Magnetic Twists of Solar Filaments

TL;DR

This paper investigates the magnetic twists in solar filaments by constructing their magnetic structures and analyzing the factors influencing flux rope twist, revealing that filament length correlates with magnetic twist.

Contribution

It introduces a method to construct filament magnetic structures using regularized Biot--Savart laws and explores how flux rope twist depends on geometric parameters.

Findings

Twist of flux ropes is proportional to axial length to minor radius ratio.

Longer quiescent filaments tend to have more twisted flux ropes.

Twist is independent of the background magnetic field strength.

Abstract

Solar filaments are cold and dense materials situated in magnetic dips, which show distinct radiation characteristics compared to the surrounding coronal plasma. They are associated with coronal sheared and twisted magnetic field lines. However, the exact magnetic configuration supporting a filament material is not easy to be ascertained because of the absence of routine observations of the magnetic field inside filaments. Since many filaments lie above weak-field regions, it is nearly impossible to extrapolate their coronal magnetic structures by applying the traditional methods to noisy photospheric magnetograms, in particular the horizontal components. In this paper, we construct magnetic structures for some filaments with the regularized Biot--Savart laws and calculate their magnetic twists. Moreover, we make a parameter survey for the flux ropes of the Titov-Demoulin-modified model to explore the factors affecting the twist of a force-free magnetic flux rope. It is found that the twist of a force-free flux rope is proportional to its axial length to minor radius ratio, and is basically independent of the overlying background magnetic field strength. Thus, we infer that long quiescent filaments are likely to be supported by more twisted flux ropes than short active-region filaments, which is consistent with observations.