  We present coordinated coronal observations of the August 21, 2017 total
solar eclipse with the Airborne Infrared Spectrometer (AIR-Spec) and the
Extreme-ultraviolet Imaging Spectrometer (EIS). These instruments provide an
unprecedented view of the solar corona in two disparate wavelength regimes, the
near to mid infrared (IR) and the extreme ultraviolet (EUV), opening new
pathways for characterizing the complex coronal plasma environment. During
totality, AIR-Spec sampled coronal IR spectra near the equatorial west limb,
detecting strong sources of Mg VIII, S XI, Si IX, and Si X in two passbands
encompassing 1.4 - 4 $\mu$m. We apply emission measure (EM) loci analysis to
these IR emission lines to test their capacity as coronal temperature
diagnostics. The density-sensitive Fe XII 186.9 \r{A}/192.4 \r{A} line pair
supplies spatially resolved, line-of-sight electron densities, supporting the
EM loci analysis. From this, we find EM loci intersections at temperatures of
$10^{6.13}$ K at 30 arcsec from the limb and $10^{6.21}$ K at 100 arcsec.
Applying the same EM loci analysis to 27 EIS emission lines associated with
seven ion species (Fe X-XIV, S X, and Si X) confirms these results, displaying
strong evidence of isothermal plasma throughout the region. However, the IR EM
loci analysis suffers from moderate uncertainties. The likely sources include:
poor signal, infrared contamination from a prominence, and photoexcitation by
continuum radiation. Regardless, we demonstrate that EUV spectral data are
valuable constraints to coronal infrared emission models, and will be powerful
supplements for future IR solar observatories, particularly DKIST.

Solar and Stellar Astrophysics