Image Right: Flying over the south pole, the IMAGE spacecraft caught these views around 7 pm ET on July 26. Click on image for movie (1.9 MB) or download a high resolution (904 KB) still. Credit: NASA / UC Berkeley
The coronal mass ejection (CME) blast that triggered the aurora took place around 10:45 am ET on July 25, traveling at roughly 1300 km per second. It took a day and a half to reach Earth, allowing NOAA to issue warnings to satellite and power grid operators. At 20 times the size of Earth, the originating sunspot was the largest seen since the fall solar storm onslaught.
Image Left: The Polar spacecraft saw both the aurora borealis and aurora australis (northern and southern lights) expanding and brightening in parallel at midnight ET, July 27. Click on the image for movie (1.4 MB), or download the higher resolution version (356 KB). Credit: NASA/The University of Iowa
The region generated several medium-sized coronal mass ejections, massive explosions with a potential force of a billion one-megaton bombs. In this case, the CMEs were probably intensified by the sunspot's proximity to 'coronal holes', voids in the solar atmosphere from which material rapidly flows away from the Sun, causing high-speed streams in the solar wind. That combined with the CME could have been enough to make this mid-sized storm more severe than it would have been.
The aurora, also known as the Northern and Southern Lights, form when solar particles and magnetic fields pump energy into the Earth's magnetic field, accelerating electrically charged particles trapped within. The high-speed particles crash into Earth's upper atmosphere (ionosphere) over the polar regions, causing the atmosphere to emit a ghostly, multicolored glow.
More Information:Exploratorium's Aurora Site