The Mighty Roots of the Aurora

If you've lived for any length of time in the northland, and if you read articles like this, then you probably know what causes the northern lights. Properly known as the Aurora Borealis, these spectacular bands and curtains of light in the night sky can be seen more frequently in Alaska than in any other state of the union, and they shine above the Interior more often than anywhere else. (Statistically, Fort Yukon is the aurora capital of the world.) So we sometimes claim them as our own, and cheechakos and Outsiders rightly expect us to be experts on this phenomenon; the ability to explain auroras is a matter of pride as well as fulfilled curiosity.

It's understood that the sun ultimately causes the aurora. The picture as we understand it was explained in this column by geophysicist Dan Swift 17 years ago. "It is almost certain," he wrote, "that the energy to power the aurora comes from the sun. From the sun there is a continuous outflow of matter in the form of electrons and nuclei of atoms, mostly hydrogen nuclei (protons). This flow, called the solar wind, streams at speeds near 400 km/sec (900,000 mph) and therefore takes several days to reach the earth, whereas light takes only eight minutes."

The solar wind powers what amounts to a gigantic electrical generator as it comes streaming in to Earth's magnetic field, and that generator provides the energy and charged particles to turn on the northern lights. The auroral light itself is emitted from molecules and atoms of Earth's atmospheric gases, in a process similar to the illumination of a neon sign or a television picture tube.

That much explanation will usually leave any questioners impressed if not downright glassy-eyed, but you may run into someone who pushes the point: "Well, I read somewhere that solar flares cause auroras and power outages and radio troubles and stuff. What's this solar wind business?"

Here's where the explanation gets interesting. A couple of years ago, you could have pointed out that solar flares, which are essentially eruptions on the sun's surface, were simply the propulsive mechanism that put gusts in the solar wind. In effect, the flares serve as the sling, and the protons and electrons are the shot.

Or so you could have said, and you were in very good company, because that's what the experts were saying too. Last year, some of them changed their tune.

According to the British journal Nature, many geophysicists now assert that both auroras and flares are manifestations of upheavals on the sun. The solar surface boils and seethes on a titanic scale, and the magnetic fields embedded within it are incessantly shifting and twisting to match. This great roiling turbulence sometimes brings oppositely directed magnetic fields together. In this so-called "magnetic reconnection," the oppositely pointing fields annihilate one another, releasing X-rays and energetic particles.

Usually the mutually annihilating fields lead to the eruption of a solar flare. Sometimes, however, a certain sort of large-scale magnetic upheaval will produce a far grander event---a coronal mass ejection. Envision a gigantic bubble of sun-stuff, subtending perhaps a quarter of the sun's circumference, bursting out and away from the solar body: that would put quite a kick in the solar wind.

Coronal mass ejections can be observed only from space-borne coronographs, observing instruments that black out the sun's brilliant disk so its outer layers can be studied. Thus, they're fairly recent additions to knowledge of the sun's behavior, and scientists are still figuring out what they do. But they've found some pretty good evidence that their great speed plus the strength and shape of their magnetic fields are what generate large geomagnetic storms and dramatic auroral displays.

So that's this year's explanation. Trust me, it'll silence any Outsider who asks, "When are they turning on the northern lights?"