A network of cameras deployed around the Arctic to understand the trigger mechanism for the magnetospheric substorms responsible for the beautiful light display called the aurora borealis – also known as the Northern Lights – has discovered that sometimes the vast curtains of aurora borealis collide, producing a stunning outburst of light.
The reason no one on Earth has ever noticed these collisions before is that they occur on such a vast scale it takes a network of sensitive cameras spread across thousands of miles to get the whole picture. Luckily NASA and the Canadian Space Agency had created just such a network of cameras for the Time History of Events and Macroscale Interactions during Substorms (THEMIS) mission.
The THEMIS mission was designed to solve the mystery of where and when substorms begin. It consists of a fleet of five identical satellites, strategically placed in key positions in the magnetosphere, in order to isolate the point of substorm origin. While the spacecraft sampled charged particles and electromagnetic fields from above, 20 all-sky imagers (ASIs) were deployed across the Alaskan and Canadian Arctic to photograph auroras from below. Capturing the action from both sides would allow the researchers to put together a coherent, large-scale movie that would reveal the substorms’ cause and effect.
When UCLA researcher Toshi Nishimura assembled continent-wide movies from the individual ASI cameras the team was stunned.
"Our jaws dropped when we saw the movies for the first time," said space scientist Larry Lyons of UCLA, a member of the team that made the discovery. "These outbursts are telling us something very fundamental about the nature of auroras." Lyons recalls, "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 researchers believe the explosions of light are a sign of something dramatic happening in Earth’s “plasma tail” – a tail millions of kilometers long that points away from the sun made up of charged particles captured mainly from the solar wind that is held together by the Earth’s magnetic field. Because the same magnetic field that holds the plasma tail together also connects it to the Earth’s polar regions, watching the Northern Lights can reveal much about what is happening in the plasma tail.
By putting together data from ground-based cameras, ground-based radar, and the THEMIS spacecraft, the team says they now have a nearly complete picture of what causes explosive auroral substorms.
The explosions of light were found to occur when a broad curtain of almost immobile auroras collides with a smaller knot of fast-moving auroras that rush in from the north. Lyons believes the smaller, fast-moving knot is associated with a stream of relatively lightweight plasma jetting through the tail that gets started in the outer regions of the plasma tail and moves rapidly inward toward Earth. The fast knot of auroras moves in synch with this stream.
Meanwhile, the broad curtain of auroras is connected to the stationary inner boundary of the plasma tail and 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, which is mirrored by a collision of auroras over the poles.
Readings from the radars located in Poker Flat, Alaska, and Sondrestrom, Greenland combined with data from the THEMIS spacecraft suggest that this theory is correct. The radars have detected echoes of material rushing through Earth's upper atmosphere just before the auroras collide, while the THEMIS spacecraft have flown through the plasma tail and confirm the existence of lightweight flows rushing toward Earth.
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