On the origin of the solar hemispherical helicity rules: Simulations of the rise of magnetic flux concentrations in a background field

TL;DR

This study uses simulations to explore how magnetic flux concentrations rise in the Sun's interior, revealing a bias that explains the hemispheric helicity rules observed in solar active regions.

Contribution

The paper introduces a new simulation-based mechanism that accounts for the solar hemispheric helicity rules by considering flux tube interactions with a background magnetic field.

Findings

Flux tubes with twist aligned with the background field are more likely to rise.

The bias explains the observed hemispheric helicity rules in solar active regions.

The mechanism accounts for the scatter and cycle variation in the rules.

Abstract

Solar active regions and sunspots are believed to be formed by the emergence of strong toroidal magnetic flux from the solar interior. Modeling of such events has focused on the dynamics of compact magnetic entities, colloquially known as "flux tubes", often considered to be isolated magnetic structures embedded in an otherwise field-free environment. In this paper, we show that relaxing such idealized assumptions can lead to surprisingly different dynamics. We consider the rise of tube-like flux concentrations embedded in a large-scale volume-filling horizontal field in an initially quiescent adiabatically-stratified compressible fluid. In a previous letter, we revealed the unexpected major result that concentrations that have their twist aligned with the background field at the bottom of the tube are more likely to rise than the opposite orientation (for certain values of the parameters). This bias leads to a selection rule which, when applied to solar dynamics, is in agreement with the observations known as the solar hemispheric helicity rule(s) (SHHR). Here, we examine this selection mechanism in more detail than was possible in the earlier letter. We explore the dependence on the parameters via simulations, delineating the Selective Rise Regime (SRR), where the bias operates. We provide a theoretical model to predict and explain the simulation dynamics. Furthermore, we create synthetic helicity maps from Monte Carlo simulations to mimic the SHHR observations and demonstrate that our mechanism explains the observed scatter in the rule and its variation over the solar cycle.