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Approximately 1,000 times a day, thunderstorms release flashes of some of the most high-energy light found naturally on Earth. These events, called ground-based gamma-ray flashes, last less than a millisecond and produce gamma rays that are tens of millions of times more energetic than visible light.Credit: NASA’s Goddard Space Flight Center
A flash of lightning. Roll of thunder. These are the sights and sounds of a normal storm. But sometimes strange things happen above the clouds. Our Fermi Gamma-ray Space Telescope has discovered bursts of gamma rays, some of the most energetic forms of light in the universe, coming from thunderstorms. It turns out that gamma rays usually come from objects with crazy extreme physics, like neutron stars and black holes. So why does Fermi see them coming from thunderstorms?
Thunderstorms occur when warm, moist air near the ground begins to rise and encounters cooler air. As warm air rises, moisture condenses into water droplets. The upwardly moving water droplets collide with the downwardly moving ice crystals, stripping them of electrons and creating an electrostatic charge within the cloud.
Like the ends of a battery, the top of the storm is positively charged and the bottom is negatively charged. Eventually, enough opposing charge builds up to overcome the insulating properties of the surrounding air, causing a zap. Lightning strikes.
Scientists suspect that lightning reconfigures the cloud’s electric field. In some cases, this allows the electrons to hurtle toward the top of the storm at nearly the speed of light. This makes thunderstorms the most powerful natural particle accelerators on Earth.
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The updrafts and downdrafts within a thunderstorm can cause rain, snow, and ice to collide and create electrical charges that can generate lightning. Under the right conditions, fast-moving electrons can generate gamma-ray flashes on the ground.Credit: NASA’s Goddard Space Flight Center
When these electrons collide with air molecules, they emit a flash of gamma rays on Earth. This means that thunderstorms produce the most high-energy forms of light in the universe. But that’s not all. Thunderstorms can also produce antimatter. Yes, you read that correctly. In some cases, gamma rays strike atoms, producing electrons and positrons, the electron’s opposite antimatter.
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This diagram shows electrons accelerating upward from a thundercloud.Credit: NASA’s Goddard Space Flight Center
Fermi can detect ground-based gamma-ray flashes up to 500 miles (800 kilometers) from its location directly below the spacecraft. This is done using equipment called gamma-ray burst monitors, which are primarily used to monitor spectacular flashes of gamma rays coming from space.
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As shown in this diagram, interactions with matter can produce gamma rays and vice versa. High-energy electrons traveling at near the speed of light are deflected by passing close to atoms or molecules, producing gamma rays. And a gamma ray passing through the electron shell of an atom turns into her two particles: an electron and a positron.Credit: NASA’s Goddard Space Flight Center
It is estimated that there are 1,800 thunderstorms occurring on Earth at any given time. In her first 10 years in space, Fermi discovered about 5,000 gamma-ray flashes on Earth. But scientists estimate that 1,000 of these flashes occur every day. All we see are flashes from Fermi’s normal orbit within 500 miles of her, with no coverage of the United States or Europe.
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Visualization of 10 years of Fermi observations of ground-based gamma-ray flashes.Credit: NASA’s Goddard Space Flight Center
The map above shows all the flashes seen by Fermi between 2008 and 2018 (note the missing clump at the bottom of South America; it’s the South Atlantic Anomaly, a radioactive is the part of the sky that affects the spacecraft and causes data glitches).
Fermi also discovered ground-based gamma-ray flashes emanating from individual tropical weather systems. Tropical Storm Frio in 2014 produced four flashes in just 100 minutes.
