New calculations of radiative rates and electron impact excitation cross sections for Fe xi are used to derive emission line intensity ratios involving 3 s ^ { 2 } 3 p ^ { 4 } –3 s ^ { 2 } 3 p ^ { 3 } 3 d transitions in the 180–223 Å wavelength range .
These ratios are subsequently compared with observations of a solar active region , obtained during the 1995 flight of the Solar EUV Research Telescope and Spectrograph ( SERTS ) .
The version of SERTS flown in 1995 incorporated a multilayer grating that enhanced the instrumental sensitivity for features in the \sim 170–225 Å wavelength range , observed in second-order between 340 and 450 Å .
This enhancement led to the detection of many emission lines not seen on previous SERTS flights , which were measured with the highest spectral resolution ( 0.03 Å ) ever achieved for spatially resolved active region spectra in this wavelength range .
However , even at this high spectral resolution , several of the Fe xi lines are found to be blended , although the sources of the blends are identified in the majority of cases .
The most useful Fe xi electron density diagnostic line intensity ratio is I ( 184.80 Å ) /I ( 188.21 Å ) .
This ratio involves lines close in wavelength and free from blends , and which varies by a factor of 11.7 between N _ { e } = 10 ^ { 9 } and 10 ^ { 11 } cm ^ { -3 } , yet shows little temperature sensitivity .
An unknown line in the SERTS spectrum at 189.00 Å is found to be due to Fe xi , the first time ( to our knowledge ) this feature has been identified in the solar spectrum .
Similarly , there are new identifications of the Fe xi 192.88 , 198.56 and 202.42 Å features , although the latter two are blended with S viii/Fe xii and Fe xiii , respectively .