Cosmic Ray modulation by Corotating Interaction Regions

J. Calogovic1, B. Vrsnak1, M. Temmer2

1Hvar Observatory, Faculty of Geodesy, Kaciceva 26, HR-10000 Zagreb, Croatia
2Institute of Physics, University of Graz, Universitatsplatz 5, A-8010 Graz, Austria

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. The NM stations were selected to include all latitudes (3 NM stations in high, 3 in middle and 2 in low latitudes) and longitudes, covering rigidities from 0 to 12.91 GeV. In situ solar wind data (density n, speed v and magnetic field B) were taken from the Advanced Composition Explorer (ACE) database, whereas the coronal hole (CH) areas were extracted from the Solar X-Ray Imager (GOES-12). For the analysis we have chosen a period in the declining phase of solar cycle 23, covering days 25-125 of the year 2005 (25 January until 5 May), when the solar CME activity was particularly low. During the CIR periods CR decreased typically from 0.5 to 1%. The cross-correlation analysis showed a distinct anti-correlation between the magnetic field and CR, with correlation coefficient (r) ranging from -0.31 to -0.41 (mean: -0.37) and with the CR time delay of 2 to 3 days depending on NM station. Similar anti-correlations, but with the lower correlation coefficients, 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. In this case the anti-correlation has r = -0.20 to -0.39 (mean: -0.29) for the CR time lag of 5 days after the central-meridian passage of CH.