From Forbidden Coronal Lines to Meaningful Coronal Magnetic Fields

TL;DR

This paper reviews methods for measuring coronal magnetic fields via polarized light in M1 lines, emphasizing new observational strategies and the potential for breakthroughs with upcoming coronagraphs.

Contribution

It introduces promising spectral lines and approaches for coronal magnetic field diagnostics, addressing common skepticisms and proposing future observational techniques.

Findings

Identification of the [Fe XI] 789.2nm line as a prime target for magnetic field measurements.

Discussion of three new approaches: single point inversions, model comparisons, and tomography.

Potential for significant advances with next-generation coronagraphs.

Abstract

We review methods to measure magnetic fields within the corona using the polarized light in magnetic-dipole (M1) lines. We are particularly interested in both the global magnetic-field evolution over a solar cycle, and the local storage of magnetic free energy within coronal plasmas. We address commonly held skepticisms concerning angular ambiguities and line-of-sight confusion. We argue that ambiguities are in principle no worse than more familiar remotely sensed photospheric vector-fields, and that the diagnosis of M1 line data would benefit from simultaneous observations of EUV lines. Based on calculations and data from eclipses, we discuss the most promising lines and different approaches that might be used. We point to the S-like [Fe {\sc XI}] line (J=2 to J=1) at 789.2nm as a prime target line (for ATST for example) to augment the hotter 1074.7 and 1079.8 nm Si-like lines of [Fe {\sc XIII}] currently observed by the Coronal Multi-channel Polarimeter (CoMP). Significant breakthroughs will be made possible with the new generation of coronagraphs, in three distinct ways: (i) through single point inversions (which encompasses also the analysis of MHD wave modes), (ii) using direct comparisons of synthetic MHD or force-free models with polarization data, and (iii) using tomographic techniques.