Role of the Sun’s Nonaxisymmetric Open Flux in Cosmic‐Ray Modulation

Abstract

We reexamine the empirical relationship between the Sun's open magnetic flux and the cosmic-ray (CR) intensity over the solar cycle. The single parameter that correlates best with the inverted CR rate is found to be the nonaxisymmetric or longitudinally varying component of the total open flux, rather than the sunspot number or the rate of coronal mass ejections (CMEs). The nonaxisymmetric open flux in turn tracks the evolution of the Sun's equatorial dipole component, which is a function of both the strength and the longitudinal distribution of sunspot activity. Year-long peaks in the equatorial dipole strength coincide with steplike decreases in the CR intensity and with the formation of global merged interaction regions (GMIRs) in the outer heliosphere. During these periods, nonaxisymmetric open flux (in the form of low-latitude coronal holes) is created through the organized emergence of large active regions, resulting in the global injection of magnetic energy into the heliosphere. At the same time, strengthenings of the equatorial dipole are generally accompanied by large increases in the number of fast CMEs. Rotationally induced, compressional interactions between the nonaxisymmetric open flux, fast CMEs, and high-speed streams then give rise to outward-propagating diffusive barriers that extend over all longitudes and to a latitude (45°) again determined by the equatorial dipole strength.