Such auroral displays are triggered when clouds of charged particles from the sun—known as coronal mass ejections (CMEs)—slam into Earth's magnetic field. As the particles get funneled along field lines toward the Poles, they collide with molecules in Earth's atmosphere, infusing them with extra energy. The molecules in turn release the energy as light. (Also see pictures: "Multicolored Auroras Sparked by Double Sun Blast.")
On September 5 to 7 the sun sent out a series of CMEs associated with several powerful solar flares. The first particle cloud reached Earth a few days later, triggering 18 hours of auroras that were seen in the United States as far south as Maine, Michigan, Vermont, and Washington State.
Photographer Bryan Hansel used a fisheye lens to capture the extent of the glow over Northern Light Lake: "The full moon was out, and I was worried that the colors would wash out. There was also a fog building on the lake, which would later obscure the display," Hansel said in an email.
"But during this picture the northern lights intensified and were overhead and the fog stayed away. ... The light from the full moon lit up the lake. I had a couple of friends with me, and they were clapping and cheering when I took this photo."
Photograph by Bryan Hansel
Although the northern lights display wasn't at first visible to the naked eye, photographer P-M Hedén captured a faint green "aurorabow" in long-exposure pictures snapped September 9 from Vallentuna, 18.6 miles (30 kilometers) north of Stockholm, Sweden.
"I decided to do a time-lapse. For this I chose 13-second exposure, ISO 800 [at] f/3.2, and hit the button using my Canon 550D and Tamron 11-millimeter," Hedén said in an email, adding the he took 780 photos to compile his video. "I also wanted to do a star trails shot, so I stacked the photos, and this was the result."
The composite picture shows not only the faint green aurora but also moonlit clouds, circular star trails, and the path of an airplane coming in for a landing at Stockholm-Arlanda Airport.
A bright midnight aurora creates a ghostly highway in the skies over Fairbanks, Alaska, on September 12. A second CME arrived at Earth on September 11, sparking a new round of auroras on Sunday and Monday nights.
Fairbanks "was overcast and drizzly for the first round of geomagnetic storms caused by the CME," photographer Brandon Lovett commented on SpaceWeather.com. But skies cleared two days later, allowing Lovett to snap several shots of the northern lights display.
"Even with the bright harvest moon, the colors shone through quite vibrant, with active pinks and dark greens," he wrote. "There never is a dull night at latitude 65!"
"Unfortunately, the northerly sky was backlit by the full moon, so images needed longer exposure to capture the intensity," resulting in a loss of detail, photographer Nathan Biletnikoff said in an email. Still, to the naked eye, "we were graced with a beautiful display of bands and streamers—many of which would come and go."
Auroral curtains swirl over Canada's Northwest Territories in a picture taken early Monday morning.
"This image was taken at Prosperous Lake on September 12 before 3 a.m., when the harvest moon was shining from the left side of the frame," photographer Yuishi Takasaka said via email.
According to Takasaka, tourists often visit the nearby town of Yellowknife (map)—just outside the Arctic Circle—to watch and photograph the aurora borealis each September, when nights are warmer than the winter lows of -22 to -40 degrees F (-30 to -40 degrees C).