Cue the Chorus: Why Certain Northern Lights Dance
A schematic diagram showing aurora over North America and a spacecraft in space (magenta) embedded in the energetic plasma source (blue cloud). These two regions are connected by the Earth's magnetic field line. Energetic plasma interacts with waves (red) and precipitates into the upper atmosphere (blue arrows) and generate aurora. The geometry of the plasma cloud determines the aurora shape.
Credit: Science/AAAS

Scientists have long known the colorful northern lights that amaze skywatchers are more than just pretty light shows in the sky. But until now, the engine behind their diversity has been a mystery.

A new study has found a connection between pulsating aurora displays around Earth's North Pole and the intensity of so-called chorus waves, which are electromagnetic fluctuations that occur in space within our planet's magnetosphere. The findings could help paint a clearer view of the drivers behind different kinds of aurora.? [Dazzling Aurora Photos]

"Our study determined that the specific process occurring in space is responsible for some types of northern lights," Yukitoshi Nishimura, lead author of the study and a visiting scholar in the Department of Atmospheric and Oceanic Sciences at UCLA, told SPACE.com. "While scientists have only had a rough idea about what is happening in space when northern lights occur, we realized through this study what specific features in space we should focus on for further studies of the cool night sky show."

The pulsing northern lights

Pulsating auroras typically occur in the night sky around the Earth's polar regions and are characterized as a group of luminous patches that blink on and off with recurrence periods of roughly 5 to 40 seconds, Nishimura explained.

"This is different from typical aurora, which does not show such regular pulsation," Nishimura said.

Compared with typical aurora, the pulsating variety is often more difficult to see, because it is much weaker, said Richard Thorne, a professor at UCLA and one of the study's co-authors.

The team of researchers made their discovery by combining satellite and ground-based observations from NASA's THEMIS mission, which includes five space probes that sift through Earth's magnetic field, searching for the stormy beginnings of our planet's most dynamic auroras.

Electromagnetic chorus

Nishimura and his colleagues used one of the THEMIS mission's ground-based All-Sky Imagers to monitor the sky on Feb. 15, 2009. They found that chorus waves in Earth's magnetosphere, which were simultaneously detected by one of the THEMIS satellites, were directly related to changes observed in the pulsating aurora.

"We found that when the intensity of chorus waves increases, the luminosity of the aurora also correspondingly increases," Nishimura said. "The timing of these modulations matched almost perfectly."

Likewise, when the intensity of the chorus waves decreased, the luminosity of the pulsating aurora also decreased.

"It sounds like birds chirping when the signal is played through a speaker ? that's why it is called 'chorus,'" Nishimura said. "Chorus has two frequency bands. The lower frequency band interacts with energetic electrons that have appropriate energies to cause aurora when precipitating toward the upper atmosphere."

In other words, pulsating auroras are caused by particles from solar wind that strike the Earth's magnetic field and travel through the planet's magnetic field lines. When these electrons reach the upper atmosphere, they are sometimes expressed in bursts of chorus waves.

"This points to the fact that the waves in space are an important part of the magnetospheric environment," Thorne told SPACE.com. "It has been speculated for years that it could be important, but this is the first piece of information that quantifies that relationship in detail."

The results of the study will be published in the Oct. 1 issue of the journal Science. The findings could help other scientists create more accurate models of Earth's magnetic field ? something that has been problematic, because it changes drastically with fluctuations in solar activity, Nishimura said.

The study will also contribute to future studies of atmospheric dynamics and chemistry, and closer examinations of other types of northern lights.

"There are many kinds of aurora, and our study solved the problem for part of them," Nishimura said. "We should keep the ongoing research in the community active. Someday, we will be able to say not only "beautiful" but to describe what is going on in space by just looking at the sky."