NASA finds the effects of lighting extend to the Van Allen belts
- Bill Steigerwald
Goddard Space Flight Center, Greenbelt, Md
NASA FINDS LIGHTNING CLEARS SAFE ZONE IN EARTH'S RADIATION BELT
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
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: