  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.
