PLASMA TURBULENCE IN THE EQUATORIAL ELECTROJET

Plasma turbulence in the daytime and nighttime equatorial electrojet is studied with a highly sophisticated radar interferometer technique. It is shown that the outer scale of the plasma turbulence scales with the zero order plasma density gradient length, and is smaller during the day because of increased recombinational damping. Observations indicate that the horizontally propagating coherent waves at the outer scale dominate the electrojet turbulence and give rise to vertically propagating type 1 waves during strong electrojet conditions. According to the linear theory extended to the long wavelength regime the large scale primary modes are dispersive and have phase velocities considerably smaller than the mean driving electron velocity, in agreement with the interferometer observations. Vertical electron transport, a quasi-linear effect due to large scale wave action, is shown to give rise to a vertical dc current which has the right direction and magnitude to explain the up-down and possibly the east-west asymmetries observed at Jicamarca. These quasi-linear considerations also show that the first order perturbed vertical electron velocity associated with the primary mode is limited to a maximum value on the order of the mean horizontal electron velocity, which might explain why vertically propagating type 1 waves are only observed during strong electrojet conditions. Finally, although narrow spectra observed at the bottomside of the electrojet can only be accounted for by nonlinear mode coupling, it is shown that the usual type 2 spectral widths of 50 MHz radar returns cannot be explained by the nonlinear damping rates, but are indicative of the largest excursions of the local electron velocities from the mean value inside the radar scattering volume. Radar interferometer data strongly support this observation.