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HeIIOffLimb
HeIIOffLimb

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At line 1 changed 3 lines
[{ALLOW edit EISMainUsers}]
[{ALLOW view Anonymous}]
!!!He II 256 in off-limb quiet Sun spectra
!!He II 256 in off-limb quiet Sun spectra
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The off-limb spectrum being used is from 2007 March 9 20:03. Using the procedure outlined [elsewhere on the wiki|https://vsolar.mssl.ucl.ac.uk/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.
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.
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!!Fit to 256 feature
!Fit to 256 feature
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!!Si X 256.366
!Si X 256.366
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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.
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.
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!!Fe X 256.398
!Fe X 256.398
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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:
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:
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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.
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.
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!!Fe XII 256.410
!Fe XII 256.410
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!!Fe XIII 256.422
!Fe XIII 256.422
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!!Summary
!Summary
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!!!Variation of He II 256 profile across the limb
To further investigate the He II 256 line, a large raster obtained at the limb has been studied to see how the line profile varies crossing from the disk to the limb.
The raster was obtained on 2007 December 12 at 10:42 (cam_ar_limb) and uses the 2" slit to raster an area of 360" x 512". The observation covers a large active region, but the bottom of the raster covers a relatively quiet region.
He II 256 spectra were generated for 25 positions by averaging Y-pixels 5 to 40 for 25 X-pixels spaced at 7 pixel intervals, beginning with pixel 2 which is inside the limb. I.e., the X-pixels used were 2, 9, 16, 23, etc., up to 170. The spectra were creating using the routine eis_mask_spectrum by creating pixel masks for each position.
The 25 spectra that resulted are plotted in the image below. The limb is found at about X=860. The Y-axis is intensity in erg/cm2/s/sr/angstrom, the X-axis gives the wavelength in angstroms but with 256 angstroms subtracted in order to make the plots easier to read.
[{Image src = 'he2_profiles.png'}]
If we assume the He II component is on the short wavelength side, then we see it becoming weaker at X=868, which is just above the limb. By X=910 the short wavelength component is about a factor 2 smaller than the long wavelength component which we assume is the blend of the coronal lines. However, as X increases the short wavelength component remains approximately constant relative to the long wavelength component. Both components fall off in intensity with height
This behavior is not consistent with He II which we expect to drop sharply above the limb, but then remain approximately constant in intensity beyond a certain height due to the scattered light contribution. Instead it actually seems to behave like a coronal line.
The conclusion from this is that there is likely a coronal line at the wavelength of the He II 256.32 line and this is revealed when we cross the limb. Possibly this line could be one of the Si X, Fe X, Fe XII or Fe XIII lines discussed earlier if one of the lines has an incorrect wavelength. Alternatively it could be a previously unknown transition.
This page (revision-7) was last changed on 18-Apr-2021 19:12 by PeterYoungTop