Since most of the US has been dry the past several weeks, the skies have been relatively free of clouds, and so there have been ample opportunities to look at the heavens above. Of course, it's always nice to step outside on a crisp night and glimpse the Moon, a few of the planets and a couple of thousand stars, but there are other things to see that are much closer and sometimes brighter. For example, auroras.

Why can auroras be seen on Earth and not the Moon? Why Jupiter and not Mars, and maybe Jupiter's moon Io, but not Venus? The answer has to do with magnetism. The cores of Earth and Jupiter are magnetized, intensely enough for those planets to trap ions and electrons. Their magnetism also leads to large electric currents, flowing from space to the high atmosphere of those planets, which conducts electricity. Those currents create beams of fast electrons, guided along the Earth's magnetic field lines, and when these hit oxygen and nitrogen in the atmosphere (particularly oxygen), the light of the aurora is emitted. Among the planets, Jupiter has the strongest magnetic field and likelyu the brightest auroras (they've been photographed by the Hubble telescope).

Saturn is also magnetized and may have auroras. Uranus and Neptune are weaker magnets, and their magnetic poles are far from their rotation poles, making auroras less likely. The crust of the Moon and Mars contains rocks that are magnetized-intensely enough for NASA spacecraft to observe but not enough to trap or accelerate electrons. Venus does not have any measurable field of its own - Mercury does, but again, it's a weak one. As for Io, it probably isn't magnetized. But it has a thin atmosphere, created by its spectacular sulfur volcanoes, and it moves through the powerful magnetic field of its planet, Jupiter. This also causes strong electric currents between Io and Jupiter, first detected when the space probe Voyager passed nearby in March 1979. So it wouldn't be a surprise if auroras exist on Io.

By the way, if you were on the moon with a compass, it's unlikely that the weak magnetization of some areas on the moon would be strong enough to move its needle, even in the weaker gravity on the Moon. An ordinary compass is sensitive to only the horizontal part of the magnetic force, and in a complicated magnetic field, this can be highly variable - towards magnetized patches, away from them, or neither.

"Aurora Borealis" (Latin for "northern dawn") or "Northern lights" is seen around the Earth's North Pole, while its counterpart, the Aurora Australis, is seen around the South Pole. At the end of October and again last Monday, November 5th, auroras were seen all over the United States, allowing many citizens to see this phenomenon for the first time. It's too bad that many of us were probably racked out on the couch or looking at the boob tube, completely oblivious to what was going on about 60 miles or so (100 km) overhead.

Auroras are by no means rare. Satellites in space observe them all the time, but it usually only appears around the polar circles, rarely in regions where most of humanity lives. It takes a large disturbance of the Earth's magnetic field, a magnetic storm, for that to happen, like the one that began in the early evening hours (for the US) on the 5th of November. The US is also fortunate in that the north magnetic pole is located in northern Canada (about 77 degrees north latitude), and it's thus closer to American cities than cities in Europe or Asia, at the same latitude. Because of this, the northern lights were observed on November 5 as far south as Georgia and Texas!

With the waning moon, and a still very active Sun, there's a decent chance that, even here in the mid latitudes, we'll be able to see the northern lights again in the coming weeks. By the way, while your outside, keep an eye out for meteors, which leads to next week's question.

A special thanks to David Sterns for his help with this question.

For more about when to see the northern lights see
http://science.nasa.gov/headlines/y2001/ast26oct_1.htm
For one report, see "Aurora over Chicago" at
http://www-spof.gsfc.nasa.gov/Education/wnovstrm.html
If you have never seen aurora, view it at
http://www-spof.gsfc.nasa.gov/Education/waurora1.html and
http://www.ptialaska.net/~hutch/aurora.html
For more about the Earth's and other planets magnetic fields see
http://www-spof.gsfc.nasa.gov/earthmag/planetmg.htm