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 ( \sim 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 ( N _ { e } ) 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 N _ { e } = 10 ^ { 8 } and 10 ^ { 11 } 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 N _ { e } = 10 ^ { 8 } and 10 ^ { 11 } 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 .