# Comparison of Two Coronal Magnetic Field Models for Reconstructing a Sigmoidal Solar Active Region With Coronal Loops

## TL;DR

This study compares two non-potential magnetic field extrapolation models for a sigmoidal solar active region, finding that the CESE-MHD-NLFFF code best matches observed coronal loops, but all models need improvement for outer loops.

## Contribution

It provides a critical comparison of two different magnetic field extrapolation codes against observed coronal loops in a solar active region.

## Key findings

- CESE-MHD-NLFFF code achieves ~10° misalignment in the core region.
- All models show ~30° misalignment in outer loops.
- CESE-MHD-NLFFF performs comparably to some forward-fitting models.

## Abstract

Magnetic field extrapolation is an important tool to study the three-dimensional (3D) solar coronal magnetic field which is difficult to directly measure. Various analytic models and numerical codes exist but their results often drastically differ. Thus a critical comparison of the modeled magnetic field lines with the observed coronal loops is strongly required to establish the credibility of the model. Here we compare two different non-potential extrapolation codes, a non-linear force-free field code (CESE-MHD-NLFFF) and a non-force-free field (NFFF) code in modeling a solar active region (AR) that has a sigmoidal configuration just before a major flare erupted from the region. A 2D coronal-loop tracing and fitting method is employed to study the 3D misalignment angles between the extrapolated magnetic field lines and the EUV loops as imaged by SDO/AIA. It is found that the CESE-MHD-NLFFF code with preprocessed magnetogram performs the best, outputting a field which matches the coronal loops in the AR core imaged in AIA 94 {\AA} with a misalignment angle of ~10 degree. This suggests that the CESE-MHD-NLFFF code, even without using the information of coronal loops in constraining the magnetic field, performs as good as some coronal-loop forward-fitting models. For the loops as imaged by AIA 171 {\AA} in the outskirts of the AR, all the codes including the potential-field give comparable results of mean misalignment angle (~30 degree). Thus further improvement of the codes is needed for a better reconstruction of the long loops enveloping the core region.