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Thread: Why is Oxygen Emission More Prominent than Nitrogen in Aurorae?

  1. #1
    Join Date
    Jun 2008

    Question Why is Oxygen Emission More Prominent than Nitrogen in Aurorae?

    Aurorae often appear mostly green, with some pink mixed in. We know the green is oxygen and the pink is nitrogen. However, O2 is only 20% of the atmosphere, while N2 is a whopping 78%. On paper you would think the latter wins out, but this is clearly not the case. What gives?
    “Of all the sciences cultivated by mankind, Astronomy is acknowledged to be, and undoubtedly is, the most sublime, the most interesting, and the most useful. For, by knowledge derived from this science, not only the bulk of the Earth is discovered, but our very faculties are enlarged with the grandeur of the ideas it conveys, our minds exalted above their low contracted prejudices.” - James Ferguson

  2. #2
    Join Date
    Mar 2007
    Falls Church, VA (near Washington, DC)
    The strength of the emission depends on the characteristics of the atoms, not just how many there are of each type.

  3. #3
    Join Date
    Jul 2005
    Couple of other things going on, too.

    The aurora occurs in the thermosphere, where gas density is so low that atmospheric components sort themselves according to their scale height, rather than turbulently mixing. Atomic oxygen (16 dalton), responsible for the green aurora, has a greater scale height than molecular nitrogen (28 dalton), responsible for the pink, so the proportion of atomic oxygen is enhanced high in the thermosphere, where it intercepts more incoming charged particles.

    Secondly, our eyes are more sensitive to green light than red light, and the aurora brightness is often down near the limit of colour vision--so we end up seeing the green, but picking up red and blue wavelengths with night vision only, so they appear white. (And a very dim aurora looks grey, or very pale green.)

    Grant Hutchison

  4. #4
    Join Date
    Jun 2003
    There is also a very shocking red aurora (sometimes called a 'blood aurora') which I've seen from my back garden in York. This is also caused by oxygen, but at an even higher altitude than the green stuff.

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