Skies lit up over the peninsula in rare fashion on May 10, when the effects of a powerful geomagnetic storm transformed the night sky and sent many residents outside to witness the display and snap photos for social media.

Known as the Northern Lights, the light show was the result of solar storms, called coronal mass ejections (CMEs), which sent waves of electrified gas particles toward earth. The light show is a phenomenon well known in far northern latitudes, but infrequently seen with such clarity and splendor in Washington state. The intensity of this geomagnetic storm created auroras that were visible in the southern United States.

The Space Weather Prediction Center issued a Severe Geomagnetic Storm Watch on May 9, which was in effect all week. It was the first such warning since 2005. The severity was confirmed the next morning after NOAA scientists observed at least five CMEs aimed at earth occurring in quick succession. 

Scientists use the Kp-index to assess aurora strength and G ranges to measure geomatic storms, which can seem confusing when they are discussed separately. The Kp index is used to predict regular auroral activity, but the measures fit on the same continuum.  The Kp-index ranges from 0-9, has Kp-1 very weak, and anything above Kp5 is a geomatic storm. A Kp-5 is a G1, a Kp-6 a G2, all the way up to a Kp-9, which is G5 and the highest geomagnetic storm.

“We can definitely see the aurora in western Washington with less intense solar storms, but it’s usually low on the horizon and fairly dim,” said Logan Howard, a meteorologist at National Weather Service in Seattle. “It isn’t uncommon to see it, but something this bright and overhead is certainly rare.”