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Questions on the Lyman Alpha Profile

  1. Apr 29, 2015 #1
    A few questions about the solar Lyman Alpha profile as presented here.

    1) This is from a 1978 study - are there more recent (higher resolution) profiles ?
    2) The scale at the bottom is confusing - how does it relate to a wavelength scale ?
    3) Where in the profile is the actual Lyman alpha line measured or is it obtained statistically ?
    4) Why twin peaks and are these replicated in Lyman beta, delta etc ?
    5) Is there any significance to the peak to peak measure (about 33 picometers as I see it) ? Spacing between Lyman Alpha (H) and Lyman Alpha (Deuterium) is of the same order of magnitude so I wondered if there was any connection.

    Any answers would be greatly appreciated.
     
  2. jcsd
  3. Apr 30, 2015 #2

    Quantum Defect

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    (1) Should be lots of measurements of the line. These measurements will be part of a bigger picture, though, like the article you link to -- they are using the line to infer other information. c.f. http://iopscience.iop.org/0004-637X/488/2/760/fulltext/fg3.gif which comes from: http://iopscience.iop.org/0004-637X/488/2/760/fulltext/36121.text.html#fg11

    (2) I have not seen this scale used. It may be something unique to astronomy, or idiosyncratic to a particular instrument.

    (3) The profile is the emission line from a source of Lyman alpha radiation. The dip is the absorption of the center part of the line.

    (4) The Lyman alpha "line" is split due to the presence of two slightly different energy upper states. The ground state is 2S_1/2, while the upper state can be 2P_1/2 or 2P_3/2. The 1/2 or 3/2 label is the total angular momentum (spin plus orbital angular momentum) of the upper state. http://en.wikipedia.org/wiki/Lyman-alpha_line. At first, I thought that this might be the origin of the splitting. But it looks like this is due to D, which is kind of a surprise to me, since D is not very abundant.

    (5) http://iopscience.iop.org/0004-637X/488/2/760/fulltext/36121.text.html#fg11 claims that the small feature is due to the same transition in Deuterium. The earlier paper appeared to not know what the other peak was, which is kind of surprising, too.
     
  4. Apr 30, 2015 #3
    Many thanks for your really detailed reply - much appreciated. Perhaps the Deuterium theory needs to be investigated further. There is a very precise terrestrial measurement (see Section 4) of the frequency difference between Lyman Alpha (H) and Lyman Alpha (D) - this translates to the wavelength difference of 33 picometers which I mention above. If one carefully examines the following more recent profile, the 33 picometer separation of peaks is again evident. Am I reading the scale correctly if I also measure +- 30 pm in the fg3.gif reference you supply ? From what I can read about energy production in the sun, one of the first stages is fusion of two hydrogen atoms to form Deuterium , then Deuterium further fuses to form Helium. So perhaps we should not be surprised to find a greater abundance of Deuterium within the Sun itself.
     
  5. Apr 30, 2015 #4

    Quantum Defect

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    The second figure you link to shows a hydrogen spectrum where there is strong absorption at the line center. This is a single peak, where the top has been reduced in size due to the presence of cooler hydrogen between light source and observer. The depression at the center of the line makes the single peak look like two. The Figure that I linked to shows the same kind of depression, except in that case, there is a depression due to H and a smaller dip/depression due to D.
     
  6. May 1, 2015 #5
    Re point 5, note that the 'small feature' in the original article I quoted is red shifted wrt H alpha. The Deuterium depression in your link is blue shifted so they are not the same. Confusion arises because in the original profile wavelength is increasing from right to left. Would be good to find a more recent high resolution Lyman Alpha profile preferably with 'crosshairs' one could move against a wavelength scale. And lines drawn

    a) at the exact point of measurement of the H Lyman alpha line/doublet
    b) at the precision measured frequency/wavelength of H Lyman alpha (there is a small discrepancy between a and b - Doppler shift ?)
    c) at the precision measured frequency/wavelength of Deuterium Lyman alpha line

    I know that line broadening occurs for a variety of reasons but given the extremely high accuracy of the terrestrially obtained 1s-2s transition frequency, one might infer that any related absorption would be over an extremely narrow frequency band for both H and D. It just seems very co-incidental that the expected difference in wavelength between Lyman Alpha H and Lyman Alpha D (33pm) falls well within the width of the broadening:

    21pm < Δλ < 36pm (from the profile I linked to).
     
