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 2 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."
To test the concept, Gurevich and his Russian colleague Anatoly Karashtin of the Radiophysical Research Institute in Nizhny Novgorod analyzed data from 3800 cloud-to-ground lightning strikes recorded in Russia and Kazakhstan. They used a radio interferometer, which measures radio waves and shows which direction they come from, and studied the radio pulses generated at the onset of lightning.
The results, reported this week in Physical Review Letters, show that storm clouds emit "hundreds or thousands" of short, strong radio pulses just before lightning strikes. Their shape, the researchers say, matches their models of runaway breakdown triggered by energetic cosmic rays. But there's a problem: Cosmic rays with enough energy are too rare to trigger all the pulses that Gurevich and Karashtin observed.
So-called hydrometeors—hail and water droplets in each cloud—may be amplifying the pulses, the researchers say. When the free electrons created by the cosmic ray particles pass near these hydrometeors, Gurevich and Karashtin propose, they trigger a flurry of microdischarges that boosts both the current and the radio pulse signal.
But lightning scientist Clive Saunders of the University of Manchester in the United Kingdom remains unconvinced that cosmic rays play a role in lightning. "They have not shown a correlation between lightning activity and the rate of arrival of cosmic rays at the earth," he says.
That rate waxes and wanes in time with the 11-year cycle of solar activity. When the solar wind is stronger, at solar maximum, it deflects more cosmic rays from Earth. If cosmic rays are behind thunderstorms, Saunders argues, the incidence of thunderstorms should follow a similar cycle. "Until they can prove this correlation, that leaves a fundamental question about the whole concept," he says.
Most researchers, Saunders says, lean toward another theory: Lightning occurs when collisions between ice crystals and hailstones in storm clouds separate enough electric charge to cause a high electric field. This process alone can ionize the air enough for a lightning bolt to form, he says—no cosmic rays required.
Dwyer says that it should be possible to settle the dispute using a detector for cosmic ray air showers known as a Cherenkov telescope. "What we really need is to have these air shower arrays that measure the cosmic ray air showers going through, measure the radio waves, measure the pulses, and put it all together," he says.