Magnetic ‘slinky effect’ may power aurora

MINNEAPOLIS / ST. PAUL–The spectacular aurora borealis displays that light up the northern nights could be powered by a gigantic “slinky” effect in Earth’s magnetic field lines, according to research performed at the University of Minnesota. Earth’s magnetic field resemble a slinky in that when “wiggled,” it undulates in waves that travel down the field lines at speeds up to 25 million miles per hour. These waves can pass energy to electrons, accelerating them along the magnetic field lines toward Earth. When the electrons hit atoms in the atmosphere, the atoms become excited and produce the colors of the aurora. Using electric and magnetic field data and images from NASA’s POLAR satellite, the researchers showed that energy from such waves is sufficient to power auroras and that statistically, the waves occur in the same locations as auroras–in a ring around the poles. The work will be published in the Jan. 17 issue of Science.

“We don’t know exactly what wiggles the field lines, but similar processes could explain the heating of the solar corona [the sun’s atmosphere], the release of energy during solar flares and the acceleration of the solar wind [a stream of charged particles from the sun],” said physics associate professor John Wygant, second author of the study. “At the edges of sunspots, other researchers have actually seen magnetic field lines waving. Understanding how such waves are caused and how they transmit energy is important to unraveling the complex processes behind larger-scale particle accelerations that occur, for example, in jets of material being ejected from black holes at the centers of galaxies.” The paper’s first author is Andreas Keiling, who headed the study while a doctoral student and, later, a research scientist at the University of Minnesota. He is now at the Center for Space Research on Radiation in Toulouse, France.

The ultimate source of energy for auroras is the solar wind. Flowing with the wind–which is mostly single protons and electrons–is a magnetic field that encounters Earth’s own field tens of thousands of miles above the planet surface. Earth is like a huge bar magnet, with magnetic field lines coming out near the poles, curving through space, and re-entering near the opposite pole. When the solar wind’s magnetic field sweeps by, it joins with some of Earth’s magnetic field lines and stretches them into space on the night side of Earth. The stretching energizes this part of the magnetic field until it suddenly “snaps” away from the solar wind and reconnects with Earth. This process, called reconnection, may send waves rippling through the magnetic field, like wiggling a slinky, said Wygant.

Energy from the waves then passes to electrons, sending them in beams along the magnetic field lines into the atmosphere. The color of the aurora depends on how deeply the electrons penetrate the atmosphere and which atoms they excite. Measurements of electrical energy at altitudes near 12,000 miles, where the electrons are accelerated, showed sufficient energy from the waves to power auroras, Wygant said.

Auroras also occur in south polar regions, where they are known as the aurora australis. Waves in the magnetic field lines are called Alfven waves, after Hannes Alfven, a Swedish physicist who helped found the field of plasma physics, said Wygant.

POLAR’s electric field measurements were performed by an instrument built by the University of California at Berkeley. Other authors of the paper are Cynthia Cattell, physics professor, University of Minnesota; Forrest Mozer, professor of physics, Berkeley; and Christopher Russell, professor of physics, UCLA. The work was supported by NASA.