It is a phenomenon that has puzzled people for thousands of years and drove Benjamin Franklin to risk his kite and life in search of answers. Now, a trio of researchers in America may have finally found a way to solve the mystery of how thunderstorms make lightning. The solution, according to Joseph Dwyer at the Florida Institute of Technology and his team, lies in using the endless rain of cosmic rays hitting the Earth’s atmosphere.

Most scientists think that lightning originates from localized pockets of intense electric fields inside thunderstorms. However, these regions are too small to be detected easily by balloons and aircraft, and the act of measuring the fields can itself initiate lightning and distort results. Currently, scientists believe that the typical electric field needed at sea level to cause a spark is about 3 MV/m but no one has yet measured a field within a thundercloud that even approaches these values.

Dwyer and his colleagues offer a solution that could instead enable scientists to study action inside thunderclouds from the ground. As cosmic ray showers — energetic protons from outer space — pass through thunderclouds, they produce a large number of electrons from hard elastic scattering with air. These electrons gain energy from the cloud’s electric fields at a higher rate than they lose it through ionization and this leads to a “runaway” of electrons.

Storming results

As these runaway electrons propagate in the strong electric field, they produce additional runaway electrons through scattering with air. A consequence of this growing “avalanche” of accelerating electrons is the emission of radio waves that can be detected from the ground. Dwyer and his team propose that these radio waves can be analyzed to reveal information about the source region.

Other researchers have welcomed the new approach. “The measurement of detailed pulse shapes in the vicinity of thunderclouds is certainly an interesting idea, despite the technical difficulties which experimentalists will encounter during this measurement,” says Nikolai Lehtinen at Stanford University in California.

Dwyer told that his team is now looking to test their theory. They will be reconstructing the electric fields along the paths of cosmic rays in an attempt to identify the localized, very high field regions that give rise to lightning. To measure both the radio pulses and the electrons, the scientists will use an “air shower array” containing particle (scintillation) detectors plus a large flat plate antenna on the ground.

These experiments will be carried out at the International Center for Lightning Research and Testing at Camp Blanding, Florida. They have built an air shower array and will use sensitive electric field antennae in order to measure the air showers and the accompanying radio pulses. “We will know more by the end of this summer,” says Dwyer. “It is quite amazing to me that in 2009, we are still struggling to understand a topic pioneered by Benjamin Franklin at a time when the US was still a British colony.”

This research was published in the Journal of Geophysical Research.