September 28, 2016
The Northern Lights may be among Earth's most stunning sights, but what's the use if most of us — outside of Canada and Alaska, anyway — can't even see them?
Curious to know more about why the natural "aurora" phenomenon is isolated to the northern and southern tips of the planet, we reached out to Steve McMillan, department head of the Department of Physics at Drexel University, for answers.
For starters, what are the Northern Lights?
The light actually comes when charged particles from solar wind hit Earth’s atmosphere. What happens is these particles are very high-speed electrons and nuclei and things like that, and when they hit the atmosphere, they break molecules and atoms up and knock out the electrons, and when the electrons finally combine back to form the molecule again — like oxygen or nitrogen or something like that — they emit light, and that’s the light we see. Energy is coming from the solar wind temporarily breaking up the molecules in the upper atmosphere and, when they re-form, which they do fairly quickly, they emit the light we see. And the colors of the light are telling us which molecule is producing it.
Is there a season for the lights? And why can't we see them in between the North and South poles?
Not really. There’s another wrinkle in this, of course, which is that these particles are charged, so they are affected by Earth’s magnetic field — and that protects most of our surface from the solar wind, our own atmosphere, from the solar wind. The charged particles tend to follow the field lines; they tend to actually orbit around the field lines, channel around the field lines, and they hit Earth's atmosphere around the poles because that is where the magnetic field lines are coming vertically down on it toward the surface. So, around the equator — you’ve probably played with a magnet and seen the picture of a magnetic field line. It comes out of one pole to one end and loops back around to the other pole — so near the equator, the field lines are running parallel to Earth’s surface, so to speak. So the charged particles are not getting into the surface, they’re just following the field lines, but near the pole is where the field lines loop down and actually go inside Earth. So, the charged particles can just follow them straight down and hit the atmosphere.
The reason you see them at the North and South poles is that's where the magnetic field line permits the particles to actually hit the atmosphere. You can see it, sort of, in New England and Canada and part of Australia, but you won’t see it at the equator.
Is it possible to see them anywhere else?
It depends. You asked if they’re seasonal, and the answer is 'not really,' because they do depend on the tilt of Earth’s axis with respect to the sun. What they depend on mostly is the solar season — the strength of the solar wind. And that depends on the sunspot cycle. These particles are actually produced by magnetic activity near the surface of the sun. So, near a solar maximum, there’s more magnetic activity and more charged particles coming from the sun, and the more charged particles there are, the likelier they are to get farther and farther south. At the time of solar maximum, you're more likely to see Aurora Borealis a little bit farther south, in Pennsylvania. But most of the time, you don’t expect to see them [except] from more northern latitudes.
Anything dangerous about the Northern Lights?
No, not really. There is danger from solar wind if you get a solar flare or something like that. Interaction with Earth's magnetic fields can produce power outages and things like that, which is serious — especially if you’re on a spacecraft or even a jet. But Northern Lights themselves are — I guess I’d say the Northern Lights themselves are not dangerous. The light is not dangerous. The high-energy particles that produce them, in sufficient quantities, can be pretty dangerous.
Are the Northern Lights something unique to Earth’s atmosphere? Could these appear elsewhere in the universe?
All you need is a — yes, it should be able to. What you need is a star with a reasonably strong wind, and most stars have winds, and the sun does not have a particularly strong one. So those would change charged particles, and you’d need a planet with a magnetic field, and you need a planet with an atmosphere. And if you didn’t have a magnetic field, you’d see effects all over. But to see them as we see them, you’d need a magnetic field to channel the particles toward the North and South poles and an atmosphere for them to interact with. I’m not sure we know if any such planet exists, in our solar system, but, of course, we see this same thing in Jupiter and Saturn, which satisfy all these things. The Hubble images, if you search Google, you’ll see Jupiter and Saturn, and the most recent missions to Jupiter, you can see a very strong aurora. You can see Jupiter’s magnetic field — which, of course, is all atmosphere, basically. That’s been seen many times.
