He i Spectropolarimetry of a Supersonic Coronal Downflow Within a Sunspot Umbra

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. using observations acquired in the He i 10830 Å triplet by the Facility Infrared Spectropolarimeter. Downflowing material accelerating along a cooled coronal loop reaches peak speeds near 200 km s−1 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 redshifted tails are also observed at the downflow terminus. From the polarized spectra, we infer longitudinal magnetic field strengths of ∼2.4 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 ∼2.8 kG as inferred using the Ca i 10839 Å 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 Solar Dynamics Observatory/Atmospheric Imaging Assembly 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.