On the Area Expansion of Magnetic Flux-Tubes in Solar Active Regions

TL;DR

This study maps the 3D area expansion factors of magnetic flux-tubes in a solar active region, revealing structured loop-like features and their variation with height, which may influence coronal structuring.

Contribution

It provides the first detailed 3D distribution of area expansion factors in a solar active region using potential field extrapolation from high-resolution magnetograms.

Findings

Loop-like structures with lower expansion factors are embedded in a smooth background.

Expansion factors generally increase with height, distinguishing the active region core.

Approximate magnetogram modeling captures large-scale flux but omits small-scale structuring.

Abstract

We calculated the 3D distribution of the area expansion factors in a potential magnetic field extrapolated from the high-resolution \textit{Hinode}/SOT magnetogram of a quiescent active region NOAA 11482. Retaining only closed loops within the computational box, we show that the distribution of area expansion factors show significant structure. Loop-like structures characterized by locally lower values of the expansion factor are embedded in a smooth background. These loop-like flux-tubes have squashed cross-sections and expand with height. The distribution of the expansion factors show overall increase with height, allowing an active region core characterized by low values of the expansion factor to be distinguished. The area expansion factors obtained from extrapolation of the SOT magnetogram are compared to those obtained from an approximation of the observed magnetogram by a series of 134 submerged charges. This approximation retains the general flux distribution in the observed magnetogram, but removes the small-scale structure in both the approximated magnetogram and the 3D distribution of the area expansion factors. We argue that the structuring of the expansion factor can be a significant ingredient in producing the observed structuring of the solar corona. However, due to the potential approximation used, these results may not be applicable to loops exhibiting twist neither to active regions producing significant flares.