Magnetic Footpoint Diffusion at the Sun and Its Relation to the Heliospheric Magnetic Field

Abstract

We discuss the physics associated with the motion of the radial magnetic field footpoints at the Sun and its relation to the heliospheric magnetic field. Using the observed photospheric convection spectrum, we construct a model for the heliospheric magnetic field and relate it to observations made by Ulysses in the outer heliosphere in the fast solar wind during solar minimum. We find excellent agreement between both the magnitude and the radial variation of the large-scale transverse magnetic variance observed by Ulysses compared to the model calculations using the observed supergranulation velocity spectrum. This suggests that the model represents the physics well. We then calculate the spatial diffusion coefficient associated with the observed magnetic footpoint motion at the solar photosphere. The calculations are based on the motion of passive additives embedded within the transverse photospheric flows. Two different models for the temporal evolution of the transverse velocity are considered. We find that the diffusion coefficient associated with these motions is in the range κ ≈ 1500-1920 km2 s-1. This is approximately 3 times larger than is commonly used in models of the long-term solar magnetic field. It is also larger than is inferred from actual measurements of the dispersion of magnetic features on the Sun. The cause of this discrepancy is not yet clear.