A thesis submitted to the University of Mumbai for the Ph.D.(Science) degree in physics under the guidance of Dr. Geeta VichareThe thesis concludes with the following list of major conclusions drawn from the entire study. • The effect of IMF Bz turnings observed in the equatorial geomagnetic field vari ations indicates that the magnitude of northward Bz does not have the influence on the equatorial ionosphere whereas, the response signatures are mainly con trolled by the magnitudes of southward Bz during both northward and southward turnings. • The equivalent current vectors reveal a clockwise (anticlockwise) ionospheric cur rent loop in the afternoon (morning) sector during the main impulse (MI) of the negative pressure impulse (SI−). This indicates an ionospheric twin-cell-vortex current system (DP2) due to field-aligned currents (FACs) associated with the dusk-to-dawn convection electric field during the MI of the SI−. • The stochastic variations of EEJ and Sq are in a cross-talk with a net flow of information from EEJ to Sq station for data having time resolutions between 30-120 min. This suggests that the variations in the magnetic field of EEJ are reflected in Sq, indicating EEJ and Sq are coupled systems. • The physical processes responsible for the geomagnetic storms and Forbush de creases are different, even though they originate due to common interplanetary disturbance like ICMEs and CIRs. For geomagnetic storms, reconnection is the major process whereas, FDs are mainly caused by inhibited diffusion of cosmic rays into the ICMEs due to enhanced IMF and Vsw. It is observed that expan sion of ICME contributes in early recovery phase whereas the gradual variation of solar wind speed beyond ICME boundaries is responsible for the long duration of FD recovery. • The developed artificial neural networks are capable in forecasting global SYMH, ASYH indices and locally observed geomagnetic field variations in Indian sector about an hour before, provided the real-time upstream solar wind data is available.