by Ann » Fri Oct 08, 2010 12:01 am
Let me explain once again how I could be so sure that this aurora could not be monochromatic and dominated by OIII emission at the same time.
I make a note of the color of any object I see, certainly of every brightly colored object. I don't remember the color of everything I see, but I remember a lot, and I always know if a particular object has a color that is unusual or unexpected for this particular object.
I have never seen an aurora in real life, but you can't spend a lot of time looking at astronomical objects without seeing a lot of aurora images, too. I always note the color of the aurorae. And since blue is far and away my favorite color, I am always actively looking for blue things. I am always asking myself, could this be a blue object? When it comes to aurorae, I have never been able to say a definite "yes" to that question. At best, multicolored aurorae have looked a bit blue in places, but I have never been able to trust that color. On the other hand, many aurorae have looked green. Quite often they have been a sort of "in-between-green", neither yellow-green nor blue-green. Several times, however, they have been strongly yellow-green, so much so that their yellow-green color has positively "jumped out" at me. Their intense yellow-green color has made a strong impression on me.
But they have never looked truly blue. Never.
I am well aware that you can never trust the color balance of an individual image. The color balance can always be off. But I know, I trust, that if a particular object is often seen to be a monocolored "in-between-green", and sometimes it is an intensely monocolored yellow-green, then I can be sure that this particular object can be a monocolored green, and its shade of green is can be either "in between" or yellowish, but i can never be bluish. Because if the "true" color of the object was really blue-green, then I would sooner or later see a picture of this object where the color balance was sufficiently off for the blue-green color to look blue. In other words, if monocolored blue-green aurorae were the least bit common, then I would have seen a picture of what looked like a monocolored blue auroa. But I have never seen that, never. And I know that I haven't, because if I had, I would definitely remember it. Therefore, I would go so far as to say that monocolored aurorae dominated by strongly blue-green OIII emission either don't exist at all, or at least they are extremely uncommon.
Chris said that the aurora over Alaska was monocolored, since the camera or film reacted to its monochromacity by producing concentric rings. I have no reason whatsoever to doubt Chris' assessment that the aurora is monochromatic. I have seen many images of green aurorae that have definitely looked monochromatic to me.
But I strongly doubt the existence of monochromatic OIII aurorae, like I said. By contrast, I know that many planetary nebulae are dominated by OIII emission. I also know that a Google search will turn up a planetary nebula that looks more blue than green. I'm not talking about those popular creatively-colored narrowband images of planetaries, but probable broadband ones. This is a typical broadband image of a planetary dominated by OIII emission, where the image makes the planetary look blue:
I know that the color balance is off here. I can see that it is. I know that it is, because astroimagers who are serious about their colors almost always make their OIII nebulae look greenish. But an image like this one confirms my suspicion that the "true" color of an OIII nebula is close enough to the blue part of the spectrum for a few astroimages to make the nebula look blue. The more blue-looking planetaries I can find, the more certain I can be that OIII emission is indeed blue-green in color. I remember very well that I have seen other broadband astroimages of blue-looking planetaries than the one I posted above.
But I have, as I said, never seen an aurora look like that. Never. Therefore I'm sure that aurorae can't be dominated by OIII emission. I strongly, strongly doubt that big, bright, monochromatic OIII aurorae even exist.
And if someone tells me that a bright yellow-looking aurorae is both monochromatic and dominated by OIII emission, I'm not going to believe them. If an aurora looks yellow, it must either be a mixture of two different kinds of emission, or else the color balance of the picture must be severely off. Indeed, the color balance of the picture seemed definitely too red. So the aurora may well be green instead of yellow. I know that monocolored yellow-green auroae definitely exist, and they are even pretty common. So this could be a monocolored yellow-green aurora. If so, its dominant color should correspond to a wavelength of about 560nm, because the yellow-green aurorae I have seen appear to "cluster" around a wavelength close to 560nm, based on the average appearance of them.
Elementary, my dear Watson.
