A Study of Relation Between Cosmic Ray Intensity and Occurrence of Forbush Decreases During Solar Cycle 24

Forbush decreases (Fd) are transient, short-term reductions in the intensity of galactic cosmic rays that reach the Earth’s surface. Coronal mass ejections (CMEs) hurl huge volumes of magnetized plasma into interplanetary space often referred to as ICMEs or ejecta. They are an important component of solar wind and can cause enhanced geomagnetic activity when they interact with the Earth’s magnetosphere. When the eject have an average speed greater than the upstream solar wind speed they create a shock. The large IMF variations due to interplanetary shocks cause depression in the cosmic ray intensity (CRI) called Forbush Decrease (FD). Large Fds caused by fast CMEs are specifically associated with energetic X-ray flares. In the present paper, the author has studied seven largest Forbush decrease events selected from Moscow Neutron Monitor Station during a period of 1986-2015, i.e., 25th solar cycle. The analysis of CRI data with interplanetary magnetic field, its southward component Bz, solar wind velocity, Kp and Dst indices shows that all the three phenomena- solar, interplanetary and geomagnetic are connected to FD. The relationship between interplanetary parameters and FDs is discussed in detail. Moreover the solar cycle effect is found to be slightly shifted for large FDs as the frequency of occurrence of major FD events is more in the descending phase of the solar cycle. In the present study, we have studied the relation between Forbush decrease and solar activity. Sunspot numbers proved a reliable and easily available solar parameter, particularly in cosmic ray modulation. To carry out study, data was taken from Neutron Monitors. Number of Forbush decrease events along with sunspot number Rz was plotted. The overall results are encouraging that shows a prominent relationship between occurrence of Forbush decrease events and 11 year sunspot solar activity cycles.