Information courtesy of NASA. Additional editing by Phil Duckworth
Spring is here and as well as daffodils, warmth from the afternoon sun, and the days drawing slightly longer is the sight of the Aurora Borealis. Spring is aurora season.
For reasons not fully understood by scientists, the weeks around the spring equinox are prone to Northern Lights. Canadians, Scandinavians, Alaskan husky dogs - all they have to do is look up to see the amazing sight of green curtains of light dancing across the night sky.
Auroras are caused by solar activity, but how could one season yield more auroras than another?
"There's a great deal we don't understand about auroras," says UCLA space physicist Vassilis Angelopoulos. For instance, "Auroras sometimes erupt with little warning and surprising intensity. We call these events 'sub-storms', and they are a big mystery."
What triggers the eruptions? Where is sub-storm energy stored? (It has to gather somewhere waiting to power the outburst.)
And, of course, why springtime?
To answer these questions NASA deployed five spacecraft named THEMIS to study auroras.
View our aurora gallery here
An aurora is a strong geomagnetic storm with side-effects ranging from satellite malfunctions in orbit to power outages on the ground. Telecommunications, air traffic, power grids and GPS systems are all vulnerable. So in a society that relies increasingly on these technologies, understanding these storms is vital.
Launched in February 2007, THEMIS has already observed one geomagnetic storm with a total energy of five hundred thousand billion (5 x 10^14) Joules, which is the same as the energy of a magnitude 5.5 earthquake. The storm moved twice as fast as was thought possible, crossing an entire polar time zone in 60 seconds.
THEMIS may have found the storm's power supply:
"The satellites have detected magnetic 'ropes' connecting Earth's upper atmosphere directly to the sun," says Dave Sibeck, project scientist for the mission at the Goddard Space Flight Center…
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