Download Study clarifies how gamma rays generated in thunderclouds

October 5, 2007
Study clarifies how gamma rays generated
in thunderclouds
Scientists investigate puzzle of how clouds produce high-energy radiation usually found
only in nuclear reactors, exploding stars
During large winter thunderstorms in January this year, radiation detectors at a nuclear
power plant in Japan detected gamma rays, a high-energy form of radiation normally
associated with nuclear reactors and exploding stars. Now a team of Japanese
scientists believe they have shed light on the radiation bursts, known as terrestrial
gamma-ray flashes. This work will be published online on 19 October 2007 in Physical
Review Letters
Gamma rays are a very high-energy form of radiation that occurs as a result of nuclear
reactions, such as are found in nuclear reactors and in space during a supernova, when
a giant star explodes; the resulting bursts of gamma rays are frequently detected from
Earth. This radiation is hard to block, and can cause serious damage to living tissue, a
property that makes it useful in medicine, for example in the treatment of cancer.
The team of scientists from RIKEN's Discovery Research Institute in Wako and the
University of Tokyo installed gamma-ray detectors at a nuclear reactor by the Sea of
Japan in Niigata Prefecture. They discovered that there were two kinds of gamma-ray
bursts during the storms.
The first was a very short burst that occurred during lightning flashes, which the
researchers confirmed with detectors for the visible light and electrical field produced by
the lightning discharge, and showed that the gamma-ray burst occurred at the same
time as the lightning.
The second type was more mysterious, a burst of gamma rays lasting around 40
seconds that appeared to come from the thunderclouds themselves, just before a
cloud-to-ground lightning flash.
The scientists now believe they understand the mechanism of how such bursts are
generated. Every day, the Earth is bombarded by cosmic rays from space, energetic
particles such as electrons and protons that can come from a variety of sources. Some
come from our own sun, others from within our own galaxy and others from the farthest
reaches of the universe.
Earth's atmosphere blocks most cosmic rays, but some especially energetic ones get
through. If these particles strike air molecules within the cloud, they can become 'seed
particles,' sending electrons spinning off at near light speeds.
Winter thunderclouds near Niigata contain powerful electric fields, with the top and
bottom being positively charged and the center negatively charged. The electrons
produced by the cosmic rays striking air molecules in the center of the cloud, are
accelerated by the powerful electric field inside the thundercloud toward the
positively-charged top and bottom edges of the cloud.
When this happens they can create an avalanche of other near-light-speed electrons as
they slam into the nuclei of air molecules, releasing still more electrons. In the collisions,
the electrons release their kinetic energy in the form of gamma rays, a process known
as bremsstrahlung.
Gamma rays produced in this way are only released in a narrow cone. This means that
they must be headed in exactly the right direction for the radiation detector to detect
them. As a result, they are detected fairly rarely.
Scientists hope that studying these thundercloud-related gamma rays will help them
understand the behavior of particles accelerated by extremely strong magnetic fields,
like those found in particle accelerators used in high-energy physics research.
Original work:
H.Tsuchiya, T. Enoto, S. Yamada, et al.,
Detection of high-energy gamma rays from winter thunderclouds.
Physical Review Letters, published online on 19 October 2007
For more information, please contact:
RIKEN Public Relations Office
Email: [email protected]