Sunspots are the hallmark of solar activity. High resolution spectropolarimetry is rapidly progressing toward a better physical understanding of small-scale structures in sunspot penumbra and umbra. However, our understanding of the magnetic structure of sunspots in the chromosphere and corona is lagging behind. In our modeling of magnetic fields in the corona, we still rely on various extrapolation methods, which do not include a realistic atmosphere nor are they constrained by the observations. Our ISSI team will explore different approaches to address this deficiency.

Nonlinear force-free field reconstructions that employ routinely available full-disk photospheric vector magnetograms as bottom boundary conditions represent the state-of-the-art of coronal magnetic field modeling. Such reconstructions, however, are not unique and suffer from an inconsistency between a force-free coronal magnetic field and non-force-free photospheric boundary condition, from which the coronal reconstruction is performed. Realistic time-dependent MHD models could help greatly, but are not expected to be routinely available any time soon. The use of chromospheric vector magnetograms can aid the coronal part of the magnetic model, but does not help to build the magnetic model between the photospheric and chromospheric  levels.

Our ISSI team will use a combination of state-of-the-art modeling with existing and near future high-resolution observations (e.g. from new DKI 4-meter aperture Solar Telescope, DKIST) to evaluate existing approaches in modeling the chromospheric and coronal magnetic fields and identify key failure points in such modeling. The goal is to integrate newly available chromospheric and/or coronal magnetic field data with the vector photospheric magnetograms to improve the magnetic field reconstructions. We will select several well-observed active regions, which have both chromospheric and coronal magnetic field diagnostics from optical and radio spectropolarimetry, construct their 3D coronal magnetic field models using various complementary techniques, and validate these models using the observations. As a result of this effort we will create better constrained modelsof the coronal magnetic field, available for public use in the form of data cubes, codes, and scientific publications.