Green lightning may be caused by positive charges, or by camera lens

SAN FRANCISCO — A zigzag of green lightning that flashed above a volcano in 2008 could have gotten its color from a cluster of positive charges.

The freak bolt, snapped by a photographer during an eruption of Chile’s Chaitén volcano, may be a different type of lightning than the kind sent down from thunderclouds, suggested atmospheric physicist Arthur Few of Rice University in Houston at the American Geophysical Union’s fall meeting on December 9.

In thunderstorms, negative charges jump from clouds to the ground in a white-hot streak. But a green color could emerge if the bolt formed from positive charges in the volcano’s ash plume, Few said. Positive charges would attract electrons that could excite nearby oxygen molecules. When oxygen molecules calm down, they emit green light.

But there may be a simpler explanation. Since Few reported his idea, several photographers have suggested to him that the color might be a trick played by the camera lens. Flip the photograph’s largest white lightning bolt upside down and take its mirror image, they say, and the result looks just like the green streak.

The camera, Few said, could somehow have recorded a green ghost of the brighter white flash.