Space weather application of forbush decrease events

Abstract: Forbush decrease (FD) is sudden decrease in the counting rates of neutron monitors, caused by fast Coronal Mass Ejections (CMEs) mostly associated with intense X-ray solar flares. The geomagnetic storm is known to occur simultaneously with large Forbush decrease events. Both phenomena have common origin; therefore, study of FD can be used to anticipate an impending storm. We have analyzed 61 FD events of magnitude> = 3.5%, registered by Moscow Neutron Monitoring Station, occurred during the 23rd solar cycle. Temporal evolution of individual FD event is studied along with the variation in interplanetary magnetic field intensity, solar wind velocity and geomagnetic activity indices- Dst index. It is found that besides the known enhanced disturbances in the magnetic field of the Earth, the FDs in majority of events are accompanied by abrupt increase in the solar wind velocity. Though the correlation coefficient between the magnitude of FD and Dst index is 0.3, but 91 % of FD events (55 out of 61) are associated with geomagnetic storms. The onset time of FD is found to be few hours earlier than that of the storm; this fact can be used to forecast a geomagnetic storm which is the known space weather application of FD event