Coronal Force-Free Field Extrapolations

Abstract

Solar corona is the source region of many explosive events, e.g., flares and CMEs, which play a leading role in causing adverse space weather. All the solar eruptions are energized from the magnetic fields, which is generated below the photosphere and transported into the upper atmosphere, i.e., the corona. Consequently, the coronal magnetic field holds the key to understanding the origin of the energetic events. Due to the absence of direct measurement, the three-dimensional magnetic field in the solar corona is usually “extrapolated” in numerical ways from the photosphere where a two-dimensional surface of magnetic vectors can be routinely measured. Up to the present, the nonlinear force-free field (NLFFF) model dominates the physical models for magnetic field extrapolation in the low corona. This chapter briefly reviews the present methods for NLFFF extrapolation with emphasis on the CESE–MHD–NLFFF code.