Sudden short-duration decreases in cosmic ray flux, known as Forbush decreases (FDs), are mainly caused by interplanetary disturbances. In this investigation, an attempts have been made to associated Forbush decreases associated with intense geomagnetic storms and their relation with drastic solar features and solar wind plasma parameters during the period of 2003-2012.From the data analysis, it is observed that all the Forbush decreases (Fds) are associated with halo and partial halo coronal mass ejections (CMEs), and most of them (85.71%) are higher speed CMEs with speed >1000Km/s. A large positive correlation with a correlation coefficient of 0.72 has been found between the magnitude of Forbush decreases (Fds) and the speed of associated CMEs. Further, all these Forbush decreases (Fds) have been identified as being associated with sudden storms commencements and are related to disturbances in solar wind plasma parameters. Positive correlations with correlation coefficient 0.49 have been determined between magnitudes of Forbush decreases (Fds), and peak values of associated disturbances in solar wind plasma temperature, 0.58 between the magnitude of Forbush decreases (Fds) and peak values of associated disturbances in solar wind plasmavelocity.0.67 and 0.65 between the magnitude of Forbush decreases and peak valuesand magnitude of associated disturbances in solar wind plasma pressure. We have also determined a large positive correlation with correlation coefficient 0.72 between magnitude of Forbush decreases and magnitude of associated intense geomagnetic storms. It is concluded that Forbush decrease are caused by coronal mass ejections and interplanetary disturbances that they generate.
Keywords: – Sudden Storm Commencements (SSC), Forbush decreases (Fds), Solar wind disturbances.
Vijay Shankar Saket1, Ragni Verma2, Chitranshi Tiwari3, P. L. Verma4*