He I spectropolarimetry of a supersonic coronal downflow within a sunspot umbra

TL;DR

This paper presents spectropolarimetric observations of a supersonic downflow in a sunspot umbra, revealing high magnetic field strengths and evidence of a radiative shock caused by the downflow impacting the lower atmosphere.

Contribution

It provides the first detailed spectropolarimetric analysis of a supersonic coronal downflow in a sunspot umbra, including measurements of magnetic fields and shock diagnostics.

Findings

Downflow speeds reach ~200 km/s

Magnetic field strength in emission core is ~2.4 kG

Evidence of radiative shock at the downflow impact site

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$^{-1}$ 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 red-shifted tails are also observed at the downflow terminus. From the polarized spectra, we infer longitudinal magnetic field strengths of ${\sim}2.4$ 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 ${\sim}2.8$ 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 (${\sim}$10 kK), while a differential emission measure analysis using SDO/AIA data indicates significant increases in radiative cooling for temperatures between $\sim$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.