E. Alam Kherani

The Acoustic Gravity Wave Induced Disturbances in the Equatorial Ionosphere

The role of acoustic gravity waves (AGWs) to excite atmospheric and ionospheric disturbances is examined in this work. These waves are launched in the atmosphere by tropospheric thermal sources and convective activity. An alternative fully time-spatial dependent nonlinear wave equation of acoustic gravity wave is derived and solved numerically using implicit finite-difference scheme. Their propagation in the atmosphere through mesopause thermal duct and lower thermosphere density duct, the role of nonlinear viscous effect to limit the amplitude of these waves in the density duct and to allow them to escape to higher altitude where they attain large amplitude in the bottomside F region Ionosphere, and the role of the mean zonal wind to reduce their amplitude are investigated in present study. To study AGW induced disturbances in the equatorial Ionosphere, the AGW model is coupled with hydromagnetic equations in Ionosphere. This coupling is explored in the context of the collisional interchange instability (CII) in the F region leading to the formation of equatorial F region plasma bubbles. To do so, AGW model is coupled with the CII model and simultaneously solved numerically. The possible role of the AGW to act as a seeding perturbation for equatorial plasma bubbles under varying nature of mean zonal wind and tropospheric thermal source are also investigated.

Coupling Processes in the Equatorial Spread F/Plasma Bubble Irregularity Development

The plasma convection pattern of the evening sector equatorial ionosphere sets the condition for the plasma structuring through instability processes leading to the Equatorial Spread F (ESF)/plasma bubble irregularity development and evolution. Vertical coupling through upward propagating atmospheric waves controls/modifies the ionosphere-thermosphere interactive processes that eventually lead to the irregularity development. Instabilities grow by the Rayleigh-Taylor mechanism at the bottom side gradient region of a “rapidly” rising post sunset F layer in the presence of precursor conditions in terms of perturbations in plasma density, convection velocity and polarization electric fields. Field line integrated conductivity controlled by thermospheric meridional/trans-equatorial winds regulates the instability growth. The day-to-day and short term variabilities in the ESF are of major concern for space application and operational systems. Our efforts to understand such variabilities and to predict the ESF occurrence pose important scientific challenges especially because of the complexity of the diverse coupling processes that control them. There is convincing new evidences that during magnetically quiet conditions, the coupling processes due to upward propagating planetary waves and/or modulated tides, and gravity waves, with their highly variable propagation conditions, energy fluxes and periodicities control the ESF variability. Penetrating electric fields and disturbance dynamo electric fields from magnetosphere-ionosphere coupling processes also cause large degree of variability. This chapter provides a review of our current understanding of the ESF development processes and its day-to-day variability originating from the different coupling processes mentioned above.