A strong Forbush Decrease (FD) was observed by ground based neutron monitor at Oulu in the mid of May 2005. The onset of FD took place on May 13 and attained its maximum on May 15, 2005. The event was in response to 221 X-ray flares out of which 13 were of M class and they were followed by coronal mass ejection (CME). This has caused a rapid decrease in galactic cosmic ray intensity called Forbush Decrease. In the present paper the effect of various parameters such as sun spot numbers, solar wind velocity and the geomagnetic index Dst on the FD has been studied. The results clearly indicate a strong relationship between geomagnetic activity and FD on short term basis. The period under investigation is found to be full of large solar activities inspite of the declining phase of solar cycle 23.

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 295-298