Caused by STRONG geomagnetic disturbances, we didn't have any strong geomagnetic storm to create this SAR arc...
The storm on Oct. 18th was not strong. Nevertheless, it created one of the biggest and brightest SAR arcs in years. Some sky watchers could see it even though the human eye is famously insensitive to the arc's pure red color (630.0 nm).
Baumgardner doesn’t know why the arc was so bright. He says it was 10 to 30 times brighter than a G2 storm would typically produce. "We need to gather more information."
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