Chromospheric magnetic field: A comparison of He I 10830 A observations with nonlinear force-free field extrapolation

TL;DR

This study compares chromospheric magnetic field measurements with NLFFF extrapolations from the photosphere, revealing significant non-potentiality in the chromosphere that NLFFF models may underestimate, highlighting the need for direct chromospheric data.

Contribution

It provides the first quantitative comparison between observed chromospheric magnetic fields and NLFFF extrapolations, emphasizing the importance of including chromospheric measurements for accurate coronal magnetic modeling.

Findings

Chromospheric magnetic fields show larger non-potentiality than photospheric fields.

NLFFF extrapolations underestimate non-potentiality by up to 30-40 degrees in shear angle.

Including chromospheric magnetic measurements can improve NLFFF modeling accuracy.

Abstract

The nonlinear force-free field (NLFFF) modeling has been extensively used to infer the three-dimensional (3D) magnetic field in the solar corona. One of the assumptions in the NLFFF extrapolation is that the plasma beta is low, but this condition is considered to be incorrect in the photosphere. We examine direct measurements of the chromospheric magnetic field in two active regions through spectropolarimetric observations at He I 10830 A, which are compared with the potential fields and NLFFFs extrapolated from the photosphere. The comparisons allow quantitative estimation of the uncertainty in the NLFFF extrapolation from the photosphere. Our analysis shows that observed chromospheric magnetic field may have larger non-potentiality compared to the photospheric magnetic field. Moreover, the large non-potentiality in the chromospheric height may not be reproduced by the NLFFF extrapolation from the photospheric magnetic field. The magnitude of the underestimation of the non-potentiality at chromospheric heights may reach 30-40 degree in shear signed angle in some locations. This deviation may be caused by the non-force-freeness in the photosphere. Our study suggests the importance of the inclusion of measured chromospheric magnetic fields in the NLFFF modeling for the improvement of the coronal extrapolation.