When lightning makes your favorite AM radio station crackle and pop, it is also cleaning up a radiation hazard overhead.
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 new NASA research. The unexpected new result resolves a forty-year-old debate as to how the safe zone is formed and is the second big lightning result in the last month.
Image Right: Without lightning's cleansing effect producing this protective safe zone in Earth's radiation belts, it would be dangerous for satellites. The camera pulls up through the clouds to a view from space. Radio waves from lightning (white glow) travel along the Earth's magnetic field and intercept particles in the safe zone region of the Van Allen Belts (red spiral). The radio waves deflect particles there, causing them to stream down the magnetic field line (blue) and impact Earth's upper atmosphere. This process occurs again until the safe zone is clear (represented by a dimming of the red spiral). Click on image for mpg, mov (no audio), or download PRINT RESOLUTION still. Credit: NASA/Walt Feimer.
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). Understanding this phenomenon may lead to ways to reduce the hazards of operating in these zones, which potentially could be a dramatic benefit to the Vision for Space Exploration in the future.
Image Left: This data-based visualization shows the Van Allen Belts pulsing from solar particles over ten days. The gap that appears toward the end shows a cleared-out safe zone for satellites. The red ring represents the orbit of the IMAGE satellite, which dips into the safe zone every few days. Click on image for movie (no audio), or download print resolution still. Credit: NASA/Tom Bridgman
"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., lead author of a paper on this research to appear in the Journal of Geophysical Research. "Without the cleansing effect from lightning, there would be just one big, nasty radiation belt, with no easily accessible place to put satellites."
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 Earth in the "hole" of the innermost donut. The Van Allen Belt slot region would appear as a space between the inner and outer donut. The belts are actually comprised of high-speed electrically charged particles (electrons and atomic nuclei) that are 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 Earth's magnetic field is concentrated.
Image Right: The safe zone appears as a gap between the inner and outer 'donut,' beginning about 7,000 km (4,350 miles) and ending about 13,000 km (8,110 miles) above the Earth's surface. Click on image for movie (no audio). Credit: NASA/Walt Feimer
Scientists debated two theories to explain how the safe zone was cleared. The prominent theory stated that radio waves from space, generated by turbulence in the zone, cleared it. An alternate theory, now confirmed with this research, posited that 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," said Green.
The flash we see from lightning is just part of the total radiation it produces. Lightning also generates radio waves (sometimes heard as static interference on AM radio stations during thunderstorms). In the same way that visible light is bent by a prism, these radio waves are bent by electrically charged gas trapped in the Earth's magnetic field, which causes the waves to flow out into space along the Earth's magnetic field lines.
According to the lightning theory, radio waves emitted by lightning clear the safe zone by interacting with the radiation belt particles, removing a little of their energy and changing their direction. This lowers the place above the polar regions where the particles bounce (called the mirror point). Eventually, the mirror point becomes so low that 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.
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 to a few weeks, according to the team. Engineers may one day 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 worlds like Europa, a moon of Jupiter that orbits within the giant planet's intense radiation belt.
Just last December, scientist realized that this safe zone is vulnerable to space radiation, if only temporarily.
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