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The nonlinear force-free field (NLFFF) modeling has been extensively used toinfer the three-dimensional (3D) magnetic field in the solar corona. One of theassumptions in the NLFFF extrapolation is that the plasma beta is low, but thiscondition is considered to be incorrect in the photosphere. We examine directmeasurements of the chromospheric magnetic field in two active regions throughspectropolarimetric observations at He I 10830 A, which are compared with thepotential fields and NLFFFs extrapolated from the photosphere. The comparisonsallow quantitative estimation of the uncertainty in the NLFFF extrapolationfrom the photosphere. Our analysis shows that observed chromospheric magneticfield may have larger non-potentiality compared to the photospheric magneticfield. Moreover, the large non-potentiality in the chromospheric height may notbe reproduced by the NLFFF extrapolation from the photospheric magnetic field.The magnitude of the underestimation of the non-potentiality at chromosphericheights may reach 30-40 degree in shear signed angle in some locations. Thisdeviation may be caused by the non-force-freeness in the photosphere. Our studysuggests the importance of the inclusion of measured chromospheric magneticfields in the NLFFF modeling for the improvement of the coronal extrapolation.

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