LINEAR GRADIENT DRIFT INSTABILITIES IN THE DAYTIME EQUATORIAL ELECTROJET

Linear gradient drift instabilities, which are associated with type II irregularities in the equatorial electrojet, are obtained for daytime profiles using initial-value and eigenmode methods in a nonlocal theo ry. The methods are shown to be in agreement, thus resolving a recent discrepancy between the theories of Ronchi et al. [1989] and Wang and Bhattacharjee [1994]. A set of equilibrium profiles that are consisten t with observed daytime electric and current density profiles are give n and analyzed for nonlocal stability. Instabilities with horizontal w avelengths of the order of 1 km are found to be dominant over most of the electrojet, with instabilities of shorter wavelengths (of the orde r of 100 m) localized in regions of weak velocity shear. The predictio ns of linear theory are in accord with the observed preponderance of k ilometer-scale irregularities in the daytime electrojet.