The long-term variation of the Sun's open magnetic flux

The interplanetary magnetic field (IMF) has its origin in open magnetic reg ions of the Sun (coronal holes). The location of these regions and their to tal open flux Phi(open) can be inferred from current-free extrapolations of the observed photospheric field. We derive the long-term variation of Phi( open) during 1971-1998 and discuss its causes. Near sunspot minimum, the op en flux originates mainly from the large polar coronal holes, whereas at su nspot maximum it is rooted in small, lower-latitude holes characterized by very high field strengths; the total amount of open flux thus remains rough ly constant between sunspot minimum and maximum. Through most of the cycle, the variation of Phi(open) closely follows that of the Sun's total dipole strength, showing much less dependence on the total photospheric flux or th e sunspot number. However, episodic increases in large-scale sunspot activi ty lead to strengthenings of the equatorial dipole component, and hence to enhancements in Phi(open) and the IMF strength lasting typically similar to 1 yr.