  7. May 1, 2015 #6
    Just to make sure I wasn't imagining things, I downloaded some free 'crosshair' software and more carefully measured the peak to peak distance in the various Ly alpha profiles I mention above. I am getting 33 picometers every time.

    Δλ=-cΔf/f^2 with:

    Δf = 670 994 334 606 Hz (Parthey) and
    f = 2 466 061 413 187 035 (Parthey)
    c = 299792458 m/s

    Check it out!
     
  8. May 4, 2015 #7

    Ken G

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    On point (4), have we ruled out absorption by hydrogen in the upper layers of the Earth's atmosphere, known as the "geocorona"? I think it's optical depth can be a bit above unity at line center even at the altitude of that observation. That wouldn't explain the small satellite feature though.
     
  9. May 5, 2015 #8
    Here is a graphic where I line up the peaks with measurement dividers (blue dotted lines) set at 33 picometers. Is it just co-incidence that the amount the peaks are separated by just happens to be (within measurement/resolution constraints) the calculated wavelength difference between Hydrogen and Deuterium Lyman Alpha lines ?

    With apologies to Mr Artzner - hope he will forgive me using his Lyman Alpha profile to illustrate the point.

    ?temp_hash=e800b78ab2f58bbd98eb92003e77d4f6.png
     

    Attached Files:

  10. May 5, 2015 #9

    Ken G

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    Yes, that is a coincidence. I suspect the deep narrow absorption in the center is due to neutral hydrogen in the vicinity of Earth (the "geocorona", I'm not really sure why it is there), but the location of the peaks probably has nothing to do with that deep narrow absorption. It is natural for emission lines of resonance lines to have two peaks like that, even when it is only one line, because light near line center has a tough time escaping-- instead, it tends to shift frequency as it scatters around, and these small random shifts (called "Doppler diffusion") allow the photons to escape better when they get a bit away from line center, where the mean-free-paths are longer.

    More technically, the reason you see this in resonance lines is that the lower level is a ground state, so is very sharp, and this means that once a photon gets to frequencies well away from line center, where the mean-free-paths are large, they tend to stay at those frequencies, which creates those two peaks. If the lower level was not sharp, every time the photon scattered its frequency would be randomized, and then the escape advantages of being away from line center are exactly compensated by the reduced chance of being emitted away from line center, so the line shape is set by other factors.
     
  11. May 5, 2015 #10
    Many thanks for the explanation. But if - from what I've read - the sun actually produces Deuterium in the first stage of energy production by nuclear fusion, is there any particular reason why we might not suspect it exists in much greater abundance within the sun (and other stars) than elsewhere ? Or perhaps is able to leave a strong 'spectral signature' before being further fused into helium. Hence my thinking about the twin peaks separated by 33 pm. (By the way there is faint evidence of twin peaks within the twin peaks as discussed earlier in this thread !).
     
  12. May 5, 2015 #11

    Ken G

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    Stars are generally expected to reduce deuterium below the original levels in the Big Bang. Whether you see that or not depends on how well mixed the gas is, I don't really know the expected abundance of deuterium in the photosphere. If you can see it, it's a small feature, not big peaks. Those peaks are symmetric around the line center of hydrogen Lyman alpha, they are not the line centers of hydrogen and deuterium. The connection in separation is coincidental, and is not present in other double-peaked resonance lines on the Sun (like the Calcium H and K lines, look close to the center in http://www.astro.gsu.edu/CHARA/MWO/hk/Overview/Images/k2dspeco2.gif [Broken]), or in Lyman alpha on other stars that also have double-peaked resonance lines. But it's an interesting coincidence to notice!
     
