Helicity and winding fluxes as indicators of twisted flux emergence

TL;DR

This study uses magnetohydrodynamic simulations to analyze helicity and winding fluxes as direct indicators of twisted flux tube emergence in the solar atmosphere, revealing their potential to provide detailed topological information beyond traditional magnetograms.

Contribution

It introduces the use of helicity and winding fluxes as direct, quantitative indicators of twisted flux emergence, incorporating convective effects and synthetic magnetogram methods.

Findings

Helicity input shows self-similar behavior across different initial field strengths.

Negative helicity dominates during initial emergence, then shifts to positive with convection influence.

Winding flux effectively detects when twisted cores reach the photosphere.

Abstract

Evidence for the emergence of twisted flux tubes into the solar atmosphere has, so far, come from indirect signatures. In this work, we investigate the topological input of twisted flux tube emergence directly by studying helicity and winding fluxes. In magnetohydrodynamic simulations with domains spanning from the top of the convection zone to the lower corona, we simulate the emergence of twisted flux tubes with a range of different initial field strengths. One important feature of this work is the inclusion of a convectively-unstable layer beneath the photosphere. We find approximately self-similar behaviour in the helicity input for the different field strengths considered. As the tubes rise and reach the photosphere, there is a strong input of negative helicity since we consider left-handed twisted tubes. This phase is then followed by a reduction of the negative input and, for low initial field strengths, a net positive helicity input. This phase corresponds to the growing influence of convection on the field and the development of serpentine field structures during emergence. The winding flux can be used to detect when the twisted cores of the tubes reach the photosphere, giving clear information about the input of topologically complex magnetic field into the solar atmosphere. In short, the helicity and winding fluxes can provide much information about how a magnetic field emerges that is not directly available from other sources, such as magnetograms. In evaulating the helicity content of these simualtions we test numerous means for creating synthetic magnetograms, including methods which acount for both the evolving geometry and the finite extent of the photosphere. Whilst the general qualitative behaviours are same in each case, the different forms of averaging do affect the helicity and winding inputs quantitatively.