Local-time-dependent pre-IMF-shock decrease and post-shock increase of cosmic rays, produced respectively by their IMF-collimated outward and inward flows across the shock responsible for Forbush decrease

Abstract The cosmic-ray storm known as “Forbush decrease” is produced generally as a result of the transient diffusion-convection of cosmic rays caused by the passage of the interplanetary magnetic field (IMF) shock wave. It is emphasized, however, that the storm is frequently accompanied by non-diffusion-convection-type phenomena. In the present paper, the authors show the existence of such phenomena, which are dependent on local time. These are: (1) the precursory decrease of cosmic-ray intensity in front of the shock, which occurs in the morning (6–12 h), having nearly the same rigidity spectrum as that of the Forbush decrease; and (2) the post-shock increase, which belongs to the daily variation in general, but bears the following anomalous characters; a steep peak as high as the pre-storm intensity level, an extremely soft rigidity spectrum and a phase of 18–24 h in space considerably later than the usual. It is concluded that the precursory decrease is produced by the IMF-collimated outward flow of the low-density cosmic rays from the inside of the shock, and that the collimation is determined by the ratio between the ordered magnetic fields at the shock front and at the observation point. Inversely, the post-shock increase is produced by the IMF-collimated inward flow of the high-density cosmic rays from the outside. As an extreme case of the above phenomena, they also point out the existence of the IMF-guided square wave of cosmic-ray intensity with 24 h periodicity, which is produced as a result of the Earth's rotation in the unbalanced two-way flows along the magnetic lines of force connecting two separated regions occupied, respectively, by the high- and low-density cosmic rays. Finally, a serious influence of the precursory decrease on the determination of the commencement of Forbush decrease and also on the study of the precursory increase expected to appear in front of the shock wave, is discussed on the basis of definite examples.