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Arctic AurorasNorthern lights dance over the Lyngan Alps in a picture taken Tuesday night near Tromsø, Norway. The brilliant auroras were triggered by a coronal mass ejection, or CME, that hit our planet Tuesday morning. A CME is a cloud of superheated gas and charged particles hurled off the sun.On Monday, space-weather scientists reported that an especially strong solar flare had erupted from an active region on the sun, followed by the huge CME that came barreling toward our planet. The burst of activity triggered the strongest solar storm experienced since October 2003, according to experts at NOAA's Space Weather Prediction Center in Boulder, Colorado.When a CME hits Earth, the charged solar particles can interact with gases in our atmosphere to produce the northern and southern lights. Sky-watchers were put on alert for intense auroras Tuesday night through Wednesday morning.(Watch a time-lapse video of this week's auroras over Lapland.)

Photograph by Ole C. Salomonsen, arcticlightphoto.no