Cosmic rays illuminate lightning

Particles from space provide insights into charges in thunderclouds

Lightning illuminates the sky during a storm in Weld County, Colo.

ELECTRICAL SURGE  Lightning illuminates the sky during a storm in Weld County, Colo. Researchers at LOFAR in the Netherlands developed a new technique to expose the conditions inside thunderclouds that lead to lightning strikes.

Bryce Bradford/Flickr (CC BY-NC-ND 2.0)

High-speed particles from space are helping to unravel a high-voltage mystery in the clouds.

Astronomers have determined the strength of electric fields in thunderclouds by detecting the radio wave signature of cosmic ray particles striking the atmosphere. Reported April 24 in Physical Review Letters, the research offers insight into the confluence of cloud conditions that leads to lightning strikes, which atmospheric simulations cannot explain. The technique could also validate an intriguing hypothesis that cosmic rays provide the spark that triggers lightning.

Cosmic rays constantly pelt the planet. Occasionally they strike atoms in the atmosphere and generate a cascade of new particles. LOFAR, an observatory based in the Netherlands, tracks these cosmic ray showers with particle detectors and small antennas, which detect radio waves emitted mainly by the shower’s electrons. In fair weather, Earth’s magnetic field steers the electrons, leading to a predictable polarization of the radio waves. But LOFAR researchers found that during thunderstorms, particle showers had distinct radio polarization signatures because the storm clouds’ electric fields redirected the electrons. By combining observations with computer simulations, scientists inferred the structure and electric field strength — 50,000 volts per meter during one storm — of thunderclouds pierced by the particles.

RADIO RECEIVERS LOFAR antennas detect radio waves emitted by showers of particles cascading through the atmosphere. Hans Hordijk

Physicist Joseph Dwyer of the University of New Hampshire in Durham says the technique should help scientists piece together how thunderclouds build up the tremendous electric fields required for lightning. Balloons and airplanes can make similar measurements, but they are difficult to maneuver in turbulent air and may trigger lightning.

In addition to helping scientists explore thunderclouds, cosmic rays may play a central role in lightning formation by carving out conductive conduits in the air. Study coauthor Pim Schellart, a radio astronomer from Radboud University in Nijmegen, the Netherlands, and colleagues plan to compare the timing of cosmic ray showers and lightning strikes to test that possibility.

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