Making Auroras

Read about a team studying how the sun interacts with Earth’s magnetic field and the particles trapped in it, and how this type of interaction produces the northern and southern lights.

By Carl Zimmer|Wednesday, April 01, 1992
By the time Morrie Pongratz stepped outside to watch his experiment one night last July, the moon had set. But it couldn’t have competed with Pongratz’s experiment in any case. The experiment began at 3:37 a.m. eastern standard time, when a NASA satellite orbiting 270 miles overhead ejected two three-foot-long metal canisters, which then released 23 pounds of barium gas into space. Within seconds the barium blossomed into a giant yellow-green fireball, even bigger and brighter than a full moon. The release was spectacular, says Pongratz, a space physicist at Los Alamos National Laboratory. You’re on the ground in the dark, but up where the barium is, the sun is shining.

Pongratz and his colleagues are studying how the sun interacts with Earth’s magnetic field and the particles trapped in it--and in particular how this type of interaction produces the beautiful auroras commonly known as the northern and southern lights. The problem the researchers face is that you can’t see a magnetic field. That was the point of the several barium releases they carried out last summer. Casting barium over the magnetic field was like scattering iron filings around a bar magnet--suddenly the researchers could see how the field behaves.

Earth’s magnetic field is similar to a bar magnet’s: its lines of force come out of the south magnetic pole and loop back north. One difference, though, is that Earth’s field is constantly pushed around by the solar wind, a stream of electrons and atomic nuclei that flows out from the sun and carries the sun’s magnetic field with it. When the lines of that field happen to connect with the lines of Earth’s field, a strange thing happens: the solar wind tries to carry Earth’s field away. In the planet’s lee the field gets stretched long and thin. Finally, because it resists such abuse, it snaps like a rubber band.

Part of Earth’s field goes rushing downstream toward the outer planets, a great eddy in the solar wind. The other part rebounds back toward Earth, bringing trapped particles and a huge amount of energy with it. Somehow the strength of this slingshot energizes electrons close to the planet, which crash into atoms in the polar atmosphere and thus--somehow-- switch on the aurora. Pongratz’s barium clouds are indicating how.

Pongratz chose barium for the experiment not only because it would reflect sunlight well (hence the yellow glow) but also because its electrons could be easily knocked off by sunlight, thereby giving the barium atoms a positive electric charge. Like the recoiling charged particles that generate the real aurora, Pongratz’s barium ions (which glowed blue) were affected by Earth’s magnetic field. While the yellow cloud of neutral barium hurtled through space, moving with the same speed as the satellite that released it, the ionized barium was dragged by the magnetic field into a long blue wake. Some of the energy lost by the barium was transferred to electrons in the atmosphere--and those excited electrons generated an artificial aurora, albeit a faint one. The barium clouds were way brighter than the aurora, says Pongratz, so it wasn’t until we looked at the data a couple months later that we saw it.

In fact, the researchers are still analyzing the satellite data to find out exactly how the barium cloud transferred its energy. But Berkeley space physicist John Wygant believes the ionized barium did what any moving electric charge does: it sent out electromagnetic waves. As the barium crossed the lines of Earth’s magnetic field, those waves dragged the field lines along for a bit and then released them, in effect plucking the lines like guitar strings. The vibrating field lines, in turn, generated a current of electrons that crashed into atoms and caused them to emit auroral light.

That, says Pongratz, is probably how the real aurora is generated: when Earth’s stretched magnetic field snaps, the clouds of charged particles recoiling toward the planet emit electromagnetic waves that pluck the field lines and send electrons racing down the lines toward the poles. These ideas have been around for a long time, says Pongratz, but this is the first real confirmation we’ve had of them. And certainly, given that it outshone the moon, Pongratz’s evidence was as dramatic as it comes.
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