COMPARISON OF TOPSIDE AND BOTTOMSIDE IRREGULARITIES IN EQUATORIAL F-REGION IONOSPHERE

Numerical simulations have been carried out to study and compare the m echanisms of the formations of the topside and bottomside ionospheric irregularities. As is well-known, the topside ionospheric irregulariti es have always originated from the bottomside ionosphere due to Gravit ational Rayleigh-Taylor (GRT) instability. From the penetration proces s we found that the primary bubble with a horizontal scale of tens of kilometers always has a smooth shape in the bottomside ionosphere and breaks into much smaller-scale structures after penetrating into deep topside ionosphere. Since the plasma bubble is unlikely to break into irregularities in the bottomside ionosphere, the bottomside irregulari ties probably result from the turbulence in the neutral atmosphere. Ex amples of simulations are presented to demonstrate that bottomside irr egularities may exist in a region with a strong neutral wind jet and s eeding gravity wave motion. A strong neutral jet stream always has a s trong vertical wind shear, which provides a broad height range of dyna mic instability. When the atmospheric gravity wave approaches its crit ical level, its amplitude will continually grow to break into smaller- scale waves and turbulence. Then all the waves of different scales as well as the turbulence will become the seeds of the bottomside irregul arities.