Role of a Variable Meridional Flow in the Secular Evolution of the Sun’s Polar Fields and Open Flux

Abstract

We use a magnetic flux transport model to simulate the evolution of the Sun's polar fields and open flux during solar cycles 13 through 22 (1888-1997). The flux emergence rates are assumed to scale according to the observed sunspot-number amplitudes. We find that stable polarity oscillations can be maintained if the meridional flow rate is allowed to vary from cycle to cycle, with higher poleward speeds occurring during the more active cycles. Our model is able to account for a doubling of the interplanetary field strength since 1900, as deduced by Lockwood, Stamper, & Wild from the geomagnetic aa index. We confirm our earlier conclusion that such a doubling of the open flux does not imply that the base level of the total photospheric flux has increased significantly over the last century.