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 theory. 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 consistent with observed daytime electric and current density profiles are given and analyzed for nonlocal stability. Instabilities with horizontal wavelengths of the order of 1 km are found to be dominant over most of the electrojet, with instabilities of shorter wavelengths (of the order of 100 m) localized in regions of weak velocity shear. The predictions of linear theory are in accord with the observed preponderance of kilometer-scale irregularities in the daytime electrojet.