A mechanism for the dependence of sunspot group tilt angles on cycle strength

TL;DR

This paper proposes that cycle-dependent variations in sunspot tilt angles are caused by deep-seated thermal changes near the convection zone base, influencing magnetic flux emergence and cycle strength.

Contribution

It introduces a non-local, deep-seated mechanism involving thermal perturbations that explains the observed dependence of sunspot tilt angles on solar cycle strength.

Findings

A 5-20 K cooling can reproduce tilt angle variations.

Deep thermal changes influence flux tube stability.

Cycle-dependent tilt angles affect magnetic flux crossing the equator.

Abstract

The average tilt angle of sunspot groups emerging throughout the solar cycle determines the net magnetic flux crossing the equator, which is correlated with the strength of the subsequent cycle. I suggest that a deep-seated, non-local process can account for the observed cycle-dependent changes in the average tilt angle. Motivated by helioseismic observations indicating cycle-scale variations in the sound speed near the base of the convection zone, I determined the effect of a thermally perturbed overshoot region on the stability of flux tubes and on the tilt angles of emerging flux loops. I found that 5-20 K of cooling is sufficient for emerging flux loops to reproduce the reported amplitude of cycle-averaged tilt angle variations, suggesting that it is a plausible effect responsible for the nonlinearity of the solar activity cycle.