A network of ground-based cameras deployed around the Arctic in support of NASA's THEMIS mission has made a startling discovery about the aurora borealis, commonly known as the Northern Lights. Sometimes vast curtains of aurora borealis collide, producing spectacular outbursts of light.
Video images of the phenomenon were presented Dec. 17 at the fall meeting of the American Geophysical Union in San Francisco.
"Our jaws dropped when we saw the movies for the first time," said Larry Lyons, UCLA professor of atmospheric and oceanic sciences and a member of the research team that made the discovery. "These outbursts are telling us something very fundamental about the nature of auroras."
NASA and the Canadian Space Agency created the camera network in the Arctic. THEMIS (Time History of Events and Macroscale Interactions during Substorms) consists of five identical satellite probes launched in 2006 to solve a long-standing mystery: Why do auroras occasionally erupt in an explosion of light known as a substorm? Substorms are dramatic disturbances of the global magnetosphere-ionosphere system that release large amounts of solar wind energy and are associated with auroral activations.
Over the last 40 years, substorms have been studied extensively from the ground and in space. However, the sequence of events during a substorm has remained elusive and been a key subject of debate among scientists who study the physics of the near–Earth space environment.
"Using the array of ground-based imagers of the THEMIS program, we have found what appears to be the answer to this long debate, the answer having similarities to, but also important differences from, previous ideas," Lyons said.
"The initial brightening seems to have started hundreds of miles away from the eventual epicenter of the substorm, and nobody before had managed to put the two together," said Vassilis Angelopoulos, UCLA professor of Earth and space sciences and the principal investigator for THEMIS. "These results unify old and recent observations from the ground and in space in a totally new paradigm of substorm evolution."
Twenty all-sky imagers (ASIs) were deployed across the Alaskan and Canadian Arctic to photograph auroras from below while the spacecraft sampled charged particles and electromagnetic fields from above.
The breakthrough came earlier this year when UCLA researcher Toshi Nishimura assembled continent-wide movies from the individual ASI cameras.
The first movie he showed Lyons was of a pair of auroras crashing together in December 2007.
"It was like nothing I had seen before," Lyons recalled. "Over the next several days, we surveyed more events. Our excitement mounted as we became convinced that the collisions were happening over and over."
The explosions of light, they believe, are a sign of something dramatic happening in the space around Earth — specifically, in the Earth's "plasma tail." Millions of miles long and pointed away from the sun, the plasma tail is made of charged particles captured mainly from the solar wind. Sometimes called the "plasma sheet," the tail is held together by the Earth's magnetic field.
"Collisions of auroras associated with plasma coming from the deep plasma tail, with the aurora coming from the plasma in the nearest portion of the plasma tail, set up an unstable configuration," Lyons said.
The same magnetic field that holds the tail together also connects it to the Earth's polar regions. Because of this connection, watching the dance of the Northern Lights can reveal much about the plasma tail.
Nishimura, Lyons and Angelopoulos, together with Stephen Mende from the University of California, Berkeley, have identified a common sequence of events. It begins with a broad curtain of slow-moving auroras and a smaller knot of fast-moving auroras, initially far apart. The slow curtain quietly hangs in place, almost immobile, while the speedy knot rushes in from the north. The auroras collide, and an eruption of light ensues.
How does this sequence connect to events in the plasma tail? Lyons believes the fast-moving knot is associated with a stream of relatively lightweight plasma jetting through the tail. The stream gets started in the outer regions of the plasma tail and moves rapidly inward toward Earth. The fast knot of auroras moves in sync with this stream.
Meanwhile, the broad curtain of auroras is connected to the stationary inner boundary of the plasma tail and is fueled by plasma instabilities there. When the lightweight stream reaches the inner boundary of the plasma tail, there is an eruption of plasma waves and instabilities. This collision of plasma is mirrored by a collision of auroras over the poles.
National Science Foundation–funded radars located in Poker Flat, Alaska, and Sondrestrom, Greenland, confirm this basic picture. They have detected material rushing through the Earth's upper atmosphere just before the auroras collide and erupt. The five THEMIS spacecraft also agree. They were able to fly through the plasma tail and confirm the existence of lightweight flows rushing toward the Earth last year. In results reported in July 2008 in the journal Science, the THEMIS team identified the mechanism that triggers such plasma streams toward Earth, just prior to onset.
At high northern latitudes in the northern U.S. and Canada, the shimmering bands of light of the aurora borealis stretch across the sky from the east to the west. During geomagnetically disturbed periods known as substorms, the bands brighten. These multicolored light shows are generated when showers of high-speed electrons descend along magnetic field lines to strike the Earth's upper atmosphere.
The THEMIS mission is establishing for the first time when and where substorms begin, determining how the individual components of substorms interact, and discovering how substorms power the aurora borealis.
Catching the substorms as they happen are THEMIS's five satellites — with electric, magnetic, ion and electron detectors — in carefully chosen orbits around the Earth and the array of 20 ground observatories with automated, all-sky cameras located in the northern U.S. and Canada.
As the satellites are measuring the magnetic and electric fields of the plasma above the Earth's atmosphere once every four days, the ground-based observatories are imaging the auroral lights and the electrical currents from space that generate them. THEMIS was launched on Feb. 17, 2007, from Cape Canaveral, Fla.
Themis was the blindfolded Greek goddess of order and justice. In 1619 A.D., Galileo Galilei coined the term "aurora borealis" after Aurora, the Roman goddess of morning. He had the misconception that the auroras he saw were due to sunlight reflecting from the atmosphere.
For more information on the THEMIS mission, visit http://themis.ssl.berkeley.edu/ and www.nasa.gov/themis. For more about the National Science Foundation and the research it supports, visit www.nsf.gov.
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