Understanding the evolution of the Sun's open magnetic flux

The large-scale magnetic field of the Sun, including the open flux that ext ends into the interplanetary medium, originates in active regions but is re distributed over the photosphere by differential rotation, supergranular co nvection, and poleward meridional flow. We use simulations to clarify the r ole of the surface transport processes in the evolution of the total open f lux, Phi(open), which determines the strength of the radial interplanetary field component. Representing the initial photospheric field configuration by one or more bipolar magnetic regions (BMRs), we show that Phi(open), var ies approximately as the net dipole strength, determined by vectorially sum ming the dipole moments of the individual BMRs. As meridional flow carries the BMR flux to higher latitudes, the equatorial dipole component is annihi lated on a timescale similar to 1 yr by the combined effect of rotational s hearing and supergranular diffusion. The remaining flux becomes concentrate d around the poles, and Phi(open) approaches a Limiting value that depends on the axisymmetric dipole strength of the original active regions. We disc uss the implications of these results for the solar cycle evolution of Phi( open).