Solar magnetic field variations and cosmic ray modulation

Abstract

Current modeling of long-term (11-year) cosmic ray modulation incorporates as a major component the so-called “Global Merged Interaction Regions” (GMIRs). GMIRs are assumed to be formed beyond about 10 AU by the merging of systems of coronal mass ejections (CMEs) and existing interaction regions. Merged interaction regions are identified from outer heliospheric magnetic field observations as complex structures and enhancements above the average field strength. We demonstrate that the ∼1 year cosmic ray modulation events supposedly caused by GMIRs are already apparent at 1 AU, with the effects of individual major CMEs superimposed. Thus, GMIRs are unlikely to be the prime cause of long-term cosmic ray modulation. We show that the cosmic ray intensity is anti-correlated with the interplanetary magnetic field strength (IMF). The IMF shows an overall solar-cycle variation of about a factor of 2 together with episodic increases with durations of about a year which do not form in the interplanetary medium but are related to global variations of the solar photospheric magnetic field. We point out, once again, that cosmic rays are important probes of the structure of the interplanetary medium.