NASA is preparing to launch a new mission to learn more about the causes of atmospheric substorms. A substorm is a period of intense geomagnetic activity, visible to observers as a sudden brightening of the polar aurorae.

The storms might be visible to us, but scientists don't really understand what triggers them. What is known is that the Northern lights suddenly brighten when floods of highly charged electrons race down the Earth's magnetic field lines and hit the upper atmosphere. The electrons are accelerated when energy from the solar wind, stored in the Earth's magnetosphere, is "explosively released".

THEMIS is a constellation of five satellites that NASA plans to put into orbit in mid-February this year. Once spinning above us in their proper formation, they will collect coordinated measurements every four days. NASA expects the satellites will collect data from more than 30 substorms during the mission's scheduled two year lifespan.

"A substorm starts from a single point in space and progresses past the moon's orbit within minutes, so a single satellite cannot identify the substorm origin. The five-satellite constellation of THEMIS will finally identify the trigger location and the physics involved in substorms," said Vassilis Angelopoulos, THEMIS' principal investigator at the University of California, Berkeley's Space Sciences Laboratory.

He says the source of the energy releases is "a question almost as old as space physics itself". ®

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