Extended Cosmic Ray Decreases with Strong Anisotropy after Passage of Interplanetary Shocks

Abstract

The passage of an interplanetary shock and/or interplanetary coronal mass ejection often causes a rapid decrease in the Galactic cosmic-ray (GCR) flux, known as a Forbush decrease, followed by a recovery of the flux over some days. These local effects are of short duration and strongly rigidity dependent, with higher-rigidity particles exhibiting much weaker effects. In contrast, we present data for two events in which the cosmic-ray flux gradually decreased for about 1 week after shock passage, then recovering over the following week, with the highest anisotropy levels observed throughout Solar Cycle 24. These extended decreases have a weak rigidity dependence and are much more prominent in observations at higher cutoff rigidity, where the initial Forbush decrease is not clearly detected and other variations are generally weak, as we demonstrate using data from the Princess Sirindhorn Neutron Monitor at Doi Inthanon, Thailand with a cutoff rigidity of about 17 GV. We propose that these extended decrease events were initiated upon the passage of an interplanetary shock that inhibited the inflow of GCRs along the interplanetary magnetic field, possibly due to magnetic mirroring at the shock. We also discuss the general behavior of GCR anisotropy as observed at this high cutoff rigidity.