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
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<sup>8.5</sup>.
Si X 256.366#
This is the easiest line to deal with and is also by far 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 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 the CHIANTI problem 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.