Photos: Multicolored Auroras Sparked by Double Sun Blast

Green Lantern
Photograph courtesy Michael Ericsson
An 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
Published August 10, 2011
Dancing Sky
Photograph by Olivier du Tré
Glimmering green and blue auroras cover large swaths of sky above Canada on August 5. "At the biggest, about 60 percent of the sky was dancing," wrote photographer Olivier du Tré on the SpaceWeather website.
Auroras occur when energized particles from the sun reach Earth and get funneled down the planet's magnetic field lines toward the Poles. Along the way, the charged particles smash into oxygen and nitrogen atoms in our atmosphere and boost their energy, which ends up being emitted as colorful light.
(Also see pictures of unusual auroras in 2010.)
Published August 10, 2011
Poland Goes Purple
Photograph courtesy Kamila Mazurkiewicz
Kamila Mazurkiewicz doubted she'd see the northern lights near Pulawy, Poland, but August 5 auroras (pictured) showed her the possibility is "not so much unrealistic as infrequent."
The photographer rubbed her eyes in amazement as a burst of extra-energetic electrons penetrated Earth's atmosphere farther south than usual, hitting nitrogen and causing the air to glow in shades of pink and purple.
Auroras are caused by charged particles—mainly electrons—from space raining down on Earth's atmosphere, causing the air to glow. Shades of green are most common and occur when molecules of oxygen are hit, about 60 miles (97 kilometers) or more above Earth's surface.
(Also see "Aurora 'Power Surges' Triggered by Magnetic Explosions.")
Published August 10, 2011
Double Feature
Photograph courtesy Robert Berdan
As the geomagnetic storm began late on August 5, a kaleidoscope of colors bathed the skies north of Calgary, Canada.
"This past weekend's display was caused by a double coronal mass ejection, an ejection of solar wind from the surface of the sun," astronomer Raminder Singh Samra said.
Published August 10, 2011
Superior Sky Show
Photograph courtesy Greg Kretovic
Sky-watchers in regions far from traditional aurora-watching country witnessed northern lights this past weekend—such as this stunner over Michigan.
"It was amazing seeing them move from the Lake Superior horizon to overhead," photographer Greg Kretovic said.
Published August 10, 2011
Stairway to the Heavens
Photograph by Olivier du Tré
Auroras appear suspended from the stars in this picture by photographer Olivier du Tré, captured on August 5 in Cochrane, Canada. From the moment du Tré set out that night, he "knew this was going to be special," he wrote on the SpaceWeather website.
Besides sparking colorful sky shows, geomagnetic storms can adversely affect spacewalking astronauts, Earth-orbiting satellites, and even communications and power systems on the ground. (See "As Sun Storms Ramp Up, Electric Grid Braces for Impact.")
Published August 10, 2011
Curtains and Clouds
Photograph courtesy Robert Berdan
Green auroras and orange-tinted clouds from a passing storm lighted up skies near Calgary, Canada, early on August 6. (Find out how photographer Robert Berdan took this picture.)
Published August 10, 2011
Northern Reflections
Photograph by Steve McDougall, My Shot
Pictured by National Geographic My Shot user Steve McDougall, an auroral "river" flows over Garden Hill, Canada just after midnight on August 6.
Because they're closer to Earth's Poles, where charged particles of solar wind penetrate the atmosphere, the Northern Hemisphere's higher latitudes traditionally experience brighter displays than areas farther south. Last week's huge solar windstorm only added to the intensity.
"The brightness of the aurora is directly linked to the flux of solar wind, and if there is more of it, then there is an increase in the amount of interactions between the charged particles and the atoms in the upper atmosphere,” astronomer Samra said.
"With an increase in the amount of interactions of charged solar wind particles and atoms in the atmosphere, aurora appear brighter."
Published August 10, 2011
Oz Above the Clouds
Photograph by Matthew E. Moses
Ghostly green pillars hover over Lake Superior near Duluth, Minnesota, on August 5—"the most brilliant display in years!" according to photographer Matthew E. Moses. (Related: "Light Pillar Pictures: Mysterious Sky Shows Explained.")
Astronomer Raminder Samra said, "We should expect similar storms and displays of the auroras in the future" as the sun awakens from a period of record low activity and heads toward the peak of its current 11-year cycle.
More: Don't miss Andrew Fazekas's weekly sky-watching tips >>
Published August 10, 2011