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What makes the Aurora colors?

  1. Jan 12, 2014 #1

    I wonder what makes the Aurora colors.

    This is what I naively believe I know:

    Charged particles are ejected from the Sun and hits our protective magnetic field eight minutes later.

    At the poles, the magnetic flux density, B, is the greatest.

    Particles coming from the Sun are intercepted by B and gyrates around the lines of force.

    Close to the poles B is the greatest, which means a higher radial frequency of the particles.

    This excites the elements of our air (mainly Nitrogen and Oxygen).

    The ejected particles excites them in such a way that when they first hit them, they are excited but later on they lose this excited energy and sends out the difference according to hf (=hc/λ) which is the Aurora light we see.

    The Aurora is however not uniform in color.

    And gasous vacuum tubes, where air has leaked inside, glows pink.

    I therefore wonder two things:

    1) Are the diffent colors (red and green, as far as I understand) due to Nitrogen and Oxygen excitation accordig to above?
    2) How come the colors are separated in the sky (and not pink or some mixture)? This obviously means that there is some kind of gradient or separate distribution in the sky while ordinary air seems to be quite homogenous.

    On the other hand I have heard something about an ionsphere up there too...

    Best regards, Roger
    Please give me a physical explanation and not too much of a layman explanation.
    Last edited: Jan 12, 2014
  2. jcsd
  3. Jan 12, 2014 #2

    Simon Bridge

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    Please see:
    http://ffden-2.phys.uaf.edu/211.fall2000.web.projects/Christina Shaw/AuroraColors.html
    You have the basics - but the devil is in the details.
    i.e. the colors depend on altitude as well as atom, and the separation of the colors is because they happen at different altitudes and the irradiation is not uniform.

    As for the discharge tube:
    Oxygen suppresses long electron cascades in the closed environment of a discharge tube - which is why the spectrum gets weaker when there's a leak - and tends to the red end (shorter - low energy - transitions).
  4. Jan 16, 2014 #3
    Hi Simon!

    Thank you for your reply!

    Viewing this beautiful picture


    while reading the interesting information in the same nicely provided link, I come to the conclusion that Oxygen at high altitudes is responsible for the red glow and Oxygen at lower altitudes is responsible for the green glow.

    But how can this be?

    How can the same molecule/atom be responsible for two different colors?

    Can the ionization/recombination be in several steps?

    Let's think of this in layman terms. Oxygen is barely excited at higher altitudes for some reason (say that only one electron is leaving the "atom"). But at lower altitudes more energetic particles exists somehow and excites the Oxygen even more (say that two electrons leave the "atom").

    By the miracle of recombination (which is frequently used but not really understood) the Oxygen residing in the upper altitude sends out red light and the Oxygen in the lower altitude sends out green light.

    Might it perhaps be something like this?

    The only thing I then do not understand is how an atom missing one or more electrons has a higher energy state than a neutral atom of the same type.

    Because the energy of light (hf) sent out, and this is one of the few things I know, is the difference between the excited (=ionized?) energy level and its neutral/rest energy level.

    To me an excited energy level of an atom would mean that the electron still hangs in there but at a higher "mgh". Which is rediculous, right?

    I really do not understand this recombination part that "makes light".

    So what I would like here is an equation for the excited/ionized energy level as well as the rest energy or at least the difference.

    This would make my day :smile:

    Here I really do not understand what you are referring to. I have never spoken of any "discharge tube". I have only spoken of the fact that when vacuum tubes are leaking they glow pink.

    But perhaps this is what you mean with a "discharge tube"?

    Anyway, your nice explanation of the fenomena told me absolutelly nothing :wink:

    Take care!

    Best regards, Roger
  5. Jan 16, 2014 #4

    Simon Bridge

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    ...they are not exactly the same
    [displaying] Spectrum of oxygen O (two lines in the red region) and O+ (the blue and purple lines).
    -- http://www.itp.uni-hannover.de/~zawischa/ITP/atoms.html

    ... add to that: solar radiation is not all the same kind.
    Low energy radiation gets stopped high in the atmosphere, higher energy gets stopped lower.

