INTERPLANETARY MAGNETIC-FIELD COLLIMATED COSMIC-RAY FLOW ACROSS MAGNETIC SHOCK FROM INSIDE OF FORBUSH DECREASE, OBSERVED AS LOCAL-TIME-DEPENDENT PRECURSORY DECREASE ON THE GROUND

In the previous papers (Nagashima et al., 1992, 1993) the authors poin ted out the existence of the local-time-dependent precursory, decrease of cosmic rays in front of the shock wave of the interplanetary magne tic field (IMF) and interpreted it as being due to the IMF-collimated outward flow of the low-density cosmic rays across the shock from the inside of the Forbush decrease. In those papers, however, the physical properties of the collimated flow such as the direction, the collimat ion angle, and the rigidity spectrum of the constituent cosmic rays we re only estimated qualitatively owing to the lack of the simulation of the precursory decrease produced by the flow. In the present paper th ese properties are quantitatively obtained by analyzing the hourly dat a of the precursory decreases observed on January 25 and 26, 1968, at the worldwide neutron monitor stations. The obtained direction and col limation angle of the flow approximately coincide respectively with th ose expected from the IMF. This fact gives evidence for the reconfirma tion of the existence of the IMF-guided collimated flow, responsible f or the precursory decrease.