aurora's continued...

For reasons not fully understood by scientists, the weeks around the vernal equinox are prone to Northern Lights.

This is a bit of a puzzle. Auroras are caused by solar activity, but the Sun doesn't know what season it is on Earth. So 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 and others, NASA has deployed a fleet of five spacecraft named THEMIS (short for "Time History of Events and Macroscale Interactions during Substorms") specially instrumented to study auroras. Angelopoulos is the mission's principal investigator.

The THEMIS mission uses five identical spacecrafts to study auroras and the substorms that ignite them. This is an artist's concept of one of the THEMIS probes. NASA/Goddard Space Flight Center's Conceptual Image Lab [View Larger Image]

Auroras are more than just pretty lights in the sky. Underlying each display is a potent geomagnetic storm with possible side effects ranging from satellite malfunctions in orbit to power outages on terra firma. Telecommunications, air traffic, power grids and GPS systems are all vulnerable. In a society that relies increasingly on space technology, understanding these storms is vital.

Launched in February 2007, THEMIS has already observed one geomagnetic storm with a total energy of 500,000 billion (5 x 10^14) Joules. "That's approximately equivalent to the energy of a magnitude 5.5 earthquake," says Angelopoulos. "This storm moved twice as fast as anyone thought possible," crossing an entire polar time zone in 60 seconds flat!

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. "We believe that solar wind particles flow in along these ropes, providing energy for geomagnetic storms and auroras." Sibeck likens them to ropes because the magnetic fields in question are organized much like the twisted hemp of a mariner's rope. Solar wind particles flow along the ropes in whirligig trajectories leading from the Sun to Earth.