We have studied Forbush decreases magnitude ≥5% observed at Oulu super Neutron Monitor with geomagnetic storms and coronal mass ejections during the period of solar cycle 23 .We have determined that all the Forbush decreases are found to be associated with geomagnetic storms and a large positive correlation with correlation coefficient 0.63 has been found between magnitude of Forbush decreases and magnitude of associated geomagnetic storms .Further the main phase duration and recovery duration of Fds are found to be unrelated to the main phase and recovery duration of associated geomagnetic storms .From the study of Forbush decreases and coronal mass ejections, it is seen that the most of the Forbush decreases (89.13%) are found to be associated with halo and partial halo coronal mass ejections (CME) .The association rates of H type and P type CMEs are 92.68% and 7.31% respectively. Positive correlation with correlation coefficient 0.43 has been found between magnitude of Forbush decreases and speed of associated CME.
Keyword: -Forbush decreases. Geomagnetic storms and Coronal Mass Ejections.
Ragni Tamrakar1, P. L. Verma2*, Anshu Gupta3