Do cosmic rays trigger lightning?

Using a radio observatory to monitor electromagnetic bursts caused by cosmic rays, scientists have been able to measure the intense electric fields inside thunderclouds that can reveal the origin of lightning.

Although such fields have been measured before by sounding rockets or balloons, The new technique could provide a better tool for verifying whether cosmic rays themselves trigger the bolts.

"The study offers long-awaited proof that a radio-astronomy observatory can be used to probe thunderclouds," said lead researcher and astrophysicist Heino Falcke from the Radboud University Nijmegen in the Netherlands.

When a highly energetic cosmic-ray particle collides with a molecule of air, it triggers a chain reaction in which millions of electrically charged particles (mostly electrons) shower down towards the ground.

The Low Frequency Array (LOFAR) -- a network of radio antennas and particle detectors spread around five European countries -- detected radio waves that are emitted by these charged particles as they fall in large part as a result of their interaction with the geomagnetic field.

Using LOFAR's data, the team led by Falcke measured 762 of the highest-energy showers between June 2011 and September 2014.

Measurements of the electric fields in clouds could help to solve the one of biggest open questions in atmospheric science.

Lightning is a channel of electrical conduction that briefly opens up in the atmosphere and partially restores the balance of electric charges, either between different layers of a cloud or between a cloud and the ground.

But scientists do not yet understand what triggers it.

The electric fields are strong, but are not sufficient in themselves to convert air from an electric insulator to a conductor.

Some researchers have proposed that cosmic rays are the trigger.

The hope is that LOFAR can test whether cosmic rays fall at the same times as do bolts of lightning.

Of course, a bolt that strikes delicate equipment could also be destructive.

"Scientifically, you are hoping that one lightning strike would fall right in the middle of LOFAR. But then as a radio astronomer, you kind of hope it wouldn't," Falcke concluded.

The study was published in the journal Physical Review Letters.