Dark lightning

Print edition : May 17, 2013

Three images of the same thundercloud depicting a less-than-10-millisecond-long sequence of events: (left) formation within cloud of a small channel, or "leader", of electrical conductivity (yellow line) with weak emission of radio signals (ripples); (middle) a burst of both dark lightning (pink) and radio waves (larger ripples); (right) a discharge of bright lightning and more radio waves. Photo: Studio Gohde

RESEARCHERS have identified a burst of high-energy radiation known as “dark lightning” that immediately precedes a flash of ordinary lightning. The finding, which is due to be published in Geophysical Research Letters, provides evidence that the two phenomena are connected though the exact nature of the relationship is still unclear, the scientists say. “Our results indicate that both these phenomena, dark and bright lightning, are intrinsic processes in the discharge of lightning,” said Nikolai Østgaard of the University of Bergen, Norway, who led the research.

Dark lightning is a burst of gamma rays produced during thunderstorms by extremely fast-moving electrons colliding with air molecules. It is the most energetic radiation produced naturally on the earth but was unknown before 1991. While scientists now know that dark lightning naturally occurs in thunderstorms, it is not known how frequently these flashes take place and whether visible lightning always accompanies them.

In 2006, two independent satellites—one equipped with an optical detector and the other carrying a gamma-ray detector—coincidentally flew within 300 km of a Venezuelan storm as a powerful lightning bolt exploded within a thundercloud. Scientists were unaware then that a weak flash of dark lightning had preceded the bright lightning. Last year, Østgaard and his colleagues reprocessed the satellite data and discovered the associated gamma-ray flash. “We developed a new, improved search algorithm... and identified more than twice as many terrestrial gamma flashes than originally reported,” he said.

“It was fortuitous that two independent satellites passed right above the same thunderstorm right as the pulse occurred,” Østgaard said. A radio receiver located 3,000 km away at Duke University in Durham, North Carolina, detected the radio discharge. The event lasted 300 milliseconds. The researchers suspect that the flash of dark lightning was triggered by the strong electric field that developed immediately before the visible lightning. This strong field created a cascade of electrons moving at close to the speed of light. When they collided with air molecules, they generated gamma rays and lower-energy electrons that were the main electric current carrier that produced the strong radio pulse before the visible lightning.