Effects of the scatter in sunspot group tilt angles on the large-scale magnetic field at the solar surface

TL;DR

This study investigates how the random scatter in sunspot group tilt angles affects the evolution of the Sun's large-scale magnetic field, revealing it as a significant factor limiting solar activity prediction accuracy.

Contribution

The paper quantifies the impact of observed tilt angle scatter on the solar surface magnetic field using surface flux transport simulations across multiple solar cycles.

Findings

Tilt angle scatter causes an uncertainty of about 0.78 G in the axial dipole moment.

The magnitude of this uncertainty is similar across weak, medium, and strong solar cycles.

Big sunspot groups near the equator are the main contributors to the uncertainty.

Abstract

The tilt angles of sunspot groups represent the poloidal field source in Babcock-Leighton-type models of the solar dynamo and are crucial for the build-up and reversals of the polar fields in Surface Flux Transport (SFT) simulations. The evolution of the polar field is a consequence of Hale's polarity rules, together with the tilt angle distribution which has a systematic component (Joy's law) and a random component (tilt-angle scatter). We determine the scatter using the observed tilt angle data and study the effects of this scatter on the evolution of the solar surface field using SFT simulations with flux input based upon the recorded sunspot groups. The tilt angle scatter is described in our simulations by a random component according to the observed distributions for different ranges of sunspot group size (total umbral area). By performing simulations with a number of different realizations of the scatter we study the effect of the tilt angle scatter on the global magnetic field, especially on the evolution of the axial dipole moment. The average axial dipole moment at the end of cycle 17 (a medium-amplitude cycle) from our simulations was 2.73G. The tilt angle scatter leads to an uncertainty of 0.78 G (standard deviation). We also considered cycle 14 (a weak cycle) and cycle 19 (a strong cycle) and show that the standard deviation of the axial dipole moment is similar for all three cycles. The uncertainty mainly results from the big sunspot groups which emerge near the equator. In the framework of Babcock-Leighton dynamo models, the tilt angle scatter therefore constitutes a significant random factor in the cycle-to-cycle amplitude variability, which strongly limits the predictability of solar activity.