Weird Glow from Earth's Auroras Explained

Themystery behind the faint glow of light in Earth's upper atmosphere,known asthe diffuse aurora, has been found, scientists say.

Aspecial kind of radio wave is at work behind the scenes to put on theauroras thatform the northern and southern lights, a new study finds.

Theseaurora-triggeringwaves are called choruswaves because their signals soundlike a bird's dawn chorus when played through a loud speaker. Paststudies haveshown these waves can also causeEarth's auroras to pulse as well as glow diffusely.

Thenew study,detailed in the Oct. 21 issue of thejournal Nature, shows that chorus wavesare responsible for scattering electrons that are trapped in space anddumpingthem down into the atmosphere to create the diffuse aurora. [Graphic:Earth's Atmosphere Top toBottom]

Notyour average aurora

Thediffuseaurora is not the same as the discreteaurora ? more popularly known as the northern and southern lights.Discreteaurora look like fiery movingcurtains of colorful light and can be seen by the naked eye,whereas thediffuse aurora is much fainter but more extensive.

Thediffuseaurora, which typically accounts forthree-quarters of the energy input into the upper atmosphere at night,variesaccording to the season and the 11-year solar cycle. Because it is sospreadout, the diffuse aurora is not usually visible to the naked eye.

Aseparate study recently discovered that chorus waves are alsoresponsible for arare kind of aurora that flickers on and off, called the pulsatingaurora. While the types of aurorasdiffer, both studies report that choruswaves are the cause, so they support each other, said Richard Horne,a co-author on the diffuse aurorastudy, from the British Antarctic Survey in Cambridge, England.

Trappedelectrons

Thediffuseaurora is caused by electrons striking theupper atmosphere ? that much was already known ? but how they get therewas notclear. The electrons are normally trapped much higher up in the Earth'smagnetic field.

Thenewresearch used computer models to calculate howfast the electrons are scattered by different kinds of waves and howmanyelectrons would be left behind trapped in the magnetic field.

"Therealclincher was when we examined thedistribution of electrons left behind in space which resembled apancake ? veryflat and peaked in a direction across the magnetic field direction,"Hornetold OurAmazingPlanet, a sister site of SPACE.com. "Only chorus wavescould produce the 'pancake' distribution."

Hornesaid thattheir finding "is an importantone because it will help scientists to understand how the diffuseaurora leadsto changes in the chemistry of the Earth'supper atmosphere, including effects on ozone at highaltitude, which mayaffect temperature right through the atmosphere."

Thelink between chorus waves and auroras should also benefitsatellite, power grid and aviation industries,Horne said, by improving GPS navigation and high frequency radiocommunicationswith aircraft on polar routes.

• Gallery- Amazing Auroras of 2010
• AGuide forWatching Earth's Auroras
• Cuethe Chorus:Why Certain NorthernLights Dance

Brett Israel is a StaffWriter for OurAmazingPlanet,a sister site toSPACE.com.