The state of nonlinear force-free magnetic field extrapolation

Abstract

Magnetic field extrapolation is the construction of a model solution for the coronal magnetic field in active regions from magnetic boundary data originating close to the Sun's surface. The nonlinear force-free model (in which the electric current density is parallel to the magnetic field) is often adopted to describe the coronal field. The solution of the nonlinear force-free equations is a challenging computational task, and the application of codes to available boundary data has revealed a number of significant problems with nonlinear force-free extrapolation. This paper summarises the present status of coronal field extrapolation, and describes some recent developments. © 2013 Published under licence by IOP Publishing Ltd.