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Study Says Cosmic Rays Grease Lightning

Nobody knows exactly what triggers lightning bolts. Now, two Russian researchers say that these discharges of a billion volts or more could be caused by the interaction of cosmic rays — high-energy particles from outer space — with water droplets in thunderclouds.

Cosmic rays are created deep in space by powerful events such as star collisions, gamma ray bursts, and supernovae. These cataclysms accelerate charged particles — mostly protons — to very high energies. The rays zoom across space, and those that strike the upper atmosphere of Earth generate invisible but highly energetic air showers of ionized particles and electromagnetic radiation.

The idea that these air showers could cause lightning when they pass through a thundercloud has been around for two decades. In 1992, Russian physicist Alexandr Gurevich of the Lebedev Physical Institute in Moscow suggested that the high-energy particles produced by a cosmic ray strike ionize the air in thunderclouds, creating a region with a lot of free electrons. The thundercloud’s electric field accelerates the electrons almost to the speed of light, boosting them to very high energies. Then the electrons collide with atoms in the air, generating even more electrons as well as X-rays and gamma rays. This avalanche of high-energy particles in the cloud — which Gurevich calls “runaway breakdown” — provides ideal conducting conditions for lightning.

Researchers worldwide have debated Gurevich’s idea ever since he introduced it, says Joseph Dwyer, a lightning scientist at the Florida Institute of Technology in Melbourne who was not involved in the study. But Gurevich hasn’t found concrete evidence that cosmic rays are the culprits. Radio waves could provide a clue, Dwyer says: Cascades of electrons at the onset of a lightning strike should produce radio waves. “The cosmic ray community has known that cosmic rays make radio waves, and when there are thunderstorms around, it’s been seen that you get more of these radio pulses,” Dwyer says. “But no one has yet closed the loop and really shown that the air showers going through a thundercloud’s electric field making these runaway electrons are the things that are doing it.”