Coronal Magnetic Field Models

Abstract

Coronal magnetic field models use photospheric field measurements as boundary condition to model the solar corona. We review in this paper the most common model assumptions, starting from MHD-models, magnetohydrostatics, force-free and finally potential field models. Each model in this list is somewhat less complex than the previous one and makes more restrictive assumptions by neglecting physical effects. The magnetohydrostatic approach neglects time-dependent phenomena and plasma flows, the force-free approach neglects additionally the gradient of the plasma pressure and the gravity force. This leads to the assumption of a vanishing Lorentz force and electric currents are parallel (or anti-parallel) to the magnetic field lines. Finally, the potential field approach neglects also these currents. We outline the main assumptions, benefits and limitations of these models both from a theoretical (how realistic are the models?) and a practical viewpoint (which computer resources to we need?). Finally we address the important problem of noisy and inconsistent photospheric boundary conditions and the possibility of using chromospheric and coronal observations to improve the models. Keywords Sun • Corona • Magnetic Field • Active Region • MHD 1 Why Do We Need to Model the Coronal Field? Because the most spectacular and dangerous manifestations of solar activity, flares and coronal mass ejections (CMEs), occur in the corona, and are almost certainly powered by free magnetic energy stored in the coronal magnetic field, modeling and measurement of the coronal field have been urgent goals of solar physics for many years. Only in the last decade B T. Wiegelmann

The Solar Energetic Particle Event of 2010 August 14: Connectivity with the Solar Source Inferred from Multiple Spacecraft Observations and Modeling

“…Moreover, static coronal field models cannot reproduce the changing magnetic fields above the evolving active regions that are likely to give rise to CMEs, and hence may not give an accurate idea of the field line connections just prior to CME eruption and SEP onset. Limitations of coronal field models have been recently reviewed by Wiegelmann et al (2015).…”

A Study of Pre-flare Solar Coronal Magnetic Fields: Magnetic Flux Ropes

“…However, as also pointed out by the authors, such results might strongly depends on the quality or reliability of the coronal magnetic field reconstructions. Currently there are many methods available for NLFFF extrapolations from the vector magnetograms, but different methods seem to produce rather inconsistence results between each other (e.g., DeRosa et al 2009;Régnier 2013;Aschwanden et al 2014;Duan et al 2017;Wiegelmann et al 2017). Thus any results based on any single NLFFF code must be taken with cautions, and independent studies with different codes are required for a better inspection.…”

Instrumentation for solar spectropolarimetry: state of the art and prospects

“…others, are widely used in space weather programs to e.g., derive the coronal magnetic field from photospheric extrapolations. 54 To provide continuous coverage from the ground, typically a network of identical instruments located around the globe is employed. 55 Two notable examples are the Oscillations Network Group (GONG 56 ), that only measures circular polarization, and the upcoming European-funded, Solar Physics Research Integrated Network Group (SPRING 57 ) which will provide the full Stokes vector.…”