Triggered Lightning

Normally, a lightning bolt (as we talked about before) hits the ground after a long, complex chain of events that concludes with a streamer from the ground reaching up and making contact with a descending ionization channel.

Throughout history, that's the way lightning has worked. People have not always understood the mechanism, of course; lightning's often been attributed historically to the action of irate gods, and it wasn't until Benjamin Franklin applied a scientific methodology to the question that modern understanding of lightning took hold.

And today, we know that lightning mostly strikes when a rising leader touches a descending ionization channel. Mostly.

Occasionally, though, that isn't the way it happens. Occasionally, a descending ionization channel makes contact with a small solid-fuel rocket rising from the ground trailing a copper wire behind it:

This is what happens when you fire a rocket trailing a wire into a thunderhead. A copper wire is even better at conducting electricity than an ionization channel is, and ninety-nine out of a hundred lightning bolts surveyed say they'd rather discharge along it than along a leader.

Ahem. Anyway, this is called "triggered lightning," and it's a research tool for understanding lightning. When such a rocket is fired into a descending ionization channel, scientists can control precisely when and where the lightning hits, which offers them the ability to measure everything from the energy contained in the bolt to the number of individual strokes.

This image, from the University of Florida, shows a triggered lightning bolt meeting an ascending rocket, and the discharge following the wire back down to the ground. In this case, the triggered lightning was part of an experiment to investigate the way different minerals affect the formation of fulgurites, which are fused glassy tubes that form when lightning strikes sandy material. (We'll talk about those in a later post.)

These rockets make a one-way trip; the rocket, and its trailing wire, are vaporized by the lightning bolt.

A single lightning bolt is often made up of several discrete flashes, each of which is a separate electrical discharge and each of which follows so closely on the heels of the one before it that to the unaided eye they look like a single bolt.

In the high-speed photograph from Lawrence Livermore above, the result of these multiple flashes can be seen; as each flash occurs, the wind blows the glowing, superheated air sideways, producing the appearance of a sideways wall of flame moving away from the straight vertical line (which is the rocket's exhaust).