Watching the famed northern lights flicker across a night sky is a rare treat for Americans outside of Alaska. But another large solar flare on September 6 could mean more auroras for the northern U.S. and Europe, which were also treated to the spectacle on Wednesday and Thursday nights.
The flare that caused auroras on Wednesday was classified as an M5 flare, whereas the more recent flare was an X9.3. Flares are ranked on a scale of five letter groups (M is the second largest, and X flares are 10 times larger); the number afterward represents subclasses of strength. The September 6 flare is the largest seen since 2005.
The flares themselves can interfere with GPS and communications satellites and even certain radio frequencies. But they are also associated with coronal mass ejections, which are spurts of the hot, charged particles called plasma that make up the sun.
When that plasma reaches Earth's atmosphere, it collides with air molecules, passing energy along to them accompanied by a burst of light. Although the transfer happens between 50 and 300 miles above the surface of the Earth, it's visible on clear dark nights.
Typically, auroras are limited to a band circling the poles—in the Northern Hemisphere, at about the latitude of Alaska and Scandinavia. This week's coronal mass ejections, however, have made auroras occur much further south, including Edinburgh in the U.K. and Maine.
The sun's activity typically rises and falls over an 11-year cycle governed by the flipping of the sun's magnetic field (the solar equivalent of the North and South poles trading places every decade).
As the magnetic field roils and flips over the course of its cycle, it knots the sun's plasma into tangled storms called sunspots, which are the source of flares and coronal mass ejections. The magnetic activity blocks heat from rising to the sun's surface in that particular location, causing a cold splotch that looks dark from Earth.
If you still have your solar-viewing glasses from August's eclipse, you can use them during the day to see the sunspot that caused the flare.