An Observationally Constrained 3D Model

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Tuesday, October 24, 2017

A 3D Potential-Field Source-Surface Model for the Evolution of Longitude-Dependent Coronal Structures: We explore a complementary approach to traditional magnetogram-based coronal field solutions. Rosa Everson and Mausumi Dikpati apply a three-dimensional potential-field source-surface model that uses a huge set of the white-light coronal images from 40 years' observations of Mauna Loa Solar Observatory for deriving 3D coronal morphology.

Carrington rotation 1775 image
Selected images from Carrington rotation 1775 during the 1986 solar minimum. Images represent consecutive quarter turns of the rotation, with MLSO white-light images in the top row, corresponding model solutions from our technique in the center row, and magnetogram-based solutions from Wilcox Solar Observatory in the bottom row. This is the most complex rotation of the 1986 solar minimum.

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 PFSS model, we investigate the use of white-light coronal images from Mauna Loa Solar Observatory 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 non-uniqueness in our results. Future work will incorporate magnetogram-based solutions prior to applying the iterative technique.

The Astrophysical Journal, 2017 in press