  Emission-lines of magnesium at 12 microns have been observed in the spectrum
of Procyon. We reproduce the observed, disk-averaged line flux from Procyon (as
well as the observed intensity profiles from the Sun) by calculating the line
formation, relaxing the assumption of Local Thermodynamic Equilibrium. We find
that the lines in Procyon are formed in the photosphere in the same manner as
the solar lines. We discuss our modeling of these Rydberg lines and evaluate,
among other things, the importance of the ionizing flux and updated model-atom
parameters. The lines are of large diagnostic value for measurements of stellar
magnetic-fields through their Zeeman splitting. We have not, however, detected
splitting of the MgI lines in Procyon. Using simple arguments, we believe we
would have detected a magnetic field, had it been of a strength larger than
approximately 800 Gauss covering more than a quarter of the surface. We discuss
the prospects for future use of the Zeeman-sensitive, mid-infrared, MgI
emission lines as a diagnostic tool for stellar magnetic fields.
