Coronal Mass Ejections and the Solar Cycle Variation of the Sun's Open Flux

TL;DR

This paper investigates the causes of the solar cycle variation in the Sun's open magnetic flux, emphasizing the dominant role of the Sun's dipole moment over coronal mass ejections (CMEs).

Contribution

It challenges the idea that CMEs significantly contribute to the Sun's open flux variation, highlighting the primary influence of the Sun's dipole moment and the role of coronal holes.

Findings

CMEs contribute at most ~30% of the IMF strength during maximum.

Large IMF peaks in 2003 and 2014 are not caused by CMEs.

The Sun's dipole moment mainly determines the open flux variation.

Abstract

The strength of the radial component of the interplanetary magnetic field (IMF), which is a measure of the Sun's total open flux, is observed to vary by roughly a factor of two over the 11 yr solar cycle. Several recent studies have proposed that the Sun's open flux consists of a constant or "floor" component that dominates at sunspot minimum, and a time-varying component due to coronal mass ejections (CMEs). Here, we point out that CMEs cannot account for the large peaks in the IMF strength which occurred in 2003 and late 2014, and which coincided with peaks in the Sun's equatorial dipole moment. We also show that near-Earth interplanetary CMEs, as identified in the catalog of Richardson and Cane, contribute at most $\sim$30\% of the average radial IMF strength even during sunspot maximum. We conclude that the long-term variation of the radial IMF strength is determined mainly by the Sun's total dipole moment, with the quadrupole moment and CMEs providing an additional boost near sunspot maximum. Most of the open flux is rooted in coronal holes, whose solar cycle evolution in turn reflects that of the Sun's lowest-order multipoles.