NON-LINEAR EQUATORIAL SPREAD-F - SPATIALLY LARGE BUBBLES RESULTING FROM LARGE HORIZONTAL SCALE INITIAL PERTURBATIONS

Motivated by the observation of large horizontal scale length equatorial spread F 'bubbles', numerical simulations were performed of the nonlinear evolution of the collisional Rayleigh-Taylor instability in the highttime equatorial ionosphere, using large horizontal scale length initial perturbations. It is found that large horizontal scale initial perturbations evolve nonlinearly into equally large horizontal scale spread F bubbles, on a time scale as fast as that of the corresponding small horizontal scalelength perturbations previously used. Further, the level of plasma depletion inside the large-scale bubbles is appreciably higher than that of the smaller-scale bubbles, approaching 100%, in substantial agreement with the observations. This level of depletion is due to the fact that the plasma comprising the large-scale bubbles has its origin at much lower altitudes than that comprising the smaller scale bubbles. Analysis of the polarization electric fields produced by the vertically aligned ionospheric irregularities show this effect to be due to fringe fields.