When a thunderstorm happens, there's also a good chance it's emitting gamma-rays called terrestrial gamma-rays or TGFs. And now, NASA has new information, thanks to its Fermi mission, about these poorly understood events.

Around the world, thunderstorms occur. These storms can create thousands of TGFs, including some of the most energetic light rays on Earth. However, we still know little about these gamma-rays and what causes them.

NASA discovered TGFs in 1992, but it wasn't until 2012 that scientists working with the Fermi mission upgraded satellites to be more sensitive to their occurrences. It also helps that we can now look for radio waves because TGFs emit them, similar to lightning. This means that scientists can also use ground-based equipment for measuring lightning for detecting TGFs.

Scientists combined data from Fermi's Gamma-Ray Burst Monitor (GBM), with data from the Total Lightning Network, the World Wide Lightning Location Network, the Department of Atmospheric Science at the University of Wyoming Laramie and the Next Generation Weather Radar (NEXRAD) for pinpointing specific instances of TGF activity.

This combined data is now shedding light on the TGFs. Most surprisingly, scientists now know that any thunderstorm is capable of producing gamma-rays, regardless of strength.

"Remarkably, we have found that any thunderstorm can produce gamma rays, even those that appear to be so weak a meteorologist wouldn't look twice at them," says Themis Chronis, who led the research at the University of Alabama Huntsville (UAH). "All told, this study is our best look yet at TGF-producing storms, and it shows convincingly that storm intensity is not the key."

Scientists still aren't sure where TGFs come from, but the working theory is that they originate at the tops of thunderstorms because of the electric fields that reside there. Updrafts and downdrafts cause precipitation to collide and get electrically charged, with positive charges rising to the top. Lightning itself happens when the electrical field discharges. TGFs happen when a lightning bolt creates electrons that rise upwards to the top of the storm. There, air deflects the electrons, creating gamma-rays.

This new research suggests that this theory holds true, but more research is still needed.

"We suspect this isn't the full story," says Michael Briggs, assistant director of the Center for Space Plasma and Aeronomic Research at UAH. "Lightning often occurs at lower altitudes and TGFs probably do too, but traveling the greater depth of air weakens the gamma rays so much the GBM can't detect them."

[Photo Credit: NASA/Bill Ingalls]

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