An Observationally Constrained 3D Potential-field Source-surface Model for the Evolution of Longitude-dependent Coronal Structures

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Abstract
The improvement of techniques for realistically modeling the solar magnetic field has been a priority in solar physics for decades. The challenge of creating synoptic maps of the photosphere that reliably reflect conditions at all locations concurrently is a major limitation to progress in this area. White-light coronal images, which contain morphological information about the 3D corona at the solar limb, have been largely overlooked as a resource for constraining or correcting synoptic maps. We explore a complementary approach to traditional magnetogram-based coronal field solutions that makes use of these images. Applying a modified 3D potential-field source-surface (PFSS) model, we investigate the use of MLSO white-light coronal images for deriving 3D coronal morphology by empirically fitting model solutions with observations only. Applying an iterative technique to coronal image data from the solar minima preceding Cycles 22, 23, and 24, and the ascending phase of Cycle 23, we obtain model solutions as linear combinations of low order and degree spherical harmonics. We find that the 3D morphology produced by our method agrees qualitatively with traditional magnetogram-based PFSS approaches for coronas that are dipole dominated. For more complex coronas, additional constraints are needed to account for polarity and correct interpretation of coronal structures. Estimates of the relative strength of dipoles versus multipoles in the coronal field also agree with traditional methods, but the contributions of specific multipoles do not, revealing nonuniqueness in our results. Future work will incorporate magnetogram-based solutions prior to applying the iterative technique.

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