Friday, July 25, 2008

Propagation: NASA Finds Cause for Auroral Substorms

Polar substorms are sudden fluctuations in the aurora caused by bursts of charged particles in periods of solar-terrestrial disturbance. These cause sudden changes in already disturbed radio propagation, as terrestrial magnetic fields and currents redistribute themselves.

NASA's new THEMIS, a five-satellite constellation that takes data on the earth's magnetosphere, has provided evidence that these substorms come from shifts and re-connections of the disturbed magnetosphere, far out from the planet.

Researchers have discovered that an explosion of magnetic energy a third of the way to the moon powers substorms, sudden brightenings and rapid movements of the aurora borealis, called the Northern Lights.

The culprit turns out to be magnetic reconnection, a common process that occurs throughout the universe when stressed magnetic field lines suddenly "snap" to a new shape, like a rubber band that's been stretched too far.


These observations confirm for the first time that magnetic reconnection triggers the onset of substorms. The discovery supports the reconnection model of substorms, which asserts a substorm starting to occur follows a particular pattern. This pattern consists of a period of reconnection, followed by rapid auroral brightening and rapid expansion of the aurora toward the poles. This culminates in a redistribution of the electrical currents flowing in space around Earth.