We analyzed the relationship between the ground-based modulation of cosmic rays (CR) and corotating interaction regions (CIRs). Daily averaged data from 8 different neutron monitor (NM) stations were used, covering rigidities from Rc = 0 − 12.91 GeV. The in situ solar wind data were taken from the Advanced Composition Explorer (ACE) database, whereas the coronal hole (CH) areas were derived from the Solar X-Ray Imager onboard GOES-12. For the analysis we have chosen a period in the declining phase of solar cycle 23, covering the period 25 January–5 May 2005. During the CIR periods CR decreased typically from 0.5% to 2%. A cross-correlation analysis showed a distinct anti-correlation between the magnetic field and CR, with the correlation coefficient (r) ranging from −0.31 to −0.38 (mean: −0.36) and with the CR time delay of 2 to 3 days. Similar anti-correlations were found for the solar wind density and velocity characterized by the CR time lag of 4 and 1 day, respectively. The relationship was also established between the CR modulation and the area of the CIR-related CH with the CR time lag of 5 days after the central-meridian passage of CH.
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