Surface Flux Transport and the Evolution of the Sun’s Polar Fields

• Published in: Space science reviews Vol. 210; no. 1-4; pp. 351 - 365
• Main Author: Wang, Y.-M.
• Format: Journal Article
• Language: English
• Published: Dordrecht Springer Netherlands 01-09-2017; Springer Nature B.V
• Subjects: Aerospace Technology and Astronautics; Astrophysics and Astroparticles; Computer simulation; Diffusion effects; Evolution; Flow profiles; Fluctuations; Latitude; Magnetic flux; Meridional flow; Physics; Physics and Astronomy; Planetology; Solar cycle; Solar magnetic field; Solar surface; Space Exploration and Astronautics; Space Sciences (including Extraterrestrial Physics; Sun; White light; Solar polar fields; Solar dynamo; Solar cycle; Magnetic flux transport on Sun
• ISSN: 0038-6308; 1572-9672
• DOI: 10.1007/s11214-016-0257-0

The evolution of the polar fields occupies a central place in flux transport (Babcock–Leighton) models of the solar cycle. We discuss the relationship between surface flux transport and polar field evolution, focusing on two main issues: the latitudinal profile of the meridional flow and the axial tilts of active regions. Recent helioseismic observations indicate that the poleward flow speed peaks at much lower latitudes than inferred from magnetic feature tracking, which includes the effect of supergranular diffusion and thus does not represent the actual bulk flow. Employing idealized simulations, we demonstrate that flow profiles that peak at mid latitudes give rise to overly strong and concentrated polar fields. We discuss the differences between magnetic and white-light measurements of tilt angles, noting the large uncertainties inherent in the sunspot group measurements and their tendency to underestimate the actual tilts. We find no clear evidence for systematic cycle-to-cycle variations in Joy’s law during cycles 21–23. Finally, based on the observed evolution of the Sun’s axial dipole component and polar fields up to the end of 2015, we predict that cycle 25 will be similar in amplitude to cycle 24.