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.
In addition to solar modulation according to the ∼11-year sunspot cycle and the ∼22-year solar magnetic cycle, the time profile of the Galactic cosmic ray flux can also exhibit short-term (∼2-week) modulation events. These are distinct from Forbush decreases in that they are more symmetric in time and evolve over a time scale much longer than the transit of an interplanetary shock and/or coronal mass ejection (CME). Using data from the Princess Sirindhorn Neutron Monitor at the summit of Doi Inthanon, Thailand, with the world's highest effective vertical geomagnetic cutoff rigidity for a fixed station (16.7 GV), we have examined the solar diurnal anisotropy and find that it exhibited strong peaks during two short-term modulation events in 2012, which were indeed stronger than the diurnal anisotropy variation from sunspot minimum to maximum. We attribute these short-term modulation events to non-local effects of CME shocks. CME shocks (possibly single, multiple, or merged) that propagate beyond Earth inhibit the access of cosmic rays for ∼2 weeks. The direction of anisotropy enhancement favors an explanation in terms of cosmic ray diffusion perpendicular to the interplanetary magnetic field, which eventually causes the cosmic ray flux to stop decreasing and gradually recover. We contrast the physics of these short-term events with the overall ∼11-year solar modulation, which has a greater effect on the Galactic cosmic ray flux but a weaker effect on its anisotropy.
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