Theirs is the first evidence that lighting can have such a far-ranging effect -- temporarily changing the composition of the radiation belts and the ionosphere below it within an area of several hundred thousand square miles.

The findings, reported in the Dec. 1 issue of Geophysical Research Letters, suggest "lightning could be an important contributor to the loss of electrons from the Earth's radiation belt, and thus helps us better understand the Earth's near-space environment," said Umran Inan, a professor of electrical engineering in Stanford's STARLab (Space, Telecommunications and Radioscience Laboratory).

Inan and graduate student Michael Johnson made the observations by deploying a network of very low frequency radio receivers in the Midwest that are able to sense changes in the ionosphere -- the conducting portion of the Earth's atmosphere that begins some 40 miles above ground. The observed lightning-associated disturbances are remarkably consistent, they said, with independent theoretical predictions of radiation belt precipitation from lightning published by recent graduate David Lauben, a former student of Inan's.

Scientists have known for several decades that the electromagnetic waves from lightning cause electrons to rain out of the radiation belt, "but this result indicates it occurs on a huge scale at night," Inan said. "It also must occur during the day, but the effect on the ionosphere is relatively small compared to solar ionization. At these mid-latitudes, we now think the nighttime ionosphere may be dominated by these lightning effects."

Electrons trapped in the Earth's magnetic field come in from the sun as solar wind, he said, and are accelerated in the radiation belts where they stay trapped bouncing from one end of the magnetic field line to the other, until some other process comes along to release them. Lightning performs that function by launching waves upward nearly along the Earth's magnetic field lines, scattering high energy particles in both momentum and pitch angle along the route.

Deflected from their trapped orbit, the electrons precipitate out into the atmosphere where they produce light, X-rays and ionization.

Previously, researchers thought effective precipitation of electrons was only caused by waves traveling in tiny tubes of ionization along magnetic field lines called ducts, with lightning only affecting those trapped electrons that are confined to these narrow paths in space.

The new finding indicates that electromagnetic waves from lightning populate large regions of the radiation belts from which they precipitate electrons, which means they could potentially influence loss rates of trapped particles on a global scale.

Not every lightning flash behaves this way, Inan said. He and Johnson are now trying to determine when it does and doesn't. "You can imagine in a thunderstorm, particularly after a solar blast has built up the particles in the radiation belt, that you could get these splashes of particles precipitating out every second, which would have a large effect on the ionosphere," Inan said.

The team has found dozens of examples. But they report in detail on the effects from a few strikes that were particularly well located for monitoring by their receiver network, including one strike in a group of flashes that lit up the sky near Austin, Texas, in the early morning hours of Oct. 18, 1998.

The precipitated energetic electrons began raining down about a second after the strike and over a huge area beginning several hundred miles to the north of Austin and ending in South Dakota. "The effect may have kept on going further north," Johnson said, "but that's how far we were able to track it from our existing network."

The size, location and temporal evolution of the particle precipitation area were remarkably similar to those predicted in computer simulations that Lauben had conducted beforehand for his doctoral thesis, Inan said.

The ground-based receiver network, called HAIL for Holographic Array for Ionospheric Lightning research, is operated by the Stanford team in collaboration with high school students and science teachers in nine schools from New Mexico to Wyoming. The receivers are able to track changes in an area above the range of weather balloons but below the range of satellites by monitoring the effects of the changes on exceptionally stable very low frequency radio signals used for U.S. Navy communications, Johnson said.

"The ground acts as one metal plate at these frequencies and the ionosphere acts as another, but the ionosphere occasionally changes its properties because of activity such as lightning, solar flares or gamma ray bursts, which in turn changes the waves propagating underneath it," he said.

The high school students help maintain the receivers, do their own research projects and send data over the Internet for analysis by Inan's research group. Inan, Johnson and others in the group now have given more than 40 talks on solar-terrestrial physics and have obtained funding to sponsor three student-teacher pairs to attend the American Geophysical Union fall meeting in San Francisco.