Secondary irregularities in the equatorial electrojet

Although the two-stream instability theory, discussed by Farley, has been quite successful in accounting for many observed features of the plasma-wave irregularities produced in the equatorial electrojet, increased equipmental sensitivity and improved experimental techniques of observing the power spectrum of radio echoes from the electrojet have resulted in the resolution of weaker irregularities not predicted by that theory. In particular, such irregularities are observed both when a VHF radar located near the magnetic equator is directed vertically and when the radar is directed obliquely in the equatorial plane. In the latter case, the irregularities have lower Doppler shifts than are suggested by the theory. Experimental spectra are presented that demonstrate features consistent with the first-order two-stream theory for observations of a well-developed electrojet at oblique angles of incidence. However, the relative contribution of the weaker irregularities can be resolved in spectra for incidence angles approaching the zenith or, when the electrojet is comparatively weak, at most angles. The spectral width obtained at vertical incidence is shown to be narrow for a weak electrojet and to be broadened as the electrojet becomes stronger. The weaker irregularities probably result from the nonlinear interaction among the stronger irregularities, as is demonstrated by Dougherty and Farley in a companion paper. Experimental results are presented that indicate an asymmetry not predicted by either theory; i.e. the upward irregularities (observed at angles of incidence west of the zenith) are stronger than the downward irregularities (observed at angles of incidence east of the zenith). This asymmetry may be accounted for by the variation with height of the physical properties of the region of instability.