Ann
Let me explain once again how I could be so sure that this aurora could not be monochromatic and dominated by OIII emission at the same time.
I make a note of the color of any object I see, certainly of every brightly colored object. I don't remember the color of everything I see, but I remember a lot, and I always know if a particular object has a color that is unusual or unexpected for this particular object.
I have never seen an aurora in real life, but you can't spend a lot of time looking at astronomical objects without seeing a lot of aurora images, too. I always note the color of the aurorae. And since blue is far and away my favorite color, I am always actively looking for blue things. I am always asking myself, could this be a blue object? When it comes to aurorae, I have never been able to say a definite "yes" to that question. At best, multicolored aurorae have looked a bit blue in places, but I have never been able to trust that color. On the other hand, many aurorae have looked green. Quite often they have been a sort of "in-between-green", neither yellow-green nor blue-green. Several times, however, they have been strongly yellow-green, so much so that their yellow-green color has positively "jumped out" at me. Their intense yellow-green color has made a strong impression on me.
But they have never looked truly blue. Never.
I am well aware that you can never trust the color balance of an individual image. The color balance can always be off. But I know, I trust, that if a particular object is often seen to be a monocolored "in-between-green", and sometimes it is an intensely monocolored yellow-green, then I can be sure that this particular object can be a monocolored green, and its shade of green is can be either "in between" or yellowish, but i can never be bluish. Because if the "true" color of the object was really blue-green, then I would sooner or later see a picture of this object where the color balance was sufficiently off for the blue-green color to look blue. In other words, if monocolored blue-green aurorae were the least bit common, then I would have seen a picture of what looked like a monocolored blue auroa. But I have never seen that, never. And I know that I haven't, because if I had, I would definitely remember it. Therefore, I would go so far as to say that monocolored aurorae dominated by strongly blue-green OIII emission either don't exist at all, or at least they are extremely uncommon.
Chris said that the aurora over Alaska was monocolored, since the camera or film reacted to its monochromacity by producing concentric rings. I have no reason whatsoever to doubt Chris' assessment that the aurora is monochromatic. I have seen many images of green aurorae that have definitely looked monochromatic to me.
But I strongly doubt the existence of monochromatic OIII aurorae, like I said. By contrast, I know that many planetary nebulae are dominated by OIII emission. I also know that a Google search will turn up a planetary nebula that looks more blue than green. I'm not talking about those popular creatively-colored narrowband images of planetaries, but probable broadband ones. This is a typical broadband image of a planetary dominated by OIII emission, where the image makes the planetary look blue:
[img]http://www.astroimages.org/ccd/n7662cm2.jpg[/img]
I know that the color balance is off here. I can see that it is. I know that it is, because astroimagers who are serious about their colors almost always make their OIII nebulae look greenish. But an image like this one confirms my suspicion that the "true" color of an OIII nebula is close enough to the blue part of the spectrum for a few astroimages to make the nebula look blue. The more blue-looking planetaries I can find, the more certain I can be that OIII emission is indeed blue-green in color. I remember very well that I have seen other broadband astroimages of blue-looking planetaries than the one I posted above.
But I have, as I said, never seen an aurora look like that. Never. Therefore I'm sure that aurorae can't be dominated by OIII emission. I strongly, strongly doubt that big, bright, monochromatic OIII aurorae even exist.
And if someone tells me that a bright yellow-looking aurorae is both monochromatic and dominated by OIII emission, I'm not going to believe them. If an aurora looks yellow, it must either be a mixture of two different kinds of emission, or else the color balance of the picture must be severely off. Indeed, the color balance of the picture seemed definitely too red. So the aurora may well be green instead of yellow. I know that monocolored yellow-green auroae definitely exist, and they are even pretty common. So this could be a monocolored yellow-green aurora. If so, its dominant color should correspond to a wavelength of about 560nm, because the yellow-green aurorae I have seen appear to "cluster" around a wavelength close to 560nm, based on the average appearance of them.
Elementary, my dear Watson.
Ann