    Last edited by a moderator: May 7, 2017
  13. May 6, 2015 #12
    From "Investigation of Deuterium in the Sun":

    If, nevertheless, the hypothesis of thermonuclear reactions as the chief source of solar energy is maintained (and there is at present no pressing need to relinquish this hypothesis), we must conclude that nuclear processes (possibly nuclear disintegration, fission, or nuclear explosions) in which elements such as lithium and deuterium are formed occur in the outermost layers of the sun. The author came to this conclusion as early as 1953 (Ref 24) from a study of the development and speeial character of emission of chromospheric flares. Further studies of the emission from flares and corpuscular ejections from faculae have confirmed this point of view (Ref 25). Special articles will be devoted to this question.

    Is this a discredited theory and if not what are the ramifications in respect of solar spectra ?
     
  14. May 6, 2015 #13

    Ken G

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    I don't know what the abundance of deuterium in the Sun is, but it won't give you features like the ones you are asking about. Flares might add some, mixing might destroy some, so it's an observational question what is there-- but it's small.
     
    Last edited: May 6, 2015
  15. May 7, 2015 #14
    If what we are seeing is somehow the spectral signature of solar thermonuclear processes, then the latter is the 'equalizer'. Suppose there is a population of hydrogen atoms with high enough energy to undergo fusion. Post reaction there is the (approximately) same population of high energy deuterium exploding in all directions. Both 'high energy' species rapidly dissipate energy emitting in all allowed parts of the spectrum. And so we see the twin peaks at Lyman Alpha. Hope that's not too simplistic a notion.
     
  16. May 7, 2015 #15

    Ken G

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    The point is, the Lyman alpha peaks are well understood without even the presence of deuterium. Their separation is only coincidentally related to the spacing between deuterium and hydrogen Lyman alpha, and they are not in the right places to be related to that.
     
  17. May 7, 2015 #16
    Yes - one would need to account for a redshift of about 18.7 picometers for this theory to make any sense.

    Do we have an accurate profile for L Beta anywhere ? Can't find anything at the same resolution as L Alpha profile above.
     
  18. May 7, 2015 #17

    Ken G

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    I don't know, but it should look a lot like Lyman alpha, it has a similar optical depth but is a little more sensitive to higher temperatures.
     
  19. May 11, 2015 #18
    Re the original post I am still not clear as to the following:

    1) The scale being used. Can someone please help me locate on which precise division we can find 1215 Angstrom ( I guessed corresponding to 160 pm). Or is that not how it works? Alternatively does anyone know the precise wavelength corresponding to 0 on the scale ? I would also assume that for really accurate measurement we need to work not with pixels (as I have been doing) but with the precise measurement gradation of the spectrometer used (0.834 pm).

    2) Where precisely the measurement of the Lyman Alpha line is made ? (I guessed at the deepest part of the absorption dip).
     
  20. May 16, 2015 #19
    I think I can now answer my own questions above just for the sake of closure.

    1) Since the Lyman Alpha line lies between 1215.6 and 1215.7 Angstroms, I think we can safely say that 100 pm marked on Artzner's scale corresponds to 1215.6 Angstrom and that since wavelength is increasing from right to left, the zero of the scale corresponds to 1216.6 Angstrom.

    2) Starting at 1216.6 Angstrom I drew up a table of wavelength values decreasing each time by the step value of 0.834 pm. I identified the deepest portion of the geocoronal absorption dip at the 112th step - that is at 1216.6 - 0.00834 x 112 = 1215.66592 Angstrom. And I presume that is what would correspond to Artzner's (unstated) measurement of the Lyman Alpha line. Perhaps that should rather be given as 1215.66592 A +- 0.00834 A.
     
  21. May 16, 2015 #20
    If you read the article you linked to, it's all explained there : the sharp central absorption feature A (and thus the twin peaks) are due to hydrogen in the earth's atmosphere (geocorona), the small absorption feature B at the top of the left peak is speculated to be due to the absorption by the interstellar wind (which is shifted from the center as the earth moves with respect to it).
     
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