HF radar studies of two-stream instability during an equatorial counter-electrojet


  • M. Crochet,

  • C. Hanuise,

  • P. Broche


A very strong daytime counter-electrojet was observed on January 21, 1977, near the magnetic equator in Africa. This phenomenon was investigated for the first time using simultaneous magnetic and radar measurements. As predicted by the linear theory, irregularities associated with the two-stream instability were detected in the absence of the usual turbulence related to the cross-field effect (or type 2 irregularities). In addition, taking advantage of the refraction of the HF radio waves in the E layer, horizontal or quasi-horizontal plasma waves were detected for the first time together with the usual oblique waves. Two kinds of spectra were then distinguished from the radar observations: (1) Spectra associated with oblique waves which do not have the usual behavior of type 1 (their phase velocity is dependent on time and angle of incidence) and (2) type H spectra associated with horizontal or quasi-horizontal waves which have a smaller phase velocity (constant with time and range) and a larger amplitude than oblique waves. Among different hypotheses we presume that the quasi-linear reduction of the polarization electric field at the bottom of the instability layer explains the constant value of the phase velocity of the horizontal waves. The absence of turbulence (or type 2) would be the source of the difference observed beween oblique waves and usual type 1.