By Dr. Marc Rayman
Asked by our friends at the Lower Hudson Valley Challenger Center in Suffern, New York.
For those of you who have never seen it, the aurora borealis, also called the northern lights, is a beautiful light show that appears in the night sky. The sky may fill with shimmering, dancing curtains of blue-green light, sometimes tinged with patches of red and pink. I think they're so beautiful that I named my colorful iguana Aurora. The closer you live to the North Pole, the more likely you will see an aurora. The same cosmic spectacle is also visible near the South Pole, where it is called the aurora australis, or southern lights.
although the auroras look like Earth's own light show, the Sun is actually directing them. The Sun is always sending out a stream of electrically charged particles called the solar wind. When the particles get close to Earth, they start to feel the effect of Earth's strong magnetic field.
Earth is like a giant magnet, with its field curving all around the planet and coming together into almost a funnel shape very near each of Earth's two poles. This magnetic field is called Earth's magnetosphere. It does a good job of protecting us from the solar wind, because it bends the paths of the charged particles from the Sun, in most cases steering them away from our planet. But the magnetic field also traps some of the charged particles and channels them down toward the poles, creating an enormous flow of electricity right into Earth's atmosphere.
When the charged particles collide with the thin air 100 km or more above the ground, the gases in the atmosphere give off light like the glowing gas in a neon light tube. Nitrogen may turn red, blue, and violet, and oxygen can color the sky red and green.
The closer you are to the poles, the more often you will be able to see auroras. Certainly auroras can be seen in New York and much of the rest of the country, but in the U.S., the farther north you live, the better your chances. So whenever you're enjoying the beauty of a very dark night sky, remember to look to the north in case there is an aurora then. In the southern hemisphere, the farther south you live, the better the aurora viewing.
The Sun doesn't always shoot out the same amount of charged particles. Our star has an 11-year cycle of activity, sometimes sending out more particles, sometimes less. We are just now nearing the end of one of the intense periods of solar activity. although auroras occur every year, these times of peak activity are the best for them, and there have been many spectacular auroras in the last year. Sometimes, there are huge explosions on the Sun that fling tremendous numbers of charged particles into space. If these happen to be aimed at Earth, we can be treated to an especially marvelous display two or three days later, once the particles have raced across the space between the Sun and us.
Other planets with magnetic fields also have auroras. My favorite planet, Jupiter, has an exceptionally strong and large magnetosphere. We have been able to see auroras at the poles of Jupiter, as well as Saturn, Uranus, and Neptune with the Hubble Space Telescope, Galileo, Cassini, Voyager, and other spacecraft.
You can read more about auroras and Earth's magnetosphere and do an interesting experiment with magnetic fields in your class at school. See "Tidy Up Those Sloppy Force Fields."