The aurora has fascinated humankind since ancient times. After reading this post, you will know it’s causes and impacts, what causes its colors, what the colors tell planetary scientists, myths and some fun facts.
- Aurora Myths, Colors, and Formation
- What Causes the Aurora
- Why Has the Aurora Different Colors?
- What You Cannot WatchLow Aurora Activity
- Aurora Affects Radio Communication
- Why You Can Receive Shortwave Radio in Mid- and Low Latitudes at Night
- New Year’s Eve Fireworks under the AuroraAurora Stories
Aurora Myths, Colors, and Formation
Auroras in the northern and southern hemisphere are called aurora borealis and aurora Australis, respectively. On Alaska clear nights, one has chances to view the aurora. While the aurora is there all the time, sometimes very weak, sometimes very strong, you cannot see it during the daytime. Thus, when we moved to Alaska in July 2001, we were looking forward to fall.
My First Aurora Sighting
One night on our way home from a symphony concert, we saw our first aurora. Its greenish curtain covered the sky from one end of the horizon to the other. The aurora was dancing. It looked like a semi-opaque vertically striped curtain with uneven greenish white folds gently moving in response to the summer breeze at an open window. We stopped at the side of the road, switched off the engine and lights, and watched until the air in the car became too cold for comfort. Read how to avoid discomfort when watching the aurora.
Dogs Can Hear the Aurora
At that time, we lived in Goldstream in the suburbs of Fairbanks close to Mary Shields. During our tenure in Goldstream, she has over 80 sled dogs. In nights with strong, i.e. beautiful aurora events, her dogs would start hauling at the aurora. Dogs and some people can hear the whistling and cracking of the aurora. Thanks to the sled dogs we never missed a great display. Sometimes we would watch the dance for over an hour.
What Causes the Aurora
In the Earth’s atmosphere, an electrically charged layer exists between 80 km and 400 km height. Here, molecules of nitrogen and oxygen, and oxygen atoms absorb X-rays and ultraviolet radiation coming from the Sun. In this process, the gas becomes excited, i.e. electrons jump to a higher orbit around their nucleus. Each affected molecule loses one or more of its electrons and then becomes a positively charged ion. Because of this ionization, this atmospheric layer is called the ionosphere.
Fast-moving electrons from space collide with the oxygen and nitrogen in Earth’s ionosphere and excite the electrons of these gases. This means the electrons reach a higher orbit around their atomic nucleus. The electrons release the excess energy in form of photons, i.e. light when they return to the normal orbit. When the electrons fall back to a lower orbit around the nuclei, the emitted radiation becomes visible as aurora.
Why Has the Aurora Different Colors?
The aurora color depends on the excited gas and the exchanged amount of energy. The different colors of the aurora depend on the emitting molecules and on the solar activity. Around 100 km or so, the emission by nitrogen molecules results in reddish fringes on the lower part of the aurora curtains. Between 100 to 300 km altitudes, emission by oxygen molecules causes the most frequent yellow-greenish auroras. Above 300 km height, the oxygen atom is the most common. Its emission produces the rare red aurora that in my 13 years in Alaska I saw only once. Even higher in the ionosphere, hydrogen and helium are the most frequent atmospheric components. When they emit radiation, we obtain bluish and purplish colors.
Oxygen leads to a greenish-yellow light or a red light. Nitrogen yields blue light.
A planet with a green aurora has oxygen. - Suyn Ichi Akasofu #quote #aurora Click To Tweet
What You Cannot Watch
Both oxygen and nitrogen molecules also emit UV light. However, only specialized cameras can register this part of the spectrum. Think of these cameras like being similar to the IR cameras. We can’t see IR, but these specialized cameras can interpret the IR light in terms of visible light. The UV is just in a quite different part of the solar spectrum than the IR.
Low Aurora Activity
When you see the aurora on a regular basis, you reduce the time watching it or staying up because the aurora forecast calls for a big display. When you live in a northern country for a while, the aurora must turn pink or red for you to be excited like a kid on Christmas.
In the zero years, the frequency of aurora activity went down. It even was relatively low when it was supposed to go up again in response to the Sun’s activity. At that time, we were lucky that friends from Europe who took the effort to visit in winter got to see an aurora. Read what to pack for aurora watching.
Aurora Affects Radio Communication
When the occurrence of free electrons is huge, they influence radio-wave propagation. The ion concentration increases with altitude to a maximum concentration at about 300 km (186 miles) above mean sea level.
The impact of the aurora activity on radio-wave propagation became a big problem during WWII. The, as part of the lend-lease act male pilots flew aircrafts from Fairbanks to Russia. Women flew them from the Lower 48s to Fairbanks, but weren’t allowed to fly the combat zone leg. Note I once met one of them, and she was a tough cookie in the most positive sense.
Since radio communication was important also after the war, Fairbanks became an aurora research center. On the spaceport Poker Flat, Alaska, scientists launch rockets for research on the aurora.
Today we still have disturbances in local cell phone calls due to aurora activity.
Why You Can Receive Shortwave Radio in Mid- and Low Latitudes at Night
The ionosphere shows three distinct layers, called the D-, E-, and F-layer. Recall the ionization depends on solar radiation. After sunset, the ion concentration goes down especially in the D- and E-layer that are closer to the Earth than the F-layer.
The D-layer absorbs AM radio-waves. However, the D- and E-layer are non-existent at night because the sunbeams passing the globe do not reach them on the night side. Thus, at night, the AM radio-waves propagate to the F-layer that reflects them back to Earth. The Earth’s surface reflects the AM radio-waves as well. Consequently, at night, AM radio-waves can overcome the Earth’s curvature and AM broadcasts can be received far away from the radio station.
New Year’s Eve Fireworks under the Aurora
I always wanted to take a photo of the New Year’s Eve fireworks under the aurora. This New Year’s Eve, I finally was lucky, but unfortunately not prepared. One would need a tripod due to the long exposure needed. We saw the aurora and fireworks on our way home from a dance performance we had given. Since we had the camera with us, I just took a chance. The photo turned out acceptable given the circumstances. What do you think?
Aurora Stories
Every northern culture has its stories and legends around the Northern Lights. In many of them, the aurora is related to spirits. Some of these legends relate the whistling and crackling noise that sometimes accompanies an aurora to the voices of the spirits communicating with the people on Earth.
One legend interprets the aurora as a spirit in great demand in helping by the best shamans. In this case, the spirit is seen as alive as the men and women on Earth. The aurora is thought to come close when you whistle at it. If you spit at it, the aurora would gather in the middle and form a new display. I never tried either of them.
One Eskimo legend associates the aurora with the ballgames of the deaths. Playing ball with a walrus skull is a favorite game among Eskimos. The goal is to kick the skull in such a way that it lands with the tusk downward to stick to the ground. This ballgame of the departed souls becomes visible as the aurora.
When you liked this aurora post let your friends know about it by tweeting them I found a great post on the aurora. #aurora #Alaska Click To Tweet
References
Akasofu, Syun-Ichi, 2002. Exploring the Secrets of the Aurora, Springer, New York.
Akasofu, Syun-Ichi, 2009. The Northern Lights: Secrets of the Aurora Borealis. Alaska Northwest Books.
Mölders, N., Kramm, G., 2014. Lectures in Meteorology, Springer, New York.
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Photos of aurora with fireworks: N. Mölders
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