If you’re in the habit of looking up at the night sky, there’s a good chance you’ve seen two very interesting sights: northern lights and falling stars.
The proper name for the northern lights is “aurora borealis.” Aurora was the Roman goddess of the dawn, their version of the Greek goddess Eos; borealis basically means “north.” (Down around the south pole people see aurora australis, instead.)
From the ground, the most common auroras look like huge, shimmering, ever-shifting curtains of light, extending from east to west. Usually they’re green, although occasionally there’s also some red to them.
They’re even bigger than they look. The bottom of the curtain is about 100 kilometres up, and the upper edge can extend to 300 kilometres or higher–to the very edge of space.
From space, though, it doesn’t look like a curtain at all; instead, it’s an oval belt of light surrounding the geomagnetic pole. (That’s the point that compasses point to, as opposed to the geographic north pole, which marks the northern end of the Earth’s axis. They’re not quite the same.) The aurora isn’t perfectly centered on the geomagnetic pole; it’s offset about three degrees (330 kilometres) toward the dark side of the Earth. As a result, auroras are most commonly seen around midnight.
The aurora belt has an average radius of about 20 degrees of latitude (2,200 kilometres), though on rare occasions it spreads out as far as 60 degrees. It extends thousands of kilometers east to west, but it’s usually only about one kilometre thick from north to south.
Auroras are caused by the interaction between the solar wind and the Earth’s magnetic field. The solar wind is the highly energetic particles which the sun sheds constantly, at a rate of about one million tonnes per second. These particles streaming through the Earth’s magnetic field generate electrical charges–energetic electrons that are channeled poleward by the magnetic field. There they collide with particles in the upper atmosphere, exciting them. (I could have said “ionizing” them, but “exciting” sounds more–well, exciting.)
These excited particles shed their excess energy in the form of light. Its colour depends on what kinds of atoms are being excited. Oxygen atoms give off the common cool green glow, while nitrogen atoms give off the rarer red glow.
Last year was a particularly good one for northern lights because the sun was just coming off the peak of the 11-year sunspot cycle. Lots of sunspots means lots of solar flares, which results in a “gusty” solar wind–more particles to bump into the upper atmosphere, creating more excited atoms, creating more northern lights.
When larger things bump into the atmosphere you get something else: falling stars.
At one time people thought falling stars really were falling stars, but as we figured out what stars really are, we realized that “falling stars” must actually be something else. That “something else” is tiny bits of matter cluttering up the space through which the Earth orbits. When one of these bits of matter hits the Earth’s atmosphere, the resulting friction causes it to burn brightly (the bit of matter, that is, not the atmosphere).
These bits are classified as meteoroids, meteors or meteorites. Meteoroids are the stuff while it’s still in space; meteors are what we see burning up in the night sky, and meteorites are the really big bits that survive their fiery fall and hit the ground.
A meteor shower occurs when the Earth passes through a region where there are more meteoroids than usual. On any given night you can see a few meteors if you stare at the sky long enough, but during the height of the Perseid Meteor Shower in August, you can see one a minute. (Very rarely there are even spectacular displays with dozens a second.)
Meteor showers (the Perseid is only one of 16 important annual meteor showers) appear to be associated with comets. Comets leave a trail of debris behind them as they orbit the sun. As we pass through the comet’s orbit, we sweep up some of that debris.
Most of the meteors we see, for all of their brilliance, are no bigger than grains of sand, but they hit the atmosphere at speeds between 35 and 95 kilometres a second, which provides lots of energy for extremely bright burning. (Smaller bits of dust, called micrometeorites, float constantly down onto the Earth from space without burning up at all, at a rate estimated at 1,000 tonnes a day.)
Meteorites are quite different. About 500 large ones probably fall on Earth every year, but only about one percent are recovered. They’re usually made of either iron or stone or both. Stone ones are most common, but it’s the iron ones we usually find, because it’s hard to tell a stone meteorite from any other kind of stone. They’re not associated with comets; instead, they, like the asteroids, are probably the remnants of some long-lost “parent body” which broke up when the solar system was young.
Auroras, meteorites, stars–the night sky is always fascinating. All you have to do, as the Friendly Giant used to say, is, “Look up. Look way up.”