Local-Time-Dependent Pre-IMF-Shock Decrease of Cosmic Rays, Produced by Their IMF-Collimated Outward Flow across the Shock from the Inside of Forbush Decrease

In the previous paper (NAGASHIMA et al., 1992), the authors pointed out the existence of the local-time-dependent precursory decrease of cosmic rays in front of the shock wave of the interplanetary magnetic field (IMF), and interpreted it as being due to their IMF-collimated outward flow across the shock from the inside of the Forbush decrease (Fd). In the present paper, they report its typical example recently observed in front of the Fd on September 9, 1992, as it bears the following dominant characteristics. The precursory decrease was not only exceedingly significant but also observed twice at the same station for two days running, which could not be clearly recognized in the previous analysis without taking the average of many events, and it was observed also with the underground muon telescope at Sakashita (depth: 80Hg/cm2), indicating its harder rigidity spectrum than the previous estimate. Based on these observations, the characteristics of the collimated flow responsible for the decrease is discussed.