Emission lines of Fe xi in the 257-407 Å wavelength region observed in solar spectra from EIS/Hinodeand SERTS

Abstract

Theoretical emission-line ratios involving Fe xi transitions in the 257–407 Å wavelength range are derived using fully relativistic calculations of radiative rates and electron impact excitation cross-sections. These are subsequently compared with both long wavelength channel Extreme-Ultraviolet Imaging Spectrometer (EIS) spectra from the Hinode satellite (covering 245–291 Å) and first-order observations (∼235–449 Å) obtained by the Solar Extreme-ultraviolet Research Telescope and Spectrograph (SERTS). The 266.39, 266.60 and 276.36 Å lines of Fe xi are detected in two EIS spectra, confirming earlier identifications of these features, and 276.36 Å is found to provide an electron density (Ne) diagnostic when ratioed against the 257.55 Å transition. Agreement between theory and observation is found to be generally good for the SERTS data sets, with discrepancies normally being due to known line blends, while the 257.55 Å feature is detected for the first time in SERTS spectra. The most useful Fe xi electron density diagnostic is found to be the 308.54/352.67 intensity ratio, which varies by a factor of 8.4 between Ne= 108 and 1011 cm−3, while showing little temperature sensitivity. However, the 349.04/352.67 ratio potentially provides a superior diagnostic, as it involves lines which are closer in wavelength, and varies by a factor of 14.7 between Ne= 108 and 1011 cm−3. Unfortunately, the 349.04 Å line is relatively weak, and also blended with the second-order Fe x 174.52 Å feature, unless the first-order instrument response is enhanced.