Solar forcefree magnetic fields
Abstract
The structure and dynamics of the solar corona is dominated by the magnetic field. In most areas in the corona magnetic forces are so dominant that all nonmagnetic forces such as plasma pressure gradients and gravity can be neglected in the lowest order. This model assumption is called the forcefree field assumption, as the Lorentz force vanishes. This can be obtained by either vanishing electric currents (leading to potential fields) or the currents are coaligned with the magnetic field lines. First we discuss a mathematically simpler approach that the magnetic field and currents are proportional with one global constant, the socalled linear forcefree field approximation. In the generic case, however, the relationship between magnetic fields and electric currents is nonlinear and analytic solutions have been only found for special cases, like 1D or 2D configurations. For constructing realistic nonlinear forcefree coronal magnetic field models in 3D, sophisticated numerical computations are required and boundary conditions must be obtained from measurements of the magnetic field vector in the solar photosphere. This approach is currently a large area of research, as accurate measurements of the photospheric field are available from groundbased observatories such as the Synoptic Optical Longterm Investigations of the Sun and the Daniel K. Inouye Solar Telescope (DKIST) and spaceborn, e.g., from Hinode and the Solar Dynamics Observatory. If we can obtain accurate forcefree coronal magnetic field models we can calculate the free magnetic energy in the corona, a quantity which is important for the prediction of flares and coronal mass ejections. Knowledge of the 3D structure of magnetic field lines also help us to interpret other coronal observations, e.g., EUV images of the radiating coronal plasma.
 Publication:

Living Reviews in Solar Physics
 Pub Date:
 December 2021
 DOI:
 10.1007/s41116020000274
 arXiv:
 arXiv:1208.4693
 Bibcode:
 2021LRSP...18....1W
 Keywords:

 Solar corona;
 Magnetic fields;
 Forcefree magnetic fields;
 Astrophysics  Solar and Stellar Astrophysics
 EPrint:
 49 pages, 11 figures, Living Reviews in Solar Physics, accepted