Two major Forbush Derease events were recorded –one on 15 May and other on 9 November 2005, of magnitudes 18% and 12% respectively. We have taken the data for cosmic ray intensity from Neutron Monitor stations at Oulu and Mawson,Antartica.In the present paper an analysis of the relation of FDs to the sources and different parameters of the interplanetary medium and geospace is studied. The data corresponding to Sun Spot Numbers, Solar Wind Velocity,Dst index and Ap index have been taken along with the variation in the Cosmic Ray Intensity to find a correlation between CRI and above parameters. Results indicate that there is a strong relationship of FDs with solar activities which further influence Earth environment.The main cause for such large decrease should be intense solar flares followed by coronal mass ejectins.

Proceedings of the 30th International Cosmic Ray Conference; Rogelio Caballero, Juan Carlos D'Olivo, Gustavo Medina-Tanco, Lukas Nellen, Federico A. Sánchez, José F. Valdés-Galicia (eds.); Universidad Nacional Autónoma de México, Mexico City, Mexico, 2008; Vol. 1 (SH), pages 307-310