EQUATORIAL PLASMA BUBBLE EVOLUTION AND ITS ROLE IN THE GENERATION OF IRREGULARITIES IN THE LOWER F-REGION

Rocket mensurements from equatorial regions during equatorial spread F (ESF) consistently reveal the presence of plasma density irregulariti es in the lower F region (200-300 km) where the initial conditions inc luding the vertical electron density gradient are not favor-able for t he development of linear instability. An investigation carried out usi ng a nonlinear numerical simulation model and the results obtained fro m the Ionization Hole campaign revealed that the altitude variation of the recombination coefficient and the vertical polarization velocity due to fringe fields associated with the plasma bubble at the base of the F layer are responsible for changing the polarity of the vertical gradient in the plasma density profile in the lower F region, while th e penetration of the fringe fields is mainly responsible for the devel opment of zonal electron density gradient which plays a crucial role i n the secondary plasma instabilities resulting in the generation of ki lometer scale size irregularities in the vertical direction.