Convective Magnetic Flux Emergence Simulations from the Deep Solar Interior to the Photosphere: Comprehensive Study of Flux Tube Twist

TL;DR

This study uses numerical simulations to analyze how magnetic twist affects flux tube emergence from the solar interior to the photosphere, revealing that excessive twist leads to unrealistic magnetic helicity levels inconsistent with observations.

Contribution

The paper systematically investigates the impact of initial magnetic twist on flux emergence, providing insights into the stability and observational signatures of emerging flux tubes.

Findings

Weak twist results in rapid diffusion of photospheric flux.

Magnetic twist is conserved from the interior to the photosphere.

Excessive twist produces unrealistically high magnetic helicity.

Abstract

The emergence of magnetic flux from the deep convection zone plays an important role in the solar magnetism, such as the generation of active regions and triggering of various eruptive phenomena, including jets, flares, and coronal mass ejections. To investigate the effects of magnetic twist on flux emergence, we performed numerical simulations of flux tube emergence using the radiative magnetohydrodynamic code R2D2, and conducted a systematic survey on the initial twist. Specifically, we varied the twist of the initial tube both positively and negatively from zero to twice the critical value for kink instability. As a result, regardless of the initial twist, the flux tube was lifted by the convective upflow and reached the photosphere to create sunspots. However, when the twist was too weak, the photospheric flux was quickly diffused and not retained long as coherent sunspots. The degree of magnetic twist measured in the photosphere conserved the original twist relatively well, and was comparable to actual solar observations. Even in the untwisted case, a finite amount of magnetic helicity was injected into the upper atmosphere because the background turbulence added helicity. However, when the initial twist exceeded the critical value for kink instability, the magnetic helicity normalized by the total magnetic flux was found to be unreasonably larger than the observations, indicating that the kink instability of the emerging flux tube may not be a likely scenario for the formation of flare-productive active regions.