A spatially localized gravity driven instability is proposed to be the likely candidate for the observed kilometer scale-size density irregularities in the bottomside equatorial F region. It is noted that the classical Rayleigh-Taylor instability, which relies on the frozen-in condition (implying E ⋅ B=0) and is of the flute-type (k ⋅ B=0), is not campatible with the experimental observations of density perturbations that have a finite extent along the magnetic field lines, while dissipative localized modes, which allow the plasma to move through the magnetic field as a result of finite electrical resistivity effects are consistent with such experimental findings. These modes have three-dimensional structures with wavelengths in the east-west direction (&lgr;ϕ) of the order of a few kilometers.