Bolts from the Heavens

A new theory about lightning suggests a cosmic-ray connection

The ancient Greeks believed that lightning bolts were hurled from the heavens by a wrathful Zeus. Modern science has shown that the bolts are actually great electrical discharges between clouds and earth. A new theory by a Johns Hop kins scientist indicates that there might be some truth in the old myth after all. At a meeting of the American Geophysical Union in San Francisco last week, Physicist James W. Follin Jr. of Johns Hopkins University theorized that lightning is probably triggered by cosmic rays from deep space.

Scientists generally agree on the basic cause of lightning: the buildup of enough voltage, or electrical potential, between clouds and earth (or be tween different clouds) to overcome the resistance of the insulating layer of air between them. The buildup occurs when electrons, perhaps carried by falling water droplets, migrate to the bottom of a cloud, giving it a strong negative charge. Because like charges repel, that negative charge drives away electrons in the ground below, leaving it with an excess positive charge. Eventually, the voltage between cloud and ground becomes so great that electrons burst across the insulating air barrier, producing a brilliant flash.

Despite their basic understanding of the phenomenon, scientists have been at a loss to explain how droplets within the cloud--or any other mechanism--can generate the tremendous potential (about 100 million volts) necessary to produce lightning or why the bolt follows so jagged a path. The answers may well lie in the action of cosmic rays, which are actually protons or other atomic particles that continuously plunge toward the earth from outer space.

As Follin and his colleagues, Ernest Grey and Kwang Yu, explain it, cosmic rays act like cue balls in a kind of nuclear billiard game. When they strike and shatter atoms in the upper atmosphere, they produce a shower of subatomic bits of matter moving at great speed. When these so-called "secondary cosmic rays" collide with atoms in a cloud, they knock electrons from them. Accelerated in the cloud's electric field, these electrons avalanche toward the bottom of the cloud and pile up there.

What triggers the bolt, says Follin, are particles in the secondary cosmic showers called muons, which increase the charge with fresh electron avalanches. Finally, electrons burst from the cloud along a path of ionized (electrically charged) air. As other muons intercept the path at different angles, forming new trails, the electrons follow a jagged, steplike route to the ground.

That sets the stage for the return surge of current along the same pathway, visible as a flash of lightning.

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