5–50‐m wavelength plasma instabilities in the equatorial electrojet 1. Cross‐field conditions

Abstract

Electron density irregularities detected with coherent radars in the equatorial electrojet have been studied mostly at wavelengths between 3 and 10 m. For low drift velocities they have been interpreted by the cross‐field instability. Longer wavelengths have been extensively observed with a multifrequency HF radar system installed in Ethiopia. Data gathered for drift velocities lower than 200 m/s are presented. From 5 to 50 m the measured phase velocity varies with wavelength and elevation angle as predicted by the linear theory. Spectral width increases with wave number but is constant with elevation angle. Ratio of spectral width to mean Doppler shift is of the order of unity and indicates that strong turbulence has to be included in nonlinear theories. These results are studied in light of recent numerical simulations and theoretical works.