We present numerical simulations of the large-scale electron density irregularities in the daytime equatorial electrojet driven by the gradient drift instability. The nonlocal nonlinear two-fluid equations are integrated numerically with scales ranging from about 10 km to less than 100 m directly resolved on a 128¿128 grid, while the effects of the smaller subgrid scales are included with the use of anomalous electron mobility and diffusion coefficients [Ronchi et al., 1990a>. The instability evolves to a state in which the perturbations propagate primarily in the east-west direction with a typical horizontal wavelength of about 2 km. The output of the numerical simulations does not indicate the presence of a significant anisotropy in the power spectrum of the irregularities in the plane perpendicular to the ambient magnetic field, in contrast to the marked differences observed in physical space between the vertical and horizontal dynamics. The one-dimensional integrated density and electric field power spectra have a power law dependence with a power index ranging between -2.5 and -1.2. The numerical results are compared with in situ rocket observations by probing the simulation region along different flight paths, following both eastward and westward trajectories. Electron vertical turbulent velocity distributions are computed from the code output and are contrasted with radar backscatter data.

The features typical of the 3-m type 2 echoes (such as the broadening and asymmetry in the frequency power spectra) are also present in the computed distributions, indicating that during weak electrojet conditions the small-scale structures act as tracers of the large-scale electric field variations. A conclusion of particular note is that a purely linear nonlocal analysis (valid for wavelengths &lgr;≈1 km) leads to the result that all perturbations are eventually damped, either by shear and then diffusion or by recombination. The inclusion of nonlinear effects, however, restores the instability. In the strongly turbulent regime a nonsteady saturated state is reached, whereby the linear convection of energy via shear to high wavenumbers is countered by the nonlinear modification of the equilibrium density and electric field gradients and by mode coupling of shorter wavelengths back to long. ¿ American Geophysical Union 1991