Coronal mass ejections, open magnetic flux, and cosmic-ray modulation

Recent work indicates that the open magnetic flux from coronal holes may be a more important driver of cosmic-ray modulation than coronal mass ejections (CMEs), which originate from closed-field regions on the Sun. As evidence against this point of view, we find that the correlation coefficient (r) between the solar open flux from coronal holes and the Climax cosmic-ray intensity for the period 1967-2000 is only -0.37. We show that, for similar values of the total open flux from the Sun near solar maximum and minimum, the cosmic-ray intensity is much lower at maximum. Thus, something in addition to the open magnetic flux from coronal holes is acting to block incoming cosmic rays at solar maximum. Because they drive shocks and contain enhanced magnetic fields, CMEs are the leading candidate to play such a role. Over the 1967-2000 interval, the cosmic-ray intensity is highly anticorrelated (r = -0.80) with the sunspot number, a measure of closed fields on the Sun. For the 1979-1989 interval covered by the Solwind and SMM coronagraphs, the correlation coefficient between the CME rate and the cosmic-ray intensity is -0.61. For cycle 21, an analysis of solar wind data shows that CMEs and related shock flows accounted for similar to 40% (2.9/7.0 nT) of the average interplanetary magnetic field strength at 1 AU at solar maximum, versus 10% (0.5/5.0 nT) at solar minimum. If we break the open flux into that originating from low-latitude (less than or equal to 45 degrees) and high-latitude (>45 degrees) coronal holes on the Sun for this 34 yr interval, we find that the low-latitude flux is highly anticorrelated with cosmic-ray intensity (r = -0.76), while the open flux from high latitudes and the Climax count rate are directly correlated (r= -0.53). We suggest that the anticorrelation between low-latitude open flux and the cosmic-ray intensity occurs because CMEs open new flux to the interplanetary medium.