Lightning in clouds, only a few miles above the ground,
clears a safe zone in the radiation belts thousands of miles
above the Earth, according to NASA-funded researchers. The
unexpected result resolves a forty-year-old debate as to how
the safe zone is formed, and it illuminates how the region is
cleared after it is filled with radiation during magnetic
storms.
The safe zone, called the Van Allen Belt slot, is a potential
haven offering reduced radiation dosages for satellites that
require Middle Earth Orbits (MEOs). The research may
eventually be applied to remove radiation belts around the
Earth and other worlds, reducing the hazards of the space
environment.
“The multi-billion-dollar Global Positioning System
satellites skirt the edge of the safe zone,” said Dr. James
Green of NASA’s Goddard Space Flight Center, Greenbelt, Md.
He is the lead author of the paper about the research
published in the Journal of Geophysical Research. “Without
the cleansing effect from lightning, there would be just one
big radiation belt, with no easily accessible place to put
satellites,” he said.
If the Van Allen radiation belts were visible from space,
they would resemble a pair of donuts around the Earth, one
inside the other, with the planet in the hole of the
innermost. The Van Allen Belt slot would appear as a space
between the inner and outer donut. The belts are comprised of
high-speed electrically charged particles (electrons and
atomic nuclei) trapped in the Earth’s magnetic field. The
Earth’s magnetic field has invisible lines of magnetic force
emerging from the South Polar Region, out into space and back
into the North Polar Region. Because the radiation belt
particles are electrically charged, they respond to magnetic
forces. The particles spiral around the Earth’s magnetic
field lines, bouncing from pole to pole where the planet’s
magnetic field is concentrated.
Scientists debated two theories to explain how the safe zone
was cleared. The prominent theory stated radio waves from
space, generated by turbulence in the zone, cleared it. An
alternate theory, confirmed by this research, stated radio
waves generated by lightning were responsible. “We were
fascinated to discover evidence that strongly supported the
lightning theory, because we usually think about how the
space environment affects the Earth, not the reverse,” Green
said.
The flash we see from lightning is just part of the total
radiation it produces. Lightning also generates radio waves.
In the same way visible light is bent by a prism, these radio
waves are bent by electrically charged gas trapped in the
Earth’s magnetic field. That causes the waves to flow out
into space along the Earth’s magnetic field lines.
According to the lightning theory, radio waves clear the safe
zone by interacting with the radiation belt particles,
removing a little of their energy and changing their
direction. This lowers the mirror point, the place above the
polar regions where the particles bounce. Eventually, the
mirror point becomes so low; it is in the Earth’s atmosphere.
When this happens, the radiation belt particles can no longer
bounce back into space, because they collide with atmospheric
particles and dissipate their energy.
To confirm the theory, the team used a global map of
lightning activity made with the Micro Lab 1 spacecraft. They
used radio wave data from the Radio Plasma Imager on the
Imager for Magnetopause to Aurora Global Exploration (IMAGE)
spacecraft, combined with archival data from the Dynamics
Explorer spacecraft. IMAGE and Dynamics Explorer showed the
radio wave activity in the safe zone closely followed
terrestrial lightning patterns observed by Micro Lab 1.
According to the team, there would not be a correlation if
the radio waves came from space instead of Earth. They
concluded when magnetic storms, caused by violent solar
activity, inject a new supply of high-speed particles into
the safe zone, lightning clears them away in a few days.
Engineers may eventually design spacecraft to generate radio
waves at the correct frequency and location to clear
radiation belts around other planets. This could be useful
for human exploration of interesting bodies like Jupiter’s
moon Europa, which orbits within the giant planet’s intense
radiation belt.
The research team included Drs. Scott Boardsen, Leonard
Garcia, William Taylor, and Shing Fung from Goddard; and Dr.
Bodo Reinisch, University of Massachusetts, Lowell. For
images and information about this research on the Web, visit:
http://www.nasa.gov/vision/universe/solarsystem/image_lightning.html
