Modeling of CIR-driven modulations of galactic cosmic ray intensity obtained by the Muon detector network

The solar wind modulates Galactic Cosmic Ray (GCR) intensity. Modulations with a time scale of several days are mainly caused by Coronal Mass Ejections (CME) and solar wind stream interaction regions. The modulation caused by CME is called Forbush decrease (FD). The modulation that occurs at intervals of 27 days associated with Co-rotating Interaction Regions (CIR) is called recurrent storm. It has been considered that the mechanism of FD consists of two different processes as follows. (i) A turbulent region of solar wind magnetic field behind an interplanetary shock prevents incursion of GCR particles (shock effect). (ii) An interplanetary magnetic flux rope that has a low GCR density passes near the Earth (ejecta effect). A gradient of GCR density which is caused by an ejecta generates the flow of GCR called the B x grad n drift anisotropy. Therefore, the directional anisotropy of GCR intensity can be measured with ground-based observations [Bieber and Evenson, 1998]. Kuwabara et al. [2004] have established a model for reproducing both the temporal variations of isotropic intensity and the anisotropy of GCR measured by the Muon detector network. The shape of the GCR low density region inside ejecta is modeled with a cylinder. Since the CIR-driven GCR decrease accompanied by the directional anisotropy can be observed by the Muon detector network, it is expected that a low density region of GCR exists in the CIR structure. The mechanism of CIR-driven GCR decrease is still an open question. Using the data obtained by satellite observations, Richardson et al. [1996] investigated the relationship between the start time of GCR intensity decrease and the solar wind structure. They suggested that a turbulent region of solar wind magnetic field takes an important role in understanding the mechanism of modulation caused by CIR. However some problems are left. For example, the start time of GCR intensity decrease does not always correspond to the turbulent region of magnetic field. With these backgrounds, we assume a slab shape for the low density region caused by CIR and construct a model for reproducing temporal variations of isotropic intensity and the anisotropy of GCR measured by the Muon detector network. Details of this model will be reported. We will also discuss a relationship between the deduced low density region of GCR and the solar wind structure on the basis of some event studies of CIR- driven GCR modulations.