On the dependence of the phase velocity of equatorial irregularities on the polarization electric field and theoretical implications

Abstract

We have studied the mean Doppler shift of equatorial irregularities in the E region, using data from a coherent radar situated in Ethiopia between 1976 and 1980. We found that even though the primary two‐stream (or type I) waves are drifting at a speed close to the expected ion acoustic speed, there is a small but nonnegligible increase in the phase velocity with increasing electron drift in the electrojet. We attribute this increase to a heating of the electron gas by the large‐amplitude, low‐frequency waves that are present in the region when conditions are favorable to the production of large‐scale gradient drift instabilities. On the other hand, for the single day when the production of large‐scale gradient drift instabilities was inhibited, the behavior of the irregularities was quite different. Namely, the Doppler shift of 10‐m waves (type O waves) was usually below the linear threshold value while the spectra remained narrow and type I‐like rather than type II‐like. On the other hand, the Doppler shift followed a dependence similar to that expected from type II waves. We have tentatively attributed this type O behavior to weak mode coupling, following similar inferences from recent Condor observations made near the top of the electrojet.