Indian Journal of Radio & Space Physics Vol. 37, August 2008, pp. 237-243 Heliographic distribution of bright solar flares and association of Forbush-decreases with flares and coronal mass ejections A P Mishra*, B N Mishra, Meera Gupta & V K Mishra Department of Physics, A P S University, Rewa (MP) 486 003, India *E-mail: apm_apsu@yahoo.co.in Received 17 April 2007; revised 29 January 2008; accepted 22 April 2008 Major solar flare events have been selected to study the heliographic distribution of solar flares during solar cycle 23. The occurrence of Forbush decreases (FDs), bright solar flares; solar flare index, coronal mass ejections (CMEs), average solar magnetic field and solar wind velocity have been studied for the period 1996-2006. It is found that the solar flare index in northern and southern hemisphere represents the cumulative activity of solar flare in both hemispheres. Though, the bright solar flares are equally distributed in the entire solar region, majority of the bright solar flares responsible for FDs ( 4% magnitude) are located in the north-east region of the solar disk. Out of 41 FDs, 88% are found to be associated with halo (central position angle  360°) and partial halo (central position angles  120°) CMEs (coronal mass ejections) and 55% with bright solar flares of importance  1B. The abrupt increase in average solar magnetic field and solar wind velocity has also been found to be a necessary condition for producing FDs. The occurrence of non-recurrent type FDs are more frequent than recurrent type FDs. In latitudinal distribution, northern hemisphere is more dominant than the southern hemisphere in producing FDs. It is also found that eastern hemisphere is more effective as compared to western hemisphere to produce FDs in longitudinal distribution during the aforesaid period. Keywords: Solar flares, Coronal mass ejections, Forbush decreases, Solar cycle PACS No.: 96.60.Qe; 96.60.Ph 1 Introduction A solar flare originated from the solar atmosphere is referred to as a major solar event and an impulsive phenomenon that releases vast amount of matter and energy in a short interval. Flares also often occur in association with CMEs (coronal mass ejections) but they are not assumed to be instigator of mass ejections 1 . Flares are believed to be generated by the heating results from reconnection of field lines. Usually the solar flares associated with CMEs are of long duration and also associated with meter wavelength type II and particularly, type IV radio bursts 2 . Large Forbush decreases (FDs) are caused by fast CMEs, which may also be associated with specific solar flares. The FDs are frequently recorded near solar maximum but it may occur throughout the solar cycle. A number of investigators have studied the distribution of solar flares around the sun and some reported the combined effect of bright solar flares and CMEs responsible for the short-term decreases in galactic cosmic ray intensity 3-6 . The distribution of flares on the solar disk shows strong north-south asymmetry for all classes of flares 7 . Recently, the northern hemisphere has been reported to be dominant in general during the rising phase of cycle 23. The dominance of northern hemisphere shifted towards the southern hemisphere after the solar maximum 8 . During CMEs, a large amount of matter propels out from the sun's outer atmosphere. These episodic expulsions of mass and magnetic fields from the solar corona into the interplanetary medium may have masses of the order of 10 15 g and may liberate energies 9 up to 10 30 -10 32 ergs. The CMEs are the result of a large-scale rearrangement of solar magnetic field and they are often observed as an eruption of twisted magnetic field from the solar atmosphere 10-11 . Out of various types of CMEs, the halo CMEs are more likely to impact the Earth and incident at right angle to Earth-Sun line. The FDs in cosmic ray intensity are often associated with CMEs and interplanetary CMEs (ICMEs) 12-14 . The FDs are observed as transient depressions in the galactic cosmic ray intensity, characterized by a sudden onset, reaching a minimum within about a day and followed by a more gradual recovery phase typically lasting up to several days.