What Causes Aurora?

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Will you be seeing an aurora tonight? Yesterday, January 23, 2012, an M8.7-class flare erupted from the Sun and sent a huge wave of high-energy protons towards Earth and Mars, moving at about 8 million kilometers per hour. According to Spaceweather.com, the CME hit Earth’s magnetic field at approximately 1500 UT (10 am EST) today, and geomagnetic storms are likely in the hours ahead, as scientists say this the largest solar storm Earth has encountered since 2005.

But what is an aurora and what causes them?

(via universetoday)