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

Abstract

In the previous papers (Nagashima et al., 1992, 1993) 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 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 collimation angle, and the rigidity spectrum of the constituent cosmic rays were only estimated qualitatively owing to the lack of the simulation of the precursory decrease produced by the flow. In the present paper these properties are quantitatively obtained by analyzing the hourly data of the precursory decreases observed on January 25 and 26, 1968, at the worldwide neutron monitor stations. The obtained direction and collimation angle of the flow approximately coincide respectively with those expected from the IMF. This fact gives evidence for the reconfirmation of the existence of the IMF‐guided collimated flow, responsible for the precursory decrease.