The intensity variation of cosmic rays associated with corotating interaction regions

Long-term decrease events in the intensity of galactic cosmic rays have been investigated using solar wind parameters. We investigated distinct long-term decreases in 1993/94 near the solar minimum phase. In this period, the corotating interaction regions were observed over half a year. Recurrent increases of the solar wind speed were observed during this time period, forming corotating interaction regions. Simultaneously recurrent energetic proton events and recurrent decrease of galactic cosmic rays are measured. The baseline counts of neutron monitors was well anti-correlated with the peak speed of solar wind. It is suggested that long-lasting enhancements of solar wind velocity or corotating interaction regions formed by them can produce the long-term decrease in the galactic cosmic ray intensity.