"Lightning is the responsible wave, interacts with particles and ... literally rains into our atmosphere and gets washed out," said Jim Green, NASA research and scientist at Goddard Space Flight Center outside Washington.

In 1958, US space probes mapped two radiation belts encircling Earth, Green said. Ever since, scientists have been trying to figure out why there are two belts instead of just one and what forms a so-called slot between them.

"Lightning is the culprit," Green said.

"It's clearing out the slot region or creating this safe zone and that enables NASA ... to use this region for a variety of spacecraft to orbit and then not suffer the high doses of radiation that occurs in the other belts."

NASA wanted to know how Earth's twin radiation belts are formed, so they can apply that knowledge to interplanetary travel, because Green said, the radiation in those belts could hit an astronaut with a dose of radiation like that of US atomic weapons dropped over Hiroshima and Nagasaki, Japan in 1945.

"Everything in space is trying to kill us when we get into a spacecraft, when we go anywhere and everything is also trying to kill the spacecraft," he said.

So, the data will help NASA and communications companies place satellites.

"As we launch more spacecraft, pagers and direct TV, what orbits do we put them in?" Green said.

Radiation constantly batters Earth, from solar flares, explosions deep within our own galaxy and from other planets. Depending on the time of year, that bombardment can fill the slot between Earth's radiation belts.

Lightning clears out that path and restores order to the Earth's radiation belts in a matter of weeks -- depending on the time of year, Green said.

NASA's IMAGE satellite, which travels over Earth's North and South Poles, mapped the radiation belts and compared them with lightning strokes recorded by two weather satellites.

The length of time lightning takes to clear out this slot depends on the number of lightning strokes, Green said. The quantity depends on the time of year.

Because Europe and North America are large land masses, their summers produce lots of lightning, which occurs less frequently over oceans. South American and Australia are smaller when compared with their surrounding oceans.

So when Earth is deluged with radiation during a Southern Hemisphere summer, the smaller number of lightning strokes means that slot or safe zone between Earth's double magnetic rings takes longer to clear, Green said.

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