Global dipole moment study using optimized surface flux transport model

The importance of solar polar magnetic fields for the dynamo and for solar cycle forecasting has become increasingly clear in recent years. Polar fields are observed to be built up from active region trailing polarities by meridional flow advection and turbulent diffusion. The surface flux transport (SFT) models describing this process involve a number of free parameters and optional choices such as turbulent diffusivity, meridional flow amplitude or choice of meridional flow profile. In the past these choices were usually optimized to best reproduce the overall time-latitude pattern (butterfly diagram) of the magnetic field distribution. In this approach, mid-latitude features (plumes) are given great weight, while the smaller polar areas, observed less well due to perspective problems, have little influence. As a result, models optimized in this way often show significant disagreements with observations of the polar field, esp. regarding the timing of polar field reversals and maxima or latitudinal extent of the polar field concentration. we took the alternative approach of constraining SFT model parameters and assumptions by reducing the allowed parameter space to the domain where the phase of polar field variations and the latitudinal extent of the polar magnetic cap agree with observational constraints. Simply, if we choose that there is nothing in the sun but a simple bipolar region, placed in some latitude, and run the SFT model with optimized parameters for some time until a dipolar field is formed, one can get the relation between the initial and final field, which can be introduced to the global dipole moment as a further factor as a function of latitude.