On the high-speed plasma streams, stormtime sudden commencements and cosmic-ray intensity: Relation amongst them during epoch 1978 to 1982
We have examined the list of HSPS given by Lindbladt, Lundstedt, and Larsen (1989). The list of SSC and the cosmic-ray intensity registered at Calgary for the same epoch, 1978 to 1982, has been studied and their time variations, using the well-known Chree method of superposed epoch analysis for various groups of data have been carried out. We have shown results that indicate that SSC is an essential pre-requisite in initiating a cosmic-ray Forbush decrease, whereas the occurrence of HSPS alone does not produce a cosmic-ray decrease. Furthermore, our results also clearly reveal that SSC flares originating in a region to the east of the Sun-Earth line reveal a much larger cosmic-ray decrease than SSC associated with flares originating in a region to the west of the Sun-Earth line. Our results substantiate the study of Sarris, Dodopoulos, and Venkatesan (1989), determined for IMP 7 and 8 data (E≥ 35 MeV) for the higher neutron monitor energies. Sarris, Dodopoulos, and Venkatesan have observed that a cosmic-ray intensity decrease follows the group of SSC with flares on the east side of the Sun-spacecraft line, while the group with flares on the west side of the Sun-spacecraft line reveals little or no decrease in cosmic-ray intensity. In particular, the zero epochs for the two groups depend on the hours of onset of SSC. The group with flares on the east side (which are essentially associated with quasi-perpendicular shocks) are followed by cosmic-ray decreases while the group associated with flares on the west of the Sun-spacecraft line (which are usually associated with quasi-parallel shocks) do not reveal any appreciable decrease in cosmic-ray intensity.