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Dark Lightning -- presented by Science@NASA
NASA's Fermi Gamma Ray Space Telescope was launched in 2008
on a mission to study high-energy phenomena in our Universe.
The telescope routinely detects things like flares
powered by black holes in distant galaxies
or outbursts from massive stars going supernova.
So, in 2010, researchers were not surprised
when the telescope was hit by a beam of high-energy positrons,
the antimatter equivalent of electrons.
That's the sort of thing Fermi is out there looking for.
But they were surprised when they realized where the antimatter came from--
not from some black hole light years across the galaxy,
but rather from our own planet.
The source was a thunderstorm just 3000 miles away!
'Earth's magnetic field
seems to have corralled about 100 trillion positrons from the storm
into a tight beam
and funneled them all the way to the spacecraft,'
explains lightning expert Joseph Dwyer
of the Florida Institute of Technology.
Something was producing antimatter above the clouds of Earth,
and hurling it into space at nearly the speed of light.
But what?
Dwyer and collaborators at the NASA Marshall Space Flight Center
and the University of Alabama
believe they have figured it out.
'The answer,' says Dwyer, 'is dark lightning.'
Dark lightning may sound like an oxymoron,
but there is growing evidence that it is real.
Ordinary lightning happens
when electric fields build up inside thunderclouds.
Electrons rush from one part of the cloud to another,
to try to cancel out the growing voltage.
The flash of light we see traces the path of the charged particles,
which heat the air five times hotter than the sun.
If Dwyer's ideas are correct,
dark lightning is a competitor of ordinary lightning.
It also tries to cancel out the thunderstorm's electric fields.
The process, he says, goes something like this:
Under the right conditions,
electric fields in a thunderstorm can create a powerful avalanche of electrons
shooting upwards nearly as fast as light.
The electrons collide with air molecules,
in turn producing gamma-rays.
Earth-orbiting spacecraft have been observing gamma-ray flashes
from thunderstorms since at least the mid-1990s.
Next, the gamma-ray energy transforms into a pair of particles:
an electron and a positron.
Successive collisions between these particles
and other air molecules
create a new batch of positrons and electrons,
and the cycle repeats.
'A continuous feedback loop forms.
Like nuclear fission, it's a natural,
self-generated, self-sustained particle accelerator' says Dwyer.
Once the feedback loop gets started, he says,
it can discharge parts of a thundercloud as fast as lightning.
And because the cascading electrons
and positrons generate more gamma rays than visible light,
the whole process is practically invisible to the human eye.
Researchers once thought
that gamma-ray flashes from thunderstorms
were a weird byproduct of ordinary lightning.
Now they are thinking it is a sign of dark lightning instead.
The Gamma-ray Burst Monitor onboard Fermi
is excellent at catching these flashes.
At the American Geophysical Union meeting last month,
Valerie Connaughton of the University of Alabama in Huntsville
explained how new data processing techniques
have improved the burst monitor's performance even more.
'In mid-2010, we began testing a mode
which allows us to locate many faint gamma-ray flashes we had been missing,'
she said.
Now, team members estimate,
Fermi should be able to catch almost 1000 flashes a year.
With data like that,
researchers hope to shed new light on dark lightning,
and solve its mysteries once and for all.
For more news about dark and mysterious things in the skies of Earth,
visit science.nasa.gov.