The Aurora (Part 1: Observing)
Published on Nov 10, 2006 at 9:22 pm.
3 Comments.
Filed under aurora, skywatching.
Mary Jo recently posted about auroral forecasts. That got me to thinking that I’d say a bit about aurorae, too. (The plural is either auroras or aurorae. Both seem to be acceptable, but I like aurorae, which is the Latin form.) The aurorae are generally seen near the Earth’s magnetic poles, so they are often very far north or very far south. When in the north, we call them the Aurora Borealis, and when in the south the Aurora Australis.
The aurora is a glowing in the upper atmosphere caused by excitation of atoms by high energy particles, typically electrons. These high speed electrons slam into the oxygen and nitrogen atoms high in the atmosphere, exciting them (imparting energy to them). These atoms will then de-excite, emitting energy in the form of light. The particular wavelength (color) of light will depend upon the particular energy level transitions involved. Of course, atoms can be excited a lower levels in the atmosphere, too. But, at the lower levels of the atmosphere, where the air is denser, atoms will bump into one another before they get a chance to de-excite. These collisions will allow the excited atoms to lose energy to other atoms through collision rather than losing energy through emission. Thus, the transitions leading to these emissions are never seen at low altitudes. Therefore, we call these “forbidden” transitions (They are forbidden transitions because you don’t generally see them in the laboratory, not because they don’t happen!). At high altitudes, the gasses of the atmosphere are so rarified that the atoms can go for long enough between collisions that they get a chance to de-excite via photon emissions. Most aurorae occur at over 60 miles altitude (Note: we often think of being over 50 miles up as being in “outer space”.) The air here is so thin that it is closer to a vacuum than you normally can get in the laboratory with a common vacuum pump. The most common colors of the aurora are reds and greens from atomic oxygen. Normally, the reds are seen at higher altitudes than the greens. Personally, I’ve seen combinations of colors that have made the aurora appear orange, and I’ve heard reports of blue and purple colors. But, by far the typical aurorae are red or green.
Aurorae can appear in a variety of forms, sometimes as ribbons, and sometimes as vertical spikes, and sometimes simply as a diffuse glow. Though normally seen near the geomagnetic poles, during severe geomagentic storms (major disturbances in Earth’s magnetic field resulting from interactions between Earth’s magnetosphere and coronal mass ejections from the Sun) the aurora can be much brighter, and can sometimes be seen much farther from the geomagnetic poles. Four times I have seen the aurora borealis here in Texas. The first time was while teach an astronomy lab. This was at an observatory a ways outside of town, where the skies were darker. I noticed that there was a dull orange glow on the northern horizon. It got brighter and brighter. I was talking with one of my TA’s about this, as we were beginning to worry that there might be a brush fire in that direction heading towards us. The, suddenly the sky erupted into reddish curtains waving back and forth. I realized at once that I was seeing an aurora! The second time I walked out of the observatory building and saw what looked like red spikes shining over some trees to the north. I was thinking that they were lights from some monstrously bright tailinghts when suddenly they spikes started moving up and down, and I realized that it was an aurora. I saw another aurora similar to that one another time while teaching lab, and one final time as simply a diffuse reddish glow in the northern sky. All this was from Texas, quite far from the geomagnetic poles, but each time during a geomagnetic storm.
More aurorae can be seen when the Sun is most active. The Sun’s activity goes through an approximately 11 year cycle, and we are currently near the bottom of that cycle. But, that doesn’t mean that aurorae can’t be seen, just that they are less likely. Interestingly, aurorae are somewhat more common near the equinoxes than at other times of the year; however, there is no clear model that explains why this is the case. In fact, there is a lot that we don’t understand about aurorae. There is also a lot that we used to think that we knew about aurorae that we now realize that we were wrong about. A lot of the things that I was taught in school are now believed to be wrong!
But, with recent advances in understanding, we also have gained some ability to predict the likelihood of seeing an aurora. If you are really interested in seeing an aurora, the University of Alaska at Fairbanks has a website that gives auroral forecasts. NOAA’s Space Environment Center has all sorts of current data and spaceweather forecasts available. Another wonderful website is Spaceweather.com, which will keep you updated on not just auroral activity, but all sorts of space related news. Spaceweather.com also maintains a phone service that you can sign up for that will automatically call you on your phone if an aurora appears to be materializing in your area. This is near real time, so it is based upon current observations and conditions rather than upon forecasts. So, if you are really interested in seeing the aurora, these are fantastic resources to check out.
I guess that’s enough of a post for today. My next post will be a bit more about where the aurora comes from.
-Astroprof
(Images courtesy of NASA)
Astroprof’s Page » The Aurora (Part 2: Origin of the Aurora) on November 12, 2006 at 2:38 pm: 1
[…] « The Aurora (Part 1: Observing) […]
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[…] “Copernicus crater mistery“, no Astronomy Blog. Sobre este assunto, ler também “Lunar mistery, part II“, no Davep’s Astronomy; “The Aurora: part1 and part 2“, no Astroprof’s Page. Dois posts de fundo sobre as auroras boreais; […]
zareena_aurora on December 1, 2007 at 9:37 am: 3
this is awesome..
this page make me wondering there is a lot of things like this to be explore…
i think i ve got my new hobby now..
geee… thanx.. lol