Saturday, March 8, 2008
Aurora Australis from Mt Nelson, Tasmania on 10 February 2008. It unexpectedly appeared during group sky viewing session. The aurora shimmered and changed by the minute. The green was easily visible to the eye while the red showed up in only in this image that I took with a Canon PowerShot A550, 15 sec, ISO 800.
Aurora occur when the Sun produces a coronal mass ejection (CME). This cloud of charged particles travel out from the Sun at about 400 km/sec. When a CME brushes past the Earth, the Earth's magnetic field acts a shield to protect us but some of the particles manage to channel down through the north and south magnetic poles to collide with atoms of the Earth's upper atmosphere, about 80km up (the image shows clouds in silhouetted in the foreground). This excites atoms that produce light. The green and red come from atomic oxygen molecules.
I've see aurora twice before from Adelaide. They were fleeting lasting only few minutes. But this time I was ready with a camera and the Mount Nelson aurora obliged us with a show that lasted more than an hour!
More about aurora from Wikipedia