Speaker
Description
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 are not associated with the local passage 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 geomagnetic cutoff rigidity for a fixed station (16.7 GV), we have examined the solar diurnal anisotropy and find that it exhibits strong peaks during such short-term modulation events, which are indeed stronger than the diurnal anisotropy variation from sunspot minimum to maximum. We attribute these short-term modulation events to non-Earth directed CMEs, and propose specific CME associations for notable events. We propose that even when not directed to the observer, CMEs (possibly single, multiple, or interacting) that propagate beyond the observer can temporarily inhibit the access of cosmic rays. The local diffusion coefficient is apparently undisturbed, but the reduced inflow past the CME coupled with an unabated flux decrease near the Sun due to adiabatic deceleration leads to a temporary, strong gradient that generates the strong anisotropy. 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.
Keywords
neutron monitor, diurnal anisotropy, coronal mass ejection, solar modulation
| Subcategory | Experimental Results |
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