Saturday, September 15, 2001

What is the Aurora?

Just in the last week or so, people have been talking about the aurora, because it was visible in our night skies not too long ago. If we're lucky, we will see it again. But what is it? The aurora borealis is a beautiful display of lights that can be seen over the northern pole of the Earth. It will appear in the northern part of the sky and have a ribbon-like shape that changes and shimmers in time. It will have hues that range from very pale white to green to blue and sometimes purple. Usually it is most visible in more northern latitudes, such as Canada and Alaska, although if it is bright enough and large enough it can be visible as far south as Florida.

There is a similar phenomenon called the aurora australis in the southern hemisphere that behaves the same way. And this gives us the first clues as to where this beautiful display of lights comes from. The north and south poles of the Earth correspond roughly to the poles of the earth's magnetic field. And just as iron filings are attracted to the north and south poles of a bar magnet, the Earth attracts charged particles such as protons to its poles. These particles originate from the sun, which gives off a slow stream of these particles in what is called the solar wind.

If these charged particles strike the atmosphere, they will inevitably crash into an air molecule or atom and often the collision will eject an electron. Since air molecules and atoms like to keep their electrons they will eventually snag a new electron. That process releases light and voila you have an aurora. Because the aurora depends on the solar wind, as the solar wind changes so does the aurora. If there are lots of particles streaming from the sun, then there will be a bright aurora. This year we are lucky, the sun is very active and so will often eject lots of material into space which causes very strong and beautiful auroras.

The last time I saw the aurora was here in State College while watching for meteors. A few days before, the sun had a huge flare which released a very strong solar wind, creating a beautiful addition to the already fun hunt for meteors! If you want to try to see the aurora, the best place to be is somewhere dark and relatively flat so that you can have a good view of the horizon. You need to look towards the North.

The best way to do that is to find the Big Dipper, which looks like a huge ice cream scooper. The two stars at the edge of the scooper form a line that will point you towards the pole star or Polaris, which is due north. You also need to go out on a night when they predict that the aurora will be strong, since we don't often have a good chance of seeing it. Websites like, and will have stories with links to forecasting sites and email alerts so you can try and find the likely days of next big storm.