Recurrent Depressions of Galactic Cosmic Rays in CIRs: 22-Year Cycle

Abstract

Recurrent variations in the galactic cosmic-ray flux are produced by the interplay of diffusion and particle drifts. The enhanced scattering and consequent small diffusion in the compressed field of Corotating Interaction Regions (CIRs) causes recurrent cosmic-ray depressions at the passage of CIRs. Drift effects are primarily controlled by the structure of the heliospheric current sheet (HCS). Our earlier 3-D codes using a symmetric tilted dipole are extended to include more complex structures of the HCS. Simulation results are presented. We consider a southward displacement of the HCS during solar minimum. We also report on 3-D model simulations with a HCS resembling those observed at solar maximum (the latitudinal extension of the HCS increases and quadrupole components of the field become important as the Sun enters a more active phase). Recurrent longitudinal variations are discussed for the two polarity states of the 22-year cycle (A < 0 and A > 0). We find remarkable differences between the 26-day recurrent cosmic-ray variations predicted for A > 0 and A < 0. The magnitude of the 26-day wave may turn out larger for the A > 0, in qualitative agreement with the so far unexplained findings of Richardson et al. (1999), if the HCS happens to be placed asymmetrically.