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

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