  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.
