In May 2008, Chile's Chaiten volcano violently erupted, spewing out clouds of dirty ash and illuminating the dark sky with a most unusual kind of lightning.
Photographer Carlos Gutierrez captured the dramatic nighttime display—in which green lightning emerged from the ash cloud—in the striking image above. (See also "PHOTOS: Chile Volcano Erupts With Ash and Lightning.")
The origin of the bright green lightning bolts remained a mystery until atmospheric scientist Arthur Few, of Rice University in Houston, became curious about the phenomenon. "I thought, 'That's funny; why don't we see this in lightning storms?'" said Few at the meeting of the American Geophysical Union (AGU) in San Francisco on Monday.
Although green lightning seems unusual, Few now suspects it occurs during all thunderstorms but is concealed inside clouds.
The concealment results from the structure of storm clouds. On the inside, the clouds contain ice crystals that are either positively or negatively charged. Surges of electricity occur between positively and negatively charged regions within the cloud—lightning—but they remain inside, unseen by even the most committed storm chasers.
In contrast, volcanic ash clouds carry their electrical charges on the outside, where they are sparked by fragments of rock forcefully ejected into the air during an eruption.
Few thinks that the bright green bolts seen at the Chaiten volcano are simply "positive streamers," or a current-transferring electrical surge from a positively to a negatively charged region on the outside of the cloud.
If only we could see inside thunderclouds, he suggests, we would see green lightning more often.
Green Color Explained
But why green? The green hue is given off by electrically excited oxygen atoms, says Few. He thinks the same process paints the sky green during the vivid light shows of the aurora borealis that can dominate northern skies in winter.
This phenomenon may have also colored the tail of Comet Lovejoy as it passed overhead in November.
For the time being, however, Chaiten in Chile is the only volcano where green lightning has been photographed.
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