backscatter observations of 1-m and 3-m type 1 waves from destabilized sporadic E layers, when steep plasma density gradients are expected to be present
Summary
Numerous radar studies on equatorial, auroral, and midlatitude E-region plasma irregularities have led to the conviction that the operating mechanisms that destabilize the plasma are the modified two stream, or Farley-Buneman (FB), and the gradient-drift (GD) instabilities (e.g. see reviews by Fejer and Kelley, 1980; Farley, 1985; Riggin et al, 1987; Haldoupis, 1989; Sahr and Fejer, 1996; Moorcroft, 2002; and references cited therein). The radar experiments, which cover the HF, VHF and UHF (high, very high, and ultra high) frequency bands and observe only short-scale irregularities with wavelengths ranging from decameters to fractions of a meter, have identified a different role for each of the two instability mechanisms: The FB instability is considered responsible for the direct excitation of the so-called type 1 irregularities, i.e. short-scale plasma waves growing spontaneously when E×B electron drifts exceed a threshold that is slightly above the ion-acoustic speed. This becomes possible if steep plasma gradients exist along the ambient electric field, even when electron drifts are below the ion acoustic velocity threshold required under gradientfree conditions.
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