A NUMERICAL STUDY OF THE WIND-FIELD EFFECT ON THE GROWTH AND OBSERVABILITY OF EQUATORIAL SPREAD F

The neutral wind field effects on the development of the equatorial pl asma bubbles have been simulated by a two-dimensional time-dependent m odel similar to that developed by Zalessak and Ossakow. The results in dicate that when there exists no neutral wind, any perturbation on the bottom-side of the ionosphere density profile will be amplified by th e gravitational Rayleigh-Taylor (GRT) instability and an upwelling bub ble will form as expected. When a zonal neutral wind field exists, the uplift velocity of the bubble will be enhanced by a uniform neutral w ind, but suppressed by vertical shear of the wind field. Secondary str uctures called plumes will grow out from the side walls of the primary bubble if some secondary perturbation signal with vertical structure is seeded. We notice that patches and multiple plume structures have b een observed in the mid-latitude and low-latitude ionosphere. So we ha ve developed a two-dimensional local theory of the generalized GRT ins tability to calculate the growth rate of a small-scale perturbation si gnal in the vicinity of the primary bubble and found that the growth r ate increases with time along with the growth of the bubble. It is pro ved that the irregularities generated by some of our simulations shoul d be able to cascade into observable (e.g., by radar) turbulence: in a reasonable period of time after their generation Similar to the effec t on the growth of the primary bubble, a strong uniform wind field may enhance the growth rate of a perturbation signal in the vicinity of t he bubble, while a strong vertical wind shear will suppress the growth rate. The main point of this theory is that the growth rate of the GR T instability is controlled by the density gradients and the magnitude s of the velocity of the plasma motion, relative to the neutral wind i n both directions. In addition, our simulations have proved that a see ding wave with phase velocity matching the background wind speed will generate the fastest growing bubble, disapproving the theory of spatia l resonance.