Finite Plasma Beta Three-dimensional Magnetic Field Extrapolation Based on MHD Relaxation Method

Abstract: Three-dimensional (3D) magnetic field in the solar atmosphere provides crucial information to understand the explosive phenomenon such as solar flares and coronal mass ejections. Since it is still hard that we determine the 3D magnetic field from direct observation, a nonlinear force-free field (NLFFF) extrapolation is one of the best modeling methods that provides 3D magnetic field. However, the method is based on zero-beta assumption, i.e., the model ignores the gas pressure gradient and gravitational force. The magnetic field based on an NLFFF is not well reconstructed in high-beta region, such as in chromospheric or lower height layer and in weak field region. To overcome this problem, we need to consider the magnetohydrostatic equilibrium. In this study, we developed a finite plasma beta magnetic field extrapolation method based on magnetohydrodynamic relaxation method. In our method, we consider a force balance of the Lorentz force and the gas pressure. We tested three different schemes and extrapolated 3D magnetic field using an observational photospheric vector magnetic field of one solar active region, which is a quadrupole complex sunspot group and well studied with an NLFFF. The verification of three schemes is performed by comparing the residual force, and we concluded that our methods reduce ~4% of residual force of the previous NLFFF. We also examined the plasma beta profile along the height and found that, in the core of the active region, plasma beta reaches a local minimum of ~0.01 in the lower corona with beta ~1 at the photosphere.