Some Solar Spectroscopy with the SHG (30Apr2016)

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Some Solar Spectroscopy with the SHG (30Apr2016)

Post by p_zetner » Tue May 31, 2016 5:12 pm

Greetings Everyone.

Here are some spectroscopic studies from my 30Apr SHG imaging session.


A High Velocity Surface Feature.

This image shows two SHG images, one taken at line centre and one at 0.7A to the blue. The spectrum associated with the small absorption feature is shown. There is a large absorption spike to the blue, indicating radial velocity towards the Earth. This feature is not visible in the line centre image.
Montage_101029_00_03_proc0_ARL7x7 _duo_spec_siz.png
The second image shows a difference spectrum, obtained by subtracting the feature spectrum from the quiescent disk spectrum. A scaled quiescent disk spectrum is shown for comparison. The wavelength scale has been converted to radial velocity with zero set at the quiescent spectrum line centre of H alpha.
Feature_diff_plot_siz.png (132.06 KiB) Viewed 1132 times
It's clear to see that this (tiny) feature arises from hydrogen absorption in gas travelling away from the solar surface at about 50 km/s. Although this is fast, it's smaller than the 620 km/s (nominal) escape velocity from the Sun, so I don't think it's a CME. A second image, taken about an hour later showed no sign of the feature.

The Magnetic Field of a Sunspot.

I'm interested in setting up a solar magnetograph and wanted to test whether the spectral dispersion and resolution I'm typically using in my SHG are sufficient to resolve the magnetic (Zeeman) splitting of an absorption line. One of the typical lines used by the professionals to measure photospheric magnetic fields is the Fe I 6302.5 Angstrom line. This is an attractive line because it's narrow, photospheric, relatively isolated and has a large sensitivity to magnetic fields (large Lande g factor).

Here are spectra around this Fe line taken near a sunspot. There are two atmospheric (telluric) oxygen lines visible which can be useful for wavelength calibrations in these types of studies.
112343_1304_plots_v2.png (172.11 KiB) Viewed 1132 times
Although a clear Zeeman splitting of the Fe I 6302.5 Angstrom line is not visible (a triplet of lines would be expected), a definite broadening of this line occurs in the penumbra of the spot. I thought I would use some spectrum fitting software (Origin 8.1) to try and extract the field. Here's the result with the green curve showing the overall fit to the observed spectrum (black dots). The other coloured curves are explained below.
Plot2_Fit1_siz_revMar2017.png (175.1 KiB) Viewed 714 times
There were a number of steps involved in this fitting procedure. One: Fit line profiles (I used Lorentzians) to the isolated Fe I 6301.5 Angstrom line and the isolated telluric O2 line in the spectrum taken far from the sunspot (plot #1). Two: Use the O2 profiles to subtract the telluric component of the spectrum. (The fitted telluric spectrum is shown as the brown curve in the figure above.) Three: Use the Fe I 6301.5 Angstrom Lorentzian line profile to fit the central portion (pi component) of the Fe I 6302.5 Angstrom line. (Blue curve in the figure.) Four: Subtract the fitted pi component from the Fe I 6302.5 Angstrom line leaving behind the remaining two members (sigma components) of the Zeeman triplet. (Red curve in the figure.) The magnetic field strength is determined by the splitting between the two sigma components (inset spectrum) through the calculation in the figure below. The result of 2.1 kG is reasonable for a strong spot but is sensitive to all the steps used in the spectrum fitting procedure.
Zeeman splitting formula v2 siz revMar2017.png
Zeeman splitting formula v2 siz revMar2017.png (111.33 KiB) Viewed 714 times
As far as the success of the measurement goes, I think I will have to improve the spectral characteristics of my SHG for use as a magnetograph. On the other hand, the final instrument will make use of circular polarizers to help extract the Zeeman sigma components (and also determine the polarity of the magnetic field) so I may just be able to manage it.
Last edited by p_zetner on Wed Mar 08, 2017 3:45 pm, edited 1 time in total.

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Re: Some Solar Spectroscopy with the SHG (30Apr2016)

Post by Carbon60 » Tue May 31, 2016 8:36 pm

Impressive work, Peter. The broadening is clear to see, but not splitting, as you say. A resolution challenge. Good luck with enhancing the performance of your SHG. It's also impressive to see your analysis of the fast moving feature.

Nice science.

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Re: Some Solar Spectroscopy with the SHG (30Apr2016)

Post by Derek Klepp » Thu Jun 02, 2016 7:12 am

Thanks Peter the Physics is beyond me but I,ll come back and read this again.I only wish that more use of the sun had been made in my High School Physics classes.May have made it more relative. As an aside and as a Beekeeper I often wonder about the influence of the Solar Cycle and all associated activities on the actual individuals themselves.
Thanks for the post.

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Re: Some Solar Spectroscopy with the SHG (30Apr2016)

Post by Montana » Thu Jun 02, 2016 2:26 pm

Wow! amazing work :bow the fast moving feature is quite a catch. The rest is a bit beyond me but I thoroughly enjoyed reading it :bow2


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Re: Some Solar Spectroscopy with the SHG (30Apr2016)

Post by marktownley » Mon Jun 06, 2016 8:33 pm

Superb science there Peter!
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