Magnetic field extrapolation is a fundamental tool to reconstruct the
three-dimensional solar coronal magnetic field. However, the prevalently used
force-free field model might not be applicable in the lower atmosphere, where
plasma \b{eta} is greater than 1. In this work, we perform extrapolation in
active region 12158, based on an updated magnetohydrostatic (MHS) method. By
comparing the results with those from the force-free field method of
Current-Field Iteration in Spherical Coordinates (CFITS), we find that the
overall properties, which are characterized by the magnetic free energy and
helicity, are roughly the same after volume integral. The major differences lie
in the magnetic configuration and the twist number of magnetic flux rope (MFR).
A coherent MFR with twist around 1 is reproduced from CFITS. In another manner,
two sets of MFR, which are highly twisted and slightly coupled, are derived by
the MHS method. The latter one is better constrained by the high-resolution
observations, such as the filament fibrils, pre-eruptive braiding
characteristics and the eruptive double-J shaped hot channel. Overall, our work
shows the MHS method is more promising to reproduce the magnetic fine
structures that can well match the observations not only in the chromosphere
but also in the corona. This initiates the necessity of reconsidering the
simplification of low atmosphere for currently widely used nonlinear force-free
extrapolation method, since such assumption will not only omit the magnetic
structures at low atmosphere but also affect those obtained in the corona, and
therefore bringing in ambiguity in interpreting the solar eruption.Comment: 19 pages, 6 figures, accepted by Ap