A curtain of green auroras ripples over hoodoo rock formations near Drumheller, Canada, early Monday, Labor Day in the U.S. The same night, similar shows enlivened skies over many high-latitude countries across the Northern Hemisphere.
Last Friday a solar flare exploded off the sun, launching a giant cloud of charged gas—a coronal mass ejection, or CME—toward Earth. About 48 hours after NASA's Solar Dynamics Observatory had detected the eruption, the CME, travelling faster than a million miles an hour, slammed into Earth's magnetic field, sparking the auroras.
When a CME, or solar wind, enters the upper atmosphere, its charged particles smash into and break up gas molecules, which give off energy in the form of the so-called northern lights (or in the Southern Hemisphere, southern lights).
The colors a sky-watcher sees depend on the type of gas being hit and how high it is. For example, the green aurora pictured was the result of oxygen-atom collisions about 60 to 120 miles (100 to 200 kilometers) up.
An unusually strong burst of green auroras paints the skies over Edmonton, Canada, on September 3.
While this weekend's geomagnetic storms were essentially atmospheric fireworks, solar storms can adversely affect spacewalking astronauts, Earth-orbiting satellites, and even communications and power systems on the ground.
Northern Norway is known for great auroral displays, and this week's geomagnetic storm—pictured lighting up the Tromsø countryside—was no exception.
"I have seen a few auroras in my life, and I have been to this location a few times, but this was just sick!" astrophotographer Ole Salomonsen wrote on spaceweather.com. "I think all I can say is WOW!"
Photograph by Ole C. Salomonsen
River of Light
A winding stream of yellow and green auroras are mirrored in the Yukon River outside of Whitehorse, Canada, early Monday.
While this week's display was impressive, it did have to compete with the glare of a waning gibbous (more than half full) moon. When Earth's natural satellite is above the horizon, it can wash out all but the most intense of displays with its light.
Photograph by David Cartier, Sr.
Bright northern lights can catch people off guard, but in high-latitude locales such as Kuujjuak, Canada, auroras become a fairly common sight as nights lengthen at summer's end.
"These were just out of this world—all over the place," photographer Debbie O'Brien wrote of the recent show on spaceweather.com.
Photograph by Debbie O'Brien
A supernova-like burst of mostly purple auroras lights up Finnish countryside early Monday in a wide-angle sky shot.
"After hours of waiting for the big aurora to show from the CME, and waiting for clouds to go away, I was about to give up. Then suddenly a spark on the horizon, which just grew, and grew and grew!" photographer Ole Salomonsen wrote on spaceweather.com.
Photograph by Ole C. Salomonsen
Spectacular ribbon and burst patterns of green auroras blanket skies over Lulea, Sweden, early Monday—but not too early.
"I was surprised to still see some auroras, glad I checked, because it sparked to life in green and purple just when I went out," astrophotographer Oskar Pettersson wrote on spaceweather.com.
Photograph by Oskar Pettersson
Spears Over Norway
Like javelins of light hurled from the heavens, northern lights appear to be aimed at Kvaløya, Norway.
Sky-watchers may have noticed the uptick in auroras this past year as the sun approaches what's known as solar maximum in early 2013—a peak in activity during our star's roughly 11-year cycle of magnetic fluctuations. (Related: "Hyperactive Sun Helping to Clear Out Space Junk.")
Photograph by Fredrik Broms
A snaking green aurora greeted sky-watchers in the northern Quebec town of Ivujivik early Monday.
This weekend, for instance, photographer P-M Hedén was at the ready well before auroras kicked into gear above the lights of Stockholm, Sweden, and of passing planes—seen in a long-exposure photograph.
"From the beginning we noticed pillars and some movements, and after that, the big bow over the city," Heden said in an email to National Geographic News.