中低纬Spread-F的数值模拟

本文导出了中低纬地区电离层F区中的Rayleigh-Taylor不稳定性的线性增长率,并对一些典型情况进行了数值模拟研究.结果表明,Ravleigh-Taylor不稳定性的线性增长率随纬度的增加而降低,在中纬度地区,如果没有合适的东西向水平外电场,纯Rayleiyh-Taylor模难于增长,初始扰动将被稳定化.在这些区域,线性增长率的表达式还表明,外电场导致的E×B模的增长率虽然随纬度的增加而减小,但其减小的速率慢于R-T模增长率的减小.非线性的数值模拟则显示,中纬非线性E×B漂移不能象在赤道地区那样使电子密度减小的区域抬升到F区顶部以上,亦即在中纬地区,F顶部以上通常不出现电离层泡,但在F区较低部分可以演化出较大范围的电子密度增强区,这与中低纬地区Spread-F的某些观测特征一致.这说明Rayleigh-Taylor不稳定性与东向水平外电场一起可以成为导致中纬Spread-F发展的一种机制.

NUMERICAL SIMULATION OF SPREAD-F IN LOW AND MID-LATITUDES

In this paper, a linear growth rate for the collisional Rayleigh-Taylor instability in the ionospheric F region of low and mid-latitudes is derived and numerical simulations for certain typical cases conducted. The results show that the growth rate becomes smaller as the latitude increases. In mid-latitudes, it is hard for a pure R-T mode to grow without a proper external horizontal electric field East-Westly directed, in hose latitudes, initial disturbances will soon be stabilized. It is also shown that although the growth rate of E×B mode driven by the external electric field becomes smaller,, too, as latitude increases, the actual change rate is less than that of the R-T mode. The numerical simulation with non-linear effects included gives out the results that non-linear E×B drift can not make the initial density depletion （bubbles） rising above the F peak in mid-latitude zones, instead, in the lower part of F region, a larger area of electron enhancement can evolve. This is in agree nent with some observational characteristics of the mid-latitude spread-F. It is thought that the collisional R-T instability together with a proper East-West electric field is probably one of the mechanism leading to the development of spread-F in mid-latitudes.