A Novel Inversion Method to Determine the Coronal Magnetic Field Including the Impact of Bound-Free Absorption

The magnetic field governs the corona; hence, it is a crucial parameter to measure. Unfortunately, existing techniques for estimating its strength are limited by strong assumptions and limitations. These techniques include photospheric or chromospheric field extrapolation using potential or nonlinear force-free methods, estimates based on coronal seismology, or direct observations via, e.g., the Cryo-NIRSP instrument on DKIST, which will measure the coronal magnetic field but only off the limb. Alternately, in this work, we investigate a recently developed approach based on the magnetic-field-induced transition (MIT) of Fe x 257.261 angstrom In order to examine this approach, we have synthesized several Fe x lines from two 3D magnetohydrodynamic simulations, one modeling an emerging flux region and the second an established mature active region. In addition, we take bound-free absorption from neutral hydrogen and helium and singly ionized helium into account. The absorption from cool plasma that occurs at coronal heights has a significant impact on determining the magnetic field. We investigate in detail the challenges of using these Fe x lines to measure the field, considering their density and temperature dependence. We present a novel approach to deriving the magnetic field from the MIT using inversions of the differential emission measure as a function of the temperature, density, and magnetic field. This approach successfully estimates the magnetic field strength (up to 18% relative error) in regions that do not suffer from significant absorption and that have relatively strong coronal magnetic fields (>250 G). This method allows regions where absorption is significant to be masked.