Nothing says "I love diving headfirst into a ditch" like your hair suddenly elevating to the tingly feel of electricity. Thunderstorms are amazing from inside a building, but they're scary if you're trapped outside. And, despite a good deal of observation, an element of mystery surrounds them. For instance, we know that lightning can produce free neutrons, antimatter, and gamma rays, but we don't have much idea of how that happens.

That has partially changed thanks to an Indian muon telescope, called GRAPES-3—a classic example of a backronym. GRAPES-3 is designed to detect muons (a heavier cousin to the electron and positron) that are generated as gamma rays hit the Earth’s atmosphere. It is a relatively simple detector that has the benefit of covering a reasonable chunk of sky with good angular resolution. The detectors are also buried under a thick layer of concrete, so muons need to be quite energetic to get to them.

Prediction: Lightning with a chance of telescopes

GRAPES-3 doesn’t actually care where the muons come from; it just happily counts away. Evidently, the scientists running the detector noticed that their data would always go a bit skewiff every time a thunderstorm passed over. Instead of ignoring this, the researchers (while keeping their heads low), installed a set of electric field monitors at various distances from the observatory and started logging electric field strength every time a storm passed over. That data could be easily compared to the muon detection rate. Unsurprisingly, storms are complex beasts, resulting in a lot of data that simply couldn’t be interpreted.

Except for one storm. This is the story of that storm.

Essentially, the electric potential meters indicated that the storm had a relatively simple distribution of electric charge. That allowed the researchers to model the storm as two sheets of charge separated by a few kilometers. With that simplification, the researchers were able to model the muon production and acceleration process.

The researchers assumed that the high-energy muon flux associated with the storm was due to low-energy muons being accelerated by the static potential of the storm. To test that, they ran models of atmospheric muon generation and acceleration. The model also included other high-energy muon production mechanisms, which allowed the researchers to exclude them as the source.

The researchers showed that they could replicate their muon measurements if the peak potential of the storm was about 1.3GV (yes, that's 1.3 billion volts). These potentials are also sufficient to explain previously unexplained gamma-ray flashes from thunderstorms. By contrast, weather-balloon measurements have never measured a static potential higher than 130MV, or 10 times weaker than that.

The researchers also used the electric field meters to independently track the movement of the storm as it passed by the observatory. These changes were nicely matched by the directional changes in muon flux, confirming that they were from the storm. These changes were also modeled and found to be consistent with the very high accelerating potential passing over the observatory.

Estimating mind-boggling numbers

From there, the researchers estimated that the storm held a charge of 1100C (Coulomb is the unit of electric charge: a single electron is 1.6×10-19C) and held an energy of more than 720GJ. After this, the researchers get a bit vague. They state that the storm delivers more than 2GW of power, but I can’t really tell what that means. Essentially, it takes about six minutes for the muon flux to increase to its peak value as the storm passes over. So, I guess the researchers mean that if you could extract energy from the storm as it passed over, you would have an average power flow of about 2GW for six minutes. But I’m not sure that makes much sense.

I must admit that there is no Earth-shattering science in here, but those big numbers speak to my inner child. They also reinforce the immense power held in the atmosphere: thunderstorms are not to be trifled with, and yet it has been estimated that our planet has about 2,000 of them active at any given moment.

Physical Review Letters, 2019, DOI: 10.1103/PhysRevLett.122.105101 (About DOIs)