Unified theory of type I and II irregularities in the equatorial electrojet

Turbulent structure of the electrojet is discussed in detail, and a unified interpretation of the electrojet irregularities (type I and type II) is given in terms of nonlinear two-stream and cross field instabilities. It is emphasized that small-scale type II irregularities (3∼15 meters) can be interpreted as resulting from nonlinear decay (collapse) of large-scale irregularities (∼150 meters), which are directly excited by the cross field instability. It is shown that polarization fields associated with the large irregularities modify the straight (horizontal) electron flow substantially; electrons are constrained to make periodic excursions vertically upward and downward in accord with the period of the irregularities. Consequently, the type I irregularities arising from the two-stream instability are excited obliquely and vertically, as well as horizontally. It is also shown that the observed constant phase velocity of the type I irregularities can be explained in terms of the quasilinear stabilization. In addition, it is emphasized that one should not regard the mean Doppler velocity of the type II (3-meter) echoes Vph3II as representing the electron drift velocity when type I echoes coexist, i.e., when Vph3II > Cs, where Cs is the ion sound velocity.