Title and Abstract

Title:  Chromospheric Magnetic Field: a Comparison of He I 10830 Å Observations with Nonlinear Force-free Field Extrapolation 

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 the direct measurements of the chromospheric magnetic field in two active regions through the spectropolarimetric observations at He I 10830 Å. The results of NLFFF extrapolation from the photosphere are compared with the direct measurements. The comparisons allow quantitative estimation of the uncertainty of the NLFFF extrapolation from the photosphere. Our analysis shows that 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 the chromospheric height may reach 30-40 degree in signed shear angle. Because the non-potentiality is crucial in the MHD instability, our findings would improve the understanding of the onset mechanisms for solar flares and CMEs, which affect the environment in the solar system.