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Galaxy and occupies perhaps a quarter of the volume of the Galactic disk.Decoding the spectrum of the Galactic diffuse ionizing field is of fundamentalinterest. This can be done via direct measurements of ionization fractions ofvarious elements. Based on current physical models for the WIM we predictedthat mid-IR fine structure lines of Ne, Ar and S would be within the grasp ofthe Mid-Infrared Imager-Medium Resolution Spectrometer (MIRI-MRS), an IntegralField Unit (IFU) spectrograph, aboard the James Webb Space Telescope (JWST).Motivated thus we analyzed a pair of commissioning data sets and detected[NeII] 12.81 $\mu$m, [SIII] 18.71 $\mu$m and possibly [SIV] 10.51 $\mu$m. Theinferred emission measure for these detections is about 10 ${\rm cm^{-6} pc}$,typical of the WIM. These detections are broadly consistent with expectationsof physical models for the WIM. The current detections are limited byuncorrected fringing (and to a lesser extent by baseline variations). In duecourse, we expect, as with other IFUs, the calibration pipeline to deliverphoton-noise-limited spectra. The detections reported here bode well for thestudy of the WIM. Along most lines-of-sight hour-long MIRI-MRS observationsshould detect line emission from the WIM. When combined with opticalobservations by modern IFUs with high spectral resolution on large ground-basedtelescopes, the ionization fraction and temperature of neon and sulfur can berobustly inferred. Separately, the ionization of helium in the WIM can beprobed by NIRspec. Finally, joint JWST and optical IFU studies will open up anew cottage industry of studying the WIM on arcsecond scales.

Mid-infrared fine structure lines from the Galactic Warm Ionized Medium