He I spectropolarimetry of a supersonic coronal downflow within a sunspot umbra
arXiv:2105.12853
Abstract
We report spectropolarimetric observations of a supersonic downflow impacting the lower atmosphere within a large sunspot umbra. This work is an extension of Schad et al. 2016 using observations acquired in the He I 10830 Angstrom triplet by the Facility Infrared Spectropolarimeter. Downflowing material accelerating along a cooled coronal loop reaches peak speeds near 200 km s and exhibits both high speed emission and absorption within the umbra, which we determine to be a consequence of the strong height dependence of the radiatively-controlled source function above the sunspot umbra. Strong emission profiles close to the rest wavelengths but with long red-shifted tails are also observed at the downflow terminus. From the polarized spectra, we infer longitudinal magnetic field strengths of kG in the core portion of the He I strong emission, which we believe is the strongest ever reported in this line. Photospheric field strengths along the same line-of-sight are kG as inferred using the Ca I 10839 Angstrom spectral line. The temperatures of the highest speed He I absorption and the near rest emission are similar (10 kK), while a differential emission measure analysis using SDO/AIA data indicates significant increases in radiative cooling for temperatures between 0.5 and 1 MK plasma associated with the downflow terminus. Combined we interpret these observations in the context of a strong radiative shock induced by the supersonic downflow impacting the low sunspot atmosphere.
Accepted for publication in ApJ