Measuring the Diffusion of Solar Magnetic Flux on Large Spatio-Temporal Scales

We present an investigation of the large-scale flows that influence magnetic fields at the solar surface. The aim of this work is to accurately characterise the supergranular diffusion coefficient, D, that governs the dispersal rate of magnetic features in the photosphere. There is a disconnect between the measured rate of magnetic field dispersal (~50 - 300 km^2/s) and the value of D used in global simulations of solar magnetic field evolution (~500 - 600 km^2/s). We track the poleward motion of magnetic features in a latitude-time map and compare the poleward progression to a data-driven simulation that includes differential rotation, the meridional flow, and supergranular diffusion. We find that over a time scale of months, setting D = 100 km^2/s matches observations, but over a time scale of years, setting D = 500 km^2/s is a better match. This supports the idea that observational time scale causes the disconnect in D values, which leads us to the conclusion that the present magnetic surface flux transport model is not adequate to explain the observed evolution of the solar surface magnetic field.