!!He II 256 in off-limb quiet Sun spectra Since the cool He II 256 will become weak in off-limb spectra then this is a good place to study the contributions of the blending coronal lines. According to CHIANTI the wavelengths of the various lines are: He II 256.317\\ He II 256.318\\ Si X 256.366\\ Fe X 256.398\\ Fe XII 256.410\\ Fe XIII 256.422\\ The first thing to note is that the four coronal lines all lie on the long wavelength side of the He II lines. The two He II lines themselves can effectively be treated as a single line. The off-limb spectrum being used is from 2007 March 9 20:03. Using the procedure outlined [elsewhere on the wiki|http://msslxr.mssl.ucl.ac.uk:8080/eiswiki/attach/DataProAnalysis/gauss_pixel_masks.pdf] a spatial region has been averaged to produce a single spectrum from which emission lines can be measured. Using standard density diagnostics we find the density is about 10^8.5. !Fit to 256 feature In the off-limb spectra, the feature at 256.3 is seen to comprise of two components that can be fit with two Gaussians. I find fit parameters of 256.321 0.071 37.1\\ 256.413 0.106 139.2 (centroid, width and intensity, respectively). Based on the wavelengths above it seems, to first approximation, that the long wavelength Gaussian represents the coronal lines, and the short wavelength Gaussian the He II lines. Below we check the combined intensity of the four coronal lines. !Si X 256.366 This is the easiest line to deal with and is also the strongest of the four lines in the off-limb spectrum. 256.4 is related to the nearby 261.0 line by a branching ratio which means that the two lines have a fixed ratio in all conditions: 256.4/261.0 should be 1.12. The intensity of 261.0 is found to be 84.3, implying the intensity of 256.4 is 94.7. !Fe X 256.398 This transition is one of a number of 2P - 4D transitions in this part of the spectrum. By far the strongest is the Fe X self-blend at 257.26. Two further lines are at 266.1 and 255.4. Firstly it is interesting to check whether the unblended lines actually agree with each other. The measured intensities are: Fe X 257.296 122.8\\ Fe X 266.122 3.3\\ Fe X 255.462 2.4 The 257 self-blend is density sensitive relative to the other two lines and between 10^8 and 10^9, the 266/257 ratio is about 0.025, and 255/257 is about 0.019. The measured ratios are 0.025 and 0.014. This agreement is reasonable and suggests that CHIANTI is doing a good job of predicting the strength of the 2P - 4D transtions. Going back to the Fe X 256 line, CHIANTI predicts the 256/257 ratio is about 0.085, giving a predicted intensity for the 256.4 line of 10.4. !Fe XII 256.410 The Fe XII 256 line is a 2D - 4F transition, and there are two other nearby lines from this multiplet that are relatively insensitive to the 256 line. However both lines are blended so it takes a little work to estimate the 256 contribution. The two lines are measured to be S X + Fe XII 259.527 56.0\\ Fe XII + Fe XIII 260.007 9.3 The first line is blended with the much stronger S X line. This can easily be accounted for by noting that the S X 259.5/264.2 ratio is predicted by CHIANTI to be almost constant with a value of 0.688, implying Fe XII has an intensity of 6.5. Estimating the Fe XIII contribution to 260.0 is more tricky. The Fe XIII 251.9/260.0 ratio is density sensitive, but between 10^8 and 10^9 it is not so sensitive with a value of around 47. This implies Fe XIII makes a contribution of 1.5 to the blend at 260.0, leaving 7.8 for Fe XII. CHIANTI predicts that Fe XII 260.0/259.5 should be 0.75, yet the above numbers give 1.2, so there's a significant discrepancy here. Now CHIANTI predicts that Fe XII 256.4/259.5 should be about 2.7, and 256.4/260.0 should be about 3.5. This means that the Fe XII 256.4 will be somewhere between 17.6 and 27.3 depending on which ratio is used. !Fe XIII 256.422 This line can be estimated by making use of the stronger Fe XIII line at 251.96, however 256.42/251.96 is density sensitive. For the off-limb spectrum we know the density is low, and the 256.42/251.96 is actually relatively insensitive to density over 10^8 to 10^9, with a value of about 0.17. The 251.96 measured intensity is 69.0, and the so predicted 256.42 intensity is 11.7. !Summary Combining the four estimated intensities gives a total of either 134.4 or 144.1, depending on which estimate of the Fe XII line intensity is used. These values are actually in very good agreement with the strength of the long wavelength Gaussian in the fit to the 256.3 feature mentioned above. This suggests that the method outlined above for estimating the intensities of the coronal lines actually works quite well. __Caveat__ In estimating the intensities above, it was necessary to assume a low density of < 10^9 in order to apply some of the ratios. In particular, this was necessary for Fe X, XII and XIII. This method would need to be revised for, e.g., an active region observation.