Neutral Helium Triplet Spectroscopy of Quiescent Coronal Rain with Sensitivity Estimates for Spectropolarimetric Magnetic Field Diagnostics
On account of its polarizability and magnetic field sensitivity, as well as the role of neutral helium in partially ionized solar environments, the neutral helium triplet (orthohelium) system provides important, yet under-utilized, diagnostics of solar coronal rain. This work describes off-limb observations of coronal rain in NOAA Active Region 12468 obtained in the He I 10830 $Å$ triplet using the Massively MultipleXed Imaging Spectrograph experiment at the Dunn Solar Telescope along with co-temporal observations from NASA's Solar Dynamics Observatory and the Interface Region Imaging Spectrograph. We detect rain simultaneously in the IRIS 1400 $Å$ and 2796 $Å$ channels and in He I 10830 $Å$. The large degree of spatial coherence present between all channels agrees with previous observations of the multi-thermal nature of coronal rain. A statistical analysis of He I spectral profiles for rain identified via automated detection indicate He I line radiances are, on average, $10^{4}$ ergs cm$^{-2}$ s$^{-1}$ sr$^{-1}$; the average translational velocity is 70 km s$^{-1}$ and Doppler widths are distributed around 10 km s$^{-1}$. Based on these results, forward models of expected He I polarized signals allow us to estimate, using synthetic observables and an inversion algorithm including fits for the scattering angle constraining the material's location along-the-line of sight, the magnetic sensitivity of the upcoming National Science Foundation's Daniel K. Inouye Solar Telescope. We predict joint observations of the He I 10830 $Å$ and 5876 $Å$ multiplets, using first-light instrumentation, will provide inverted magnetic field errors of $\pm3.5$ G ($2σ$) for spatial scales of $0.5''$ ($\sim360$ km) assuming dynamically-limited integration times of 5.5 seconds.