Emission lines of FeX in active region spectra obtained with the Solar Extreme-ultraviolet Research Telescope and Spectrograph
Fully relativistic calculations of radiative rates and electron impact excitation cross-sections for FeX are used to derive theoretical emission-line ratios involving transitions in the 174-366Å 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 FeX emission features are detected for the first time in SERTS spectra, while the 3s23p52P3/2-3s23p4(1S)3d 2D3/2 transition at 195.32Å is identified for the first time (to our knowledge) in an astronomical source. The most useful FeX electron density (Ne) 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 Ne = 108 and 1011cm-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 Ne, although this requires an accurate evaluation of the instrument intensity calibration over a relatively large wavelength range. However, if the weak 324.73Å line of FeX 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 Ne determined from other diagnostic ratios in the EUVE spectra. A comparison of several theoretical extreme-ultraviolet FeX 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.