On the Observability of Optically Thin Coronal Hyperfine Structure Lines
  We present Cloudy calculations for the intensity of coronal hyperfine lines
in various environments. We model indirect collisional and radiative
transitions, and quantify the collisionally-excited line emissivity in the
density-temperature phase-space. As an observational aid, we also express the
emissivity in units of the continuum in the 0.4--0.7 keV band. For most
hyperfine lines, knowledge of the X-ray surface brightness and the plasma
temperature is sufficient for rough estimates. We find that the radiation
fields of both Perseus A and Virgo A can enhance the populations of highly
ionized species within 1 kpc. They can also enhance line emissivity within the
cluster core. This could have implications for the interpretation of spectra
around bright AGN. We find the intensity of the $^{57}$Fe XXIV {\lambda}3.068
mm to be about two orders of magnitude fainter than previously thought, at
about 20 {\mu}K. Comparably bright lines may be found in the infrared. Finally,
we find the intensity of hyperfine lines in the Extended Orion Nebula to be
low, due to the shallow sightline. Observations of coronal hyperfine lines will
likely be feasible with the next generation of radio and sub-mm telescopes.
