SPRG Seminars
April 14, 2009:
"Terrestrial Gamma-ray Flashes: Bursts of x-rays and gamma rays from Lightning"
Brian Grefenstette, Santa Cruz
Institute for Particle Physics
In 1925, C.T.R. Wilson predicted that the large electric fields inside a thunderstorm could accelerate electrons to extremely high energies. These electrons would then make x-ray and gamma-ray bremsstrahlung as they slowed in the air. This insight has proven fruitful, as x-rays and gamma-rays associated with thunderstorms have been observed by a variety of instruments located on the ground and flying on aircraft, high-altitude balloons, and satellites. Terrestrial Gamma-ray Flashes (TGFs) are tremendously bright, millisecond bursts of gamma-rays with energies that can exceed 20 MeV and are visible from space. The TGFs are observed when the satellite flies over thunderstorms, and their spectrum indicates that they are the bremsstrahlung from beams of relativistic electrons originating near cloud-top altitudes. Over the last few years a production mechanism known as relativistic breakdown has been suggested and a candidate for TGF production and may link TGFs to the currently unknown process that initiates lightning. In this talk I will discuss some of the discoveries that we have made based on the satellite data as well as review the physics of relativistic breakdown in planetary atmospheres. I will also show how new measurements, in situ and in space, may help answer some of the questions that still remain about source mechanism of TGFs and its relation to lightning.