1 of 9

Green LanternAn auroral "flame" flickers over Tibbitt Lake in Canada's Northwest Territories early on August 7. Throughout last weekend, auroras shimmered above northern countries as Earth's atmosphere was hit by its strongest geomagnetic storm in years.Two August 3 solar flares were each accompanied by giant clouds of charged gas—or coronal mass ejections—aimed at Earth, triggering the sky show. About 48 hours after NASA's Solar Dynamics Observatory had detected the first eruption, the "solar wind" slammed into Earth's magnetic field, creating a geomagnetic storm that lasted four hours.Astronomers are saying the show may not be over just yet, either.

"There is the possibility of increased auroral activity, as we may be hit by the flank of last week's storm in the coming days," said Raminder Singh Samra, an astronomer with the H.R. MacMillan Space Centre in Vancouver, Canada."So for those in Canada and northern U.S. border states, it may still be worth going outside around local midnight and keeping watch towards the northern horizon for those mysterious glows."(See more aurora pictures.)—Andrew Fazekas

Photograph courtesy Michael Ericsson