    It's an example - the effect is obvious in gas discharge tubes: look it up.

    I may have misunderstood you though:
    What are you calling "vacuum tubes" - I'm thinking either valves or CRTs.
    How do you see the pinkish glow and where?
  6. Jan 17, 2014 #5
    Very interesting information. This explains why the more high energic photons are emitted at lower altitude.

    I will.

    I was thinking of this phenomena:


    which is visible in leaking vacuum tubes. The picture is by the way fetched from another thread of mine namely:


    You may want to look it up :wink:

    Finally, I loved the link you so thankfully have supplied. Lots of interesting things to read!

    However, I am a quite lazy guy who likes these "social" discussions more than reading.

    So I like trying to understand things without reading too much "useless" information because there is lots of "noise" out there.

    You could spend a lifetime reading stuff but never feel that you grasp it all.

    And that's the fun of living.

    But I am mostly interested in plasma physics.

    And I need to focus on that before I get old :wink:

    Thanks for your reply!

    Best regards, Roger
  7. Jan 17, 2014 #6

    Simon Bridge

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    The 6L6 GC (pictured) is a guitar-amp valve
    ... it is not a vacuum tube as it contains gas (mercury vapor iirc).
    Purple-pink and orange glows are part of normal operation - but I think it's supposed to be bluer than that.

    A bad seal can produce a muddier pinkish-orange due to trace oxygen and nitrogen, as described for gas discharge tubes.
  8. Jan 17, 2014 #7


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    A 6L6 is a normal vacuum tube, the color in his tube picture is abnormal.
  9. Jan 17, 2014 #8

    Simon Bridge

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    Link reference needs to be more specific
    ... don't know what it is supposed to show or which part you are referring to.

    I don't think we are actually disagreeing though: if you've got a better answer to OPs question, I'd love to hear it... it's why I'm here after all...

    I'll take your word for it.

    I wanted to make a distinction between a "vacuum tube" (I know them as "thermionic valve"s) and a "tube designed to have a vacuum in it" - reason: see context above. I don't know what you mean by "normal" in this context.

    Maybe I'm just not being clear:
    The 6L6GC I am thinking of is an audio-frequency power pentode - commonly found, these days, in electric guitar amp boxes. (see below)

    You'll find a lot of chatter on the guitar amp groups about their blue/purple glow in operation.
    The blue glow is characteristic of tubes using mercury vapor as opposed to a simple vacuum action.
    Without the vapor, the only part that should glow at all is the heater (on the cathode).

    But I don't have one handy to check - so I could be mistaken.

    I do have my own refs though:

    A FAQ: http://www.rru.com/~meo/Guitar/Tubes/blue_glow.html

    ... also see specs.
    http://www.tubeampdoctor.com/images/File/TAD-Datasheet_6L6GC-STR_Version1.3 08.07.11.pdf
    ... notice the pic shows a clear orange glow from the heater? The purple tinge is not evident though.

    I believe the Sylvana Engineering Data Service reference in FAQ is available in docstoc:

    I agree - it should be bluer - you wouldn't normally get the 6L6GC with argon or neon, which is the other way you see pinky or reddish glows in the gas.

    There are plenty of pics of the tube in normal operation - you'll see that the exact color of the glow varies with manufacturer and use.

    I don't see a disagreement.
  10. Jan 17, 2014 #9


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    Not much of a disagreement really but a clarification.

    Normal in this case would be a "6L6 type thermionic valve" with very little residual gas. When I was younger I rebuilt a quite a few tube audio amplifiers. (some were even for guitars :wink:) Mercury vapor tubes were normally only seen in the rectifier sections or as power switches as they are very non-linear.

    As I was explaining in the other thread the 'normal' blue glow in receiving type tubes is not from gas ionization, it's from FLUORESCENCE per your link:

    A FAQ: http://www.rru.com/~meo/Guitar/Tubes/blue_glow.html

    Sorry for side-tracking the thread on Aurora, your information on them was pretty cool.
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