Whistling for lightning's rhythm.

Whistling for lightning's rhythm

From our earth-bound perspective,lightning ranks high on nature's list of random and capricious acts. But a report in the June 5 NATURE suggests there is a subtle interplay between lightning and the magnetosphere, the region of the upper atmosphere dominated by the earth's magnetic field, and that this link can control the timing between lightning strikes.

William C. Armstrong at Stanford Universityfound evidence for the lightning-magnetosphere link by studying radio signals that, when amplified, sound like the descending pitch of a whistle. Such "whistlers' are generated by lightning, but are heard in the opposite hemisphere. A lightning strike in Canada, for example, would create electromagnetic waves that travel along the earth's magnetic field lines to Antarctica, where the lines meet the earth. Along the way, the signal would be "spread out,' causing the waves of higher frequency to arrive in Antarctica before the lower-frequency signals, giving whistlers their distinctive drop in pitch.

Scientists have known that followinga lightning flash, whistlers can echo back and forth between hemispheres many times. What Armstrong has discovered, with data from Antarctica, is that on a few occasions new whistlers are heard at the same time as echoes. In one case, for example, a new whistler was recorded on the 12th echo of a previous whistler signal.

While this phenomenon is not verycommon, says Armstrong, it is intriguing because such periodic behavior probably cannot be caused by random processes. Armstrong speculates that new whistlers, and the lightning that generates them, are ultimately triggered by the first whistler in the following process: As the first whistler moves toward Antarctica, it gives northbound electrons (which are confined to move along the earth's magnetic field lines) in the magnetosphere enough energy to penetrate farther down into the atmosphere than usual. As they slow down, they produce X-rays that ionize the air, making it easier for an electric discharge to occur between thunderclouds and the upper atmosphere. As a result, lightning is released by the cloud, creating another whistler.

"Whether the details of Armstrong'sideas are borne out, only time will tell,' notes Stanford's Donald Carpenter. "But [his work] is an exciting, provocative start,' he says, and it supports a growing belief that the outer reaches of the earth are intimately connected to weather and other phenomena in the atmospheric layers closer to home.