1. A Telescope Over Egypt Caught Antimatter From a Storm in Zambia
Antimatter sounds like science fiction, but it is a real kind of matter. Many particles have an antimatter counterpart. The electron's counterpart is the positron, which has the opposite electric charge. When a positron meets an electron, both vanish and turn into gamma rays, the most energetic form of light.
Fermi is built to watch for gamma rays from space. When its burst monitor recorded a signal near 511,000 electron volts, a feature taken as the sign of an electron meeting a positron [1]. Most of the time, the storm producing the signal was right below the telescope. But on December 14, 2009, Fermi was over Egypt and the active storm was in Zambia, below the telescope's horizon, so no gamma rays from it could have reached the instrument [1].
The research team explained it with Earth's magnetic field. Joseph Dwyer, a co-author of the study, said the storm made fast electrons and positrons that "rode up Earth's magnetic field" to hit the spacecraft. The particles reached a point where their motion reversed, then traveled back and struck Fermi a second time 23 milliseconds later [1]. Michael Briggs of the Fermi team called the signals "the first direct evidence that thunderstorms make antimatter particle beams" in 2011 [1][2].
One caution: this does not mean every flash of lightning shoots out antimatter. The signal came with a brief burst from inside a storm called a terrestrial gamma-ray flash, or TGF, which is covered in section 5 [1].
2. Why Fast Electrons Can "Run Away" Inside a Thundercloud
Air is normally an insulator. Lightning needs the air to break down so that electricity can flow. In the usual explanation, this needs an electric field of about 2 million volts per meter. Yet fields measured inside thunderclouds have generally been substantially weaker than that, which has long been a mystery about how lightning begins [3].
The way out involves a surprising fact about fast electrons. As an electron moves through air, it bumps into molecules, and those bumps act like a brake. In air, the braking force on an electron weakens as its energy rises, and it is smallest at an energy of about 1.5 million electron volts [3]. In other words, for a while a faster electron is slowed down less.
Now add an electric field, like the one in a thundercloud. If an electron is fast enough that the push from the field beats the brake, it keeps speeding up instead of slowing down. This is a "runaway electron." Physics Today's authors explain that these electrons knock new fast electrons loose from air molecules, and each of those can run away too. The result is an avalanche that grows rapidly, called runaway breakdown [3].
A simple way to picture it is a snowball rolling downhill. One rolling snowball gathers more snow, and the number grows quickly. The picture only helps you think about it. In the real process, fast electrons knock other electrons loose, and single particles move almost at the speed of light.
3. Where Do the First Fast Electrons Come From? Possibly From Space
An avalanche needs something to start it: at least a few electrons already moving fast enough. One candidate comes from space. Cosmic rays are fast-moving particles from beyond our atmosphere. When one strikes the air high up, it breaks into a spray of many smaller particles, called an extensive atmospheric shower, and this spray includes fast electrons.
The Physics Today authors, who proposed this idea, draw a picture of a runaway discharge set off by such a shower in a thundercloud, in a setup at a mountain research station in Kazakhstan [3]. So the idea is not that cosmic rays "make" the storm. It is that they may supply the first sparks of speed, and the cloud's electric field does the rest.
4. What Is a Terrestrial Gamma-Ray Flash?
A terrestrial gamma-ray flash is a brief burst of gamma rays from inside a thunderstorm, linked to lightning [1]. Satellites built to study the sky, including the Compton Gamma Ray Observatory and RHESSI, found that some millisecond-long bursts were coming from thunderstorms, not from space [3]. They had been watching for cosmic events, and Earth's own weather turned up in the data.
It is estimated that about 500 of these flashes happen every day around the world, though most go undetected [1]. The Fermi team had identified 130 of them since the telescope launched in 2008 [1].
The explanation put forward in this research connects everything so far. The scientists think that near the tops of storms, the electric field drives an upward avalanche of electrons moving close to the speed of light. When air molecules deflect those electrons, they give off gamma rays. There are so many gamma rays that some change into an electron and a positron, a pair, which can leave the atmosphere and follow the magnetic field out to Fermi [1]. In this study, the observed flashes were explained as sending out electron and positron beams [1].
5. What a Winter Thunderstorm in Japan Showed on the Ground
In Japan, thunderstorms are common in winter along the coast of the Sea of Japan, not only in summer. On February 6, 2017, radiation detectors at the Kashiwazaki-Kariwa nuclear power station in Niigata, on that coast, recorded a burst of radiation from a lightning discharge. This was followed by a longer gamma-ray signal near 511,000 electron volts, which corresponds to electrons meeting positrons, the same kind of signal Fermi had recorded from space [4].
The research team, led by Teruaki Enoto of Kyoto University, reported that this showed lightning can set off nuclear reactions. The team interpreted it this way: a lightning gamma ray knocked a neutron out of an atom's center, or nucleus, leaving behind an unstable atom that later gave off a positron [4]. Enoto said the reactions in air were only the second natural source of radioactive atoms in the atmosphere, after cosmic rays. He also said the amounts are small and short-lived, with no health risk [4].
It was one lightning discharge. Enoto told ScienceAlert it remains an open question whether all storms can do this [4].
6. Do Cosmic Rays Start Lightning? Scientists Still Disagree
Cosmic rays may provide seed electrons, but that does not prove they start lightning. In 2013, Aleksandr Gurevich and Anatoly Karashtin in Russia studied radio signals from thousands of lightning strikes. They suggested that cosmic-ray showers, working together with tiny pieces of water and ice in the cloud, could help set off a discharge [5].
That is a model, not a final answer. Steven Cummer of Duke University, who was not part of the Fermi study, said in 2011 that scientists still had to work out what is special about these storms and exactly what role lightning plays [1]. How lightning begins remains a mystery, with fields in clouds generally measured as weaker than the usual breakdown value [3].
7. Counting the Seconds Between Lightning and Thunder, From Indoors
You cannot see antimatter or gamma rays, but you can estimate how far away a storm is. When you see a flash, count the seconds until you hear thunder. As a rough guide, divide the seconds by 3 for kilometers, or by 5 for miles. Do this from inside a building, away from windows, and never go outside to watch.
If you want to dig deeper, read the NASA and AGU announcement about Fermi's discovery and the Physics Today article, both listed below. What other surprises might be hiding in the data from instruments built for something else?