Radar observation of kinetic effects at meter scales for Farley-Buneman plasma waves

[1] Coherent backscatter Doppler measurements, made simultaneously at 144 MHz and 50 MHz from a common volume in the midlatitude E region ionosphere, were analyzed in order to study the phase velocity ratio of type 1 plasma irregularities at 1 m and 3 m wavelengths. In the analysis, high- resolution Doppler spectrograms were used first to identify the type 1 events and then to estimate the mean and spectral peak velocities from averaged power Doppler spectra. The simultaneous spectrogram signatures of type 1 echoes suggested a somewhat higher threshold for instability excitation at 144 MHz than at 50 MHz. Statistically, the measured 144 MHz to 50 MHz velocity ratios attain values above unity, mostly in the range from 1. 05 to 1. 14 with an overall average of 1.10. This 10% difference in the type 1 velocities at 144 MHz and 50 MHz was attributed to kinetic effects at short plasma wavelengths. For comparison, a linear kinetic model of the Farley-Buneman instability, which includes also a destabilizing plasma density gradient, was used to provide numerical estimates of type 1 phase velocities. It was found that the theoretical predictions for gradient- free Farley- Buneman waves agreed well with the observations, under the suppositions that the strongest type 1 echoes come from E region altitudes where conditions for instability are optimal and that type 1 waves have their phase velocities limited at threshold values equal to the plasma ion acoustic speed. The present study has confirmed the accuracy of the kinetic theory of the Farley- Buneman instability, which strengthens its validity and suitability for meter- scale E region irregularity studies.