Quote:
Originally Posted by vpcats
Rance - why do those northern lights look green? Please don't hit me if that's a REALLY dumb question.
Also, what's that to the left of the 2nd pic- all red and cloudy. A fire? 2nd REALLY dumb question.
Thanks.
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Aurora is very interesting phenomenon, caused by energy particles from the sun hitting particles in the earth's atomosphere. The particles become "excited" by the energy, and glow. Nitrogen is what causes the green color and oxygen is what causes the red color. I'm not sure about any other colors. I've seen some very "golden" colored aurora, for example, and in photographs it is very easy to determine that, even though we don't see it, there is a lot of very dark blue color too.
Aurora exists at both poles, and is almost exactly the same at any given moment. It peaks during the months of November and March. The activity is in a fuzzy circle or oval. One effect is that places as far south as Juneau get to see a lot of aurora, and so does much of the central part of southern Canada. But other areas that are equally far south (in the Bering Sea, for example), don't see as much.
http://www.swpc.noaa.gov/pmap/gif/pmapN.gif
The ideal place to observe aurora is in Alaska's Interior, for example around Fairbanks. That is, on a average, right about where the southern edge of the fuzzy band will be, so it is usually seen in the northern sky from Fairbanks. The Interior climate in both November and March usually means crisp cold nights with cloudless skies, which is great for watching the lights.
Here in Barrow we get just about as much auroral activity, but the number of days when the sky is clear enough to see them is limited! Also, the "Northern Lights" in Barrow are usually seen either in the southern sky (when they aren't too active) or in the west when it is moderately active. Only when activity is fairly high do we see much to the north.
Photographing the aurora isn't difficult. A wide angle lens is probably best, and setting the camera for manual control of everything will make it easier. Exposures of 30 seconds will show blurred stars! Due to the earth's rotation they will have "moved across the sky". At 15-20 seconds it isn't as noticable, and with shutter speeds faster than that it won't be obvious at all. Set the camera on a tripod or other solid support, put the focus to infinity, set 20 seconds for the shutter speed, and perhaps f/5.6 or f/8 for the aperture... and click away.
Wider aperture and longer shutter speeds might have different effects than one would expect, depending of whether the display is moving or static. If it is not moving at all, the image will be brighter with a longer shutter speed, but if it's moving the shutter speed will instead just change the area over which it has moved! So the aperture is what sets how bright it is, usually.
If there is any light from ground objects, it will usually show up much brighter than you'd expect (due to the long exposure times, so if that is a problem, try shorter exposures).
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03-18-2008, 01:02 PM
Aurora, and photography
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