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Cue the Chorus: Why Certain Northern Lights Dance

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.
(Image: © Science/AAAS)

Scientistshave long known the colorful northern lights that amaze skywatchers are morethan just pretty light shows in the sky. But until now, the engine behind theirdiversity has been a mystery.

A new studyhas found a connection between pulsating aurora displays around Earth's North Poleand the intensity of so-called chorus waves, which are electromagneticfluctuations that occur in space within our planet's magnetosphere. Thefindings could help paint a clearer view of the drivers behind different kindsof aurora.? [DazzlingAurora Photos]

"Ourstudy determined that the specific process occurring in space is responsiblefor some types of northern lights," Yukitoshi Nishimura, leadauthor of the study and a visiting scholar in the Department of Atmospheric andOceanic Sciences at UCLA, told SPACE.com. "While scientists have only hada rough idea about what is happening in space when northern lights occur, werealized through this study what specific features in space we should focus onfor further studies of the cool night sky show."

Thepulsing northern lights

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

"Thisis different from typical aurora, which does not show such regularpulsation," Nishimura said.

Compared withtypical aurora, the pulsating variety is often more difficult to see, becauseit is much weaker, said Richard Thorne, a professor at UCLA and one of thestudy's co-authors.

The team ofresearchers made their discovery by combining satellite and ground-basedobservations from NASA's THEMIS mission, which includes five space probes thatsift through Earth's magnetic field, searching for the stormy beginnings of ourplanet's most dynamic auroras.

Electromagneticchorus

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

"Wefound that when the intensity of chorus waves increases, the luminosity of theaurora also correspondingly increases," Nishimura said. "The timingof these modulations matched almost perfectly."

Likewise, whenthe intensity of the chorus waves decreased, the luminosity of the pulsatingaurora also decreased.

"Itsounds like birds chirping when the signal is played through a speaker ? that'swhy it is called 'chorus,'" Nishimura said. "Chorus has two frequencybands. The lower frequency band interacts with energetic electrons that haveappropriate energies to cause aurora when precipitating toward the upper atmosphere."

In otherwords, pulsating auroras are caused by particles from solar wind that strike the Earth's magneticfield and travel through the planet's magnetic field lines. When theseelectrons reach the upper atmosphere, they are sometimes expressed in bursts ofchorus waves.

"Thispoints to the fact that the waves in space are an important part of themagnetospheric environment," Thorne told SPACE.com. "It has beenspeculated for years that it could be important, but this is the first piece ofinformation that quantifies that relationship in detail."

The resultsof the study will be published in the Oct. 1 issue of the journal Science. The findingscould help other scientists create more accurate models of Earth's magneticfield ? something that has been problematic, because it changes drasticallywith fluctuations in solar activity, Nishimura said.

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

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

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