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Fully relativistic calculations of radiative rates and electron impact excitation cross‐sections for Fe  x  are used to derive theoretical emission‐line ratios involving transitions in the 174–366 Å wavelength range. A comparison of these with solar active region observations obtained during the 1989 and 1995 flights of the Solar Extreme‐ultraviolet Research Telescope and Spectrograph (SERTS) reveals generally very good agreement between theory and experiment. Several Fe  x  emission features are detected for the first time in SERTS spectra, while the 3s 2 3p 5   2 P 3/2 ‐‐3s 2 3p 4 ( 1 S)3d  2 D 3/2  transition at 195.32 Å is identified for the first time (to our knowledge) in an astronomical source. The most useful Fe  x  electron density ( N  e ) diagnostic line ratios are assessed to be 175.27/174.53 and 175.27/177.24, which both involve lines close in wavelength and free from blends, vary by factors of 13 between   N  e = 10 8   and 10 11  cm −3 , and yet show little temperature sensitivity. Should these lines not be available, then the 257.25/345.74 ratio may be employed to determine  N  e , although this requires an accurate evaluation of the instrument intensity calibration over a relatively large wavelength range. However, if the weak 324.73 Å line of Fe  x  is reliably detected, the use of 324.73/345.74 or 257.25/324.73 is recommended over 257.25/345.74. Electron densities deduced from 175.27/174.53 and 175.27/177.24 for the stars Procyon and α Cen, using observations from the Extreme‐Ultraviolet Explorer (EUVE) satellite, are found to be consistent and in agreement with the values of  N  e  determined from other diagnostic ratios in the EUVE spectra. A comparison of several theoretical extreme‐ultraviolet Fe  x  line ratios with experimental values for a θ‐pinch, for which the plasma parameters have been independently determined, reveals reasonable agreement between theory and observation, providing some independent support for the accuracy of the adopted atomic data.

Emission lines of Fe x in active region spectra obtained with the Solar Extreme‐ultraviolet Research Telescope and Spectrograph