Localized plasma depletion in the ionosphere and the equatorial spread F

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 =O) and is of the flute‐type ( k · B =O), is not compatible 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 (λ ϕ ) of the order of a few kilometers.