Scientists detect dark lightning linked to visible lightning
Apr 24, 2013 | Phys.org
have identified a burst of high-energy radiation known as 'dark
lightning" immediately preceding a flash of ordinary lightning. The new
finding provides observational evidence that the two phenomena are
connected, although the exact nature of the relationship between
ordinary bright lightning and the dark variety is still unclear, the
"Our results indicate that both these phenomena, dark and bright lightning,
are intrinsic processes in the discharge of lightning," said Nikolai
Østgaard, who is a space scientist at the University of Bergen in Norway
and led the research team.
He and his collaborators describe their findings in an article recently accepted in Geophysical Research Letters—a journal of the American Geophysical Union.
Dark lightning is a burst of gamma rays produced during thunderstorms by extremely fast moving electrons colliding with air molecules. Researchers refer to such a burst as a terrestrial gamma ray flash.
Dark lightning is the most energetic radiation produced naturally on
Earth, but was unknown before 1991. While scientists now know that dark
lightning naturally occurs in thunderstorms, they do not know how
frequently these flashes take place or whether visible lightning always
In 2006, two
independent satellites—one equipped with an optical detector and the
other carrying a gamma ray detector—coincidentally flew within 300
kilometers (186 miles) 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.
But last year, Østgaard and his colleagues discovered the previously unknown gamma ray burst while reprocessing the satellite data. "We developed a new, improved search algorithm…and
identified more than twice as many terrestrial gamma flashes than
originally reported," said Østgaard. He and his team detected the gamma
ray flash and a discharge of radio waves immediately preceding the
was really lucky," Østgaard said. "It was fortuitous that two
independent satellites—which are traveling at 7 kilometers per second
(4.3 miles per second)—passed right above the same thunderstorm right as
the pulse occurred." A radio receiver located 3,000 kilometers (1864
miles) away at Duke University in Durham, North Carolina detected the
The satellites' observations combined with radio-wave data provided
the information that Østgaard and his team used to reconstruct this
ethereal electrical event, which lasted 300 milliseconds.
Østgaard and his team 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 those relativistic
electrons 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.
Dark and bright lightning may be intrinsic processes in the discharge
of lightning, Østgaard said, but he stressed that more research needs
to be done to elucidate the link.
The European Space Agency is planning on launching the Atmospheric
Space Interactions Monitor (ASIM) within the next three years, which
will be able to better detect both dark and visible lightning from
space, said Østgaard, who is part of the team that is building the ASIM
Dark lightning has remained a perplexing phenomenon due to scientific
limitations and a dearth of measurements, Østgaard explained.
"Dark lightning might be a natural process of lightning that we were
completely unaware of before 1991," he noted. "But it is right above our
heads, which makes it very fascinating."
More information: "Simultaneous observations of optical lightning and terrestrial gamma ray flash from space" onlinelibrary.wiley.com/doi/10.1002/grl.50466/abstract