A Numerical Study of the Effects of Corotating Interaction Regions on Cosmic-Ray Transport

The intensity of galactic cosmic rays (GCRs) is modulated by solar activity on various timescales. In this study, we performed comprehensive numerical modeling of the solar rotational recurrent variation in GCRs caused by a corotation interaction region (CIR). A recently developed magnetohydrodynamic numerical model is adapted to simulate the background solar wind plasma with a CIR structure present in the inner heliosphere. As for the outer heliospheric plasma background, from 27 to 80 au, the Parker interplanetary magnetic field model is utilized. The output of these plasma and magnetic field models is incorporated into a comprehensive Parker-type transport model for GCRs. The local interstellar spectrum for galactic protons is transported to 80 au, specifying the outer boundary condition. The obtained solutions of this hybrid model, for studying the CIR effect, are as follows: (1) the onset of the decrease in the GCR intensity inside the CIR coincides with the increase of the solar wind speed with the intensity depression accompanied by a magnetic field and plasma density enhancement. Additionally, the CIR effect weakens with increasing heliocentric radial distance. (2) This decrease in GCR intensity also appears at different heliolatitudes and varies with changing latitude; the amplitude of the GCR depression exhibits a maximum in the low-latitude region. (3) The CIR affects GCR transport at different energy levels as well. Careful analysis has revealed a specific energy dependence of the amplitude of the recurrent GCR variation in the range of 30-2000 MeV.