The properties of terrestrial gamma ray flashes have eluded scientists for years. This new study sheds light on how they originate.

Thunderstorms are composed of multiple layers of electric charge. This charge separation creates a strong electric field, which in turn produces regular lightning.

But sometimes, thunderstorms also generate short bursts of high-energy radiation called terrestrial gamma ray flashes (TGFs). These flashes occur when the storms act like giant particle accelerators, allowing accelerated high-energy electrons to slam into air molecules and produce gamma rays, more electrons, and positrons.

Scientists remain unsure of the exact mechanisms that produce TGFs. Previous studies have uncovered some details about how TGFs are formed but have not yet measured where inside thunderstorms they are produced. However, knowing their source altitude would help determine how bright TGFs are and would shed further light on the physics of how they are made.

Cummer et al. conducted a study to directly measure the TGF source altitude. The team combined TGF measurements collected by the Gamma-ray Burst Motor (GBM) instrument on NASA’s Fermi Gamma-ray Space Telescope with ground-based measurements of the associated radio emissions.

Using the relative timing to determine which radio emissions were produced during the TGF generation and exploiting reflections of these radio emissions from the ionosphere, the authors determined that the TGFs observed were produced near 12 kilometers in altitude. This shows that TGFs are produced in the very center of thunderstorms, between the electrically charged layers. The team also found that the electromagnetic pulse produced by TGFs can be strong enough to affect the ionosphere. (Geophysical Research Letters, doi:10.1002/2014GL062196, 2014)

—Catherine Minnehan, Freelance Writer

Citation: Minnehan, C. (2015), Newly discovered properties of elusive gamma ray flashes, Eos, 96, doi:10.1029/2015EO032431. Published on 9 July 2015.