Nonlinear Rayleigh-Taylor instability in partially ionized plasma and the equatorial spread-F

The nonlinear evolution of the collisional gravitation induced RayleighTaylor (R-T) instability in the equatorialF region is investigated taking into account the finite larmor radius (FLR) effects and the complete ion inertial term in ion equation of motion. A special class of coherent weakly nonlinear modes as solutions to the wave equation describing R-T instability driven modes is obtained. The leading nonlinear effects in the wave equation are found to appear throughV _l, the ion diamagnetic drift which essentially gives the FLR corrections. It is shown that the R-T modes in the equatorialF region can evolve into coherent, nonlinear, almost sinusoidal, stationary wave structures. These structures are found to travel with a constant phase velocity and to have slightly distorted sinusoidal shapes. These results seem to have a good agreement with many of the recent rocket and satellite observations of the equatorial spreadF irregularities.