Meridional Flow and the Solar Cycle Variation of the Sun’s Open Magnetic Flux

Abstract

We simulate the evolution of the Sun's large-scale magnetic field during solar cycle 21, including the effect of surface transport processes and active region emergence. As an important new constraint on the model, we have scaled our source fluxes upward to be consistent with the average measured strength of the interplanetary magnetic field (IMF). By adopting a poleward bulk flow of amplitude ~20-25 m s-1 together with a supergranular diffusion rate of ~500 km2 s-1, we are then able to match the observed variation of the Sun's polar fields and open magnetic flux. The high meridional flow speeds, peaking at low latitudes, prevent the buildup of an overly strong axisymmetric dipole component at sunspot minimum, while accounting for the giant poleward surges of flux and accompanying polar field fluctuations observed near sunspot maximum. The present simulations also reproduce the large peak in the equatorial dipole and IMF strength recorded in 1982.