Waves and ripples of otherworldly light continue to grace the night skies, leaving residents of the northern United States and Europe in awe. The multicolored Northern Lights appear to have revived two years beyond the peak in an 11-year cycle of activity that fuels them.

Also known as aurora, the displays are typically visible only in the far north -- places like Alaska, Canada and Finland, or in the few populated regions of the far Southern Hemisphere. However, unusually powerful solar bursts sometimes overload Earth's magnetic field, sending the Northern Lights as far south as Texas on rare occasions.

"This was one of the strangest auroras we have ever seen," said Bismarck, ND resident Lyndon Anderson, who photographed a Sept. 3 show. "I saw numerous arcs in the northern sky, and the glow was overpowering."

Anderson witnessed pulsating patches of color and said early evening activity began oddly in the northwest instead of the northeast, as is customary.

"This wasn't a typical show by any means," Anderson said, "but then again, every show is unique."

A few days later, a similar event was visible again across the northern United States and Europe. Its Southern Hemisphere counterpart was spotted from Australia and New Zealand.

Painting the sky

Aurora are generated when sunspots and their twisted magnetic fields unleash huge blobs of charged particles that race toward Earth and then interact with our planet's magnetic field, which emanates from the poles. Though the Sun is well past a peak of activity that comes every 11 years, it has been unusually active in recent months, solar astronomers and aurora observers say.

The lights are a product of charged particles being funneled by Earth's magnetic field lines down into the upper atmosphere. Molecules of air glow when they are struck by the charged particles.

The color of an aurora depends on the type of atom or molecule, according to Hal Kibbey of Indiana University.

High-altitude oxygen, about 200 miles up, is the source of rare, all-red auroras, Kibbey explains. Oxygen at lower altitudes, about 60 miles up, produces a brilliant yellow-green, the brightest and most common auroral color. Ionized nitrogen molecules produce blue light, and neutral nitrogen glows red. The nitrogens create the purplish-red lower borders and rippled edges of an aurora.

Continued activity

Aurora typically result from solar events called coronal mass ejections, or CMEs.

Whether or not any given CME affects Earth depends on where it is generated on the portion of the Sun facing our planet at that moment. The Sun rotates from east to west.

"When we have a CME and it occurs on the western side of the Sun then the magnetic field lines trail backwards toward the Earth, and that sets up favorable conditions for aurora," says William Livingston, a National Solar Observatory staff scientist, emeritus.

In a recent telephone interview, Livingston explained why solar activity has been strong lately. The solar cycle typically has two peaks of activity, he said. Other scientists had said in January that the second peak, or maximum, appeared to be underway.

"The second one is typically a couple years later or so," Livingston said. "We've just gone through the second maxima, I think."

He said some of the most powerful solar eruptions, and therefore strong aurora, often occur during this second peak, but no one knows why.

Here's another thing scientists don't understand: Geomagnetic storms and the auroral lights they create tend to be stronger around the equinoxes, in early Autumn and again in Spring. Researchers have known about this seasonal variation for more than a century. It is caused partly by an alignment of Earth's magnetic field with a so-called interplanetary magnetic field, which allows more charged particles to be funneled earthward.

But there's more to the seasonal variation that scientists say still stumps them.

Space weather forecasting, done at NOAA’s Space Environment Center, cannot guarantee great aurora viewing. But forecasters do typically provide 24 to 48 hours of warning prior to a potential storm. SPACE.com provides a regularly updated Space Weather Forecast that gives advance notice of storms that might generate enhanced auroral activity.

"The aurora of the evening of Sept. 3 was very beautiful to observe, and covered a large part of the sky, even going overhead and to the south at times," said photographer Lyndon Anderson of Bismarck, North Dakota. This photo looks to the northwest. Lights on the horizon are from Washburn, ND. Look close and you can see the Big Dipper's stars. Click to enlarge

"Just past sunset, the sky over Tomahawk, Wisconsin became vibrantly alive with an absolutely stunning Auroral display," said photographer Carol Lakomiak. "In contrast to the still-blue twilight, patches of crimson appeared as I took this 15 second exposure. Click to enlarge

More Photos

• Aurora Photo Gallery from 2001
• John Russell's Aurora Gallery from 2001
• Science Tuesday Archives and Astronomy News Headlines by Topic