engleski

Cosmic ray modulation by ICME-driven shocks

Forbush decreases are decreases in the cosmic ray count rate which last typically for about a week, and were first reported by Scott E. Forbush in 1937. They can be caused by corotating interaction regions (CIRs) or interplanetary coronal mass ejections (ICMEs). We study modulation of cosmic ray flux in the downstream region (so called “sheath region”) of ICME-driven oblique fast-mode magnetohydrodynamical shocks, where magnetic field contains components both parallel and normal to the shock front. In particular, we consider two effects: magnetic mirror at the shock and the convective compression of the downstream region. Magnetic mirror effect decreases the particle flux, while compression increases it. Since the magnetic mirror effect is stronger, the flux in the downstream region is smaller than in the upstream region. The results can be applied to CIRs too. The calculated flux decreases have somewhat larger amplitudes than observed, indicating that other mechanisms, like, e.g., diffusion, play a significant role too.

ICMEs, shocks, Forbush decrease

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