“The sight filled the northern sky; the immensity of it was scarcely conceivable. As if from Heaven itself, great curtains of delicate light hung and trembled. Pale green and rose-pink, and as transparent as the most fragile fabric, and at the bottom edge a profound and fiery crimson like the fires of Hell, they swung and shimmered loosely with more grace than the most skillful dancer. Lyra thought she could even hear them: a vast distant whispering swish.”
This is Philip Pullman’s description of the aurora borealis in his award-winning novel, “Northern Lights.” The little girl Lyra, seeing the lights for the first time, is moved almost to tears by their beauty and wonders to herself: What makes the aurora glow? (She also thinks that she can hear noise from the lights, but more of that later.)
The mystery of what powers the aurora was essentially solved by the pioneering Norwegian scientist Kristian Birkeland about 100 years ago. Birkeland demonstrated that the solar wind, a massive flux of protons and electrons streaming out of the sun, interacted with the Earth’s magnetic field to create the atmospheric illuminations. His ideas caught on all over Europe, except for England.
The Royal Society, then the world’s leading scientific organization, refused to accept Birkeland’s thesis, and during his lifetime he never achieved the recognition he deserved. Yet today, Birkeland’s masterwork, “The Norwegian Aurora Polaris Expedition 1902-03,” is recognized as an extraordinary and prophetic research piece. And last week in Science a paper was published that describes in far more detail what Birkeland proposed at the beginning of the 20th century.
The Northern Lights are powered by a gigantic “Slinky” effect in the lines of Earth’s magnetic field, say a team of scientists headed by Andreas Keiling, now at the Center for Space Research on Radiation in Toulouse, France. The group’s research was performed at the University of Minnesota.
“I am aware of Birkeland and his work,” Keiling said in an e-mail interview. “All auroral researchers know his work very well. He is most famous for predicting electrical currents above the aurora. These currents flow along the Earth’s magnetic field lines and are known as ‘Birkeland currents’ in recognition of his work.”
Earth’s magnetic field resembles a Slinky — the coil-like children’s toy — in that when “wiggled” it undulates in waves that travel down the field lines at speeds of up to 15 million kph. 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 (that is, they rise to a higher energy state) 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 the Slinky waves powers auroras. They also showed that the waves occur in the same locations as auroras: in a ring around the poles.
“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,” said University of Minnesota physicist John Wygant, an 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 ultimate source of energy for auroras is the solar wind. Flowing with the stream of positively charged protons and negatively charged electrons is a magnetic field that encounters Earth’s own field tens of thousands of kilometers 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. The Polar satellite made measurements of electrical energy at altitudes between 25,000 and 38,000 km, where the electrons are accelerated, and showed there was sufficient energy from the waves to power auroras. This means that at least part of the energy behind the auroras is more distant than previously thought.
The work also illustrates why the centuries-old legends that the aurora generates noise are unlikely. Like the tellers of those legends, Lyra, the little girl in “Northern Lights,” thinks that as the aurora shimmers there is an accompanying swishing sound. But the distance between an observer and the Lights is so great that there would be a considerable delay before any noise was audible.
In any case, the air is so thin that high in the atmosphere that sound can’t travel through it — that’s why in space no one can hear you scream.