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whats the story about helium D3 line?

Posted: Mon Dec 08, 2014 4:21 pm
by highfnum
I notice a few shots in the helium line
what is significance of this line?
where is it exactly I know its near sodium

Re: whats the story about helium D3 line?

Posted: Mon Dec 08, 2014 9:03 pm
by Merlin66
http://adsabs.harvard.edu/full/1964ZA.....60..207B
To quote:
- It is absent in the normal solar spectrum
- Can flare in absorption or emission
- produced by a different mechanism to Ha

Since it's discovery in 1868 the D3 was thought to be a new element - Helium. It was only in 1895 that the gas was finally identified.
Hope this helps.

Re: whats the story about helium D3 line?

Posted: Tue Dec 09, 2014 5:16 am
by swisswalter
Hi Ken

an interesting story

Re: whats the story about helium D3 line?

Posted: Tue Dec 09, 2014 4:27 pm
by p_zetner
The He I D3 Line is somewhat of a favourite of mine!

Here are some of the facts I'm familiar with.
Line Centre: 5875.7 Angstroms.
Linewidth in absorption features (plage regions) is ~ 0.5 Angstroms fwhm.

This line is generally invisible on the solar disk (absorption is too weak) except in regions of plage and filaments where absorption is enhanced. The correlation between He absorption features and H alpha features can be seen in the spectroheliograms I posted here in July 2014:

http://solarchat.natca.net/viewtopic.php?f=8&t=12392

The line is very bright at the limb and in prominences. There is a bright D3 limb "band" visible in telescopes with very high angular resolution. Measurements show that this band corresponds to He D3 emission from a shell with a sharply defined lower edge about 1400km above the photosphere.

The mechanism of He D3 absorption is interesting (to me!). Extreme ultraviolet (EUV) radiation from the corona shines downward, illuminating the He from above. This EUV light is energetic enough to ionize the He. Recombination of the ions and free electrons then produces neutral He in excited states of high excitation energy (~20eV) from which we see absorption at the D3 line as well as the He IR (10830 Angstroms) line. The observed shell in limb emission is consistent with this idea. The bottom of the shell corresponds to the maximum depth that the coronal EUV radiation can penetrate (downwards). Also, because radiation from the corona is required, He D3 absorption features don't appear where coronal holes are present. He IR images are used as "proxy" maps of coronal holes, the only tool to map coronal holes from the ground. I guess He D3 images could serve the same purpose but the D3 absorption is much weaker than the IR absorption. Here is a comparison of one of my He D3 images with a professional He 10830A IR image:

http://solarchat.natca.net/viewtopic.php?f=8&t=11761

As opposed to other chromospheric gases, He is optically thin and can be used (in emission, at the limb) to measure chromospheric magnetic fields by a phenomenon called the Hanle effect. The Hanle effect is responsive to much weaker fields than the Zeeman effect and such measurements have given the first accurate picture of chromospheric magnetic fields.

Finally, the D3 line has also been used as a diagnostic of solar flare dynamics. The additional information this line provides is of great interest to the experts but beyond my understanding, at the moment.

Cheers.
Peter.

Re: whats the story about helium D3 line?

Posted: Tue Dec 09, 2014 4:40 pm
by highfnum
thx for replies

if you don't normally see line how did you record it?

Re: whats the story about helium D3 line?

Posted: Tue Dec 09, 2014 4:44 pm
by p_zetner
One of the image processing steps involves subtracting an on-band image from a nearby continuum image.

Re: whats the story about helium D3 line?

Posted: Tue Dec 09, 2014 8:39 pm
by Derek Klepp
Peter and Ken thanks for the explanations.

Re: whats the story about helium D3 line?

Posted: Tue Dec 09, 2014 9:06 pm
by swisswalter
Hi Peter

thank you very much for the explanations. I love that stuff

Re: whats the story about helium D3 line?

Posted: Tue Dec 09, 2014 11:30 pm
by marktownley
Great info folks!

Re: whats the story about helium D3 line?

Posted: Fri Dec 12, 2014 10:52 am
by Marcello
Hi Peter,
this is very interesting and your explanation very clear, thanks!