Contact:
Kathleen O’Toole, News Service
(650) 725-1939; e-mail: kathleen.otoole@stanford.edu
11/30/99
Lightning upsets space weather several states away, researchers find
Within roughly a second of a single lightning flash in Texas, electrons can precipitate out of the Earth’s radiation belts
onto the upper atmosphere above an area spanning Oklahoma to South Dakota,
researchers at Stanford have found.
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.
The Dec. 1 article in Geophysical Research Letters is available on the web at
http://www.agu.org/GRL/articles/1999GL010706/node1.html
A color illustration, named lightning.jpg, accompanies this release. It is available through the Stanford News Service
anonymous ftp site at ftp://36.15.0.227/images .
[NOTE: A smaller version of the illustration is at
http://www.stanford.edu/dept/news/pr/99/991130lightning.html]
