Plasma Bullets and the Northern Lights

Southern Lights. Photo credit: J. Dana Hrubes

NASA reported the news today:

Duck! Plasma bullets are zinging past Earth.

That's the conclusion of researchers studying data from NASA's five THEMIS spacecraft. The gigantic bullets, they say, are launched by explosions 1/3rd of the way to the Moon and when they hit Earth—wow. The impacts spark colorful outbursts of Northern Lights called "substorms."

"We have discovered what makes the Northern Lights dance," declares UCLA physicist Vassilis Angelopoulos, principal investigator of the THEMIS mission. The findings appear online in the July 24 issue of Science Express and in print August 14 in the journal Science.

This is good news, although I hadn't realized they didn't already know what caused the Northern Lights.

The discovery came on what began as a quiet day, Feb 26, 2008. Arctic skies were dark and Earth's magnetic field was still. High above the planet, the five THEMIS satellites had just arranged themselves in a line down the middle of Earth’s magnetotail—a million kilometer long tail of magnetism pulled into space by the action of the solar wind.

That's when the explosion occurred.

I posted a while back on the magnetotail and how it affects the moon, but now we can see how it affects the earth as well.

Although the explosion happened inside Earth's magnetic field, it was actually a release of energy from the sun. When the solar wind stretches Earth's magnetic field, it stores energy there, in much the same way energy is stored in a rubber band when you stretch it between thumb and forefinger. Bend your forefinger and—crack!—the rubber band snaps back on your thumb. Something similar happened inside the magnetotail on Feb. 26, 2008. Over-stretched magnetic fields snapped back, producing a powerful explosion. This process is called "magnetic reconnection" and it is thought to be common in stellar and planetary magnetic fields.

The blast launched two "plasma bullets," gigantic clouds of protons and electrons, one toward Earth and one away from Earth. The Earth-directed cloud crashed into the planet below, sparking vivid auroras observed by some 20 THEMIS ground stations in Canada and Alaska. The opposite cloud shot harmlessly into space, and may still be going for all researchers know.

An artist's concept of the THEMIS satellites lined up inside Earth's magnetotail with an explosion between the 4th and 5th satellites.

Pretty. Darn. Cool!