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How a Space Telescope's Accidental Discovery Overturned Everything we Thought we Knew About Lightning Storms

Monday, March 11, 2019

The GRAPES-3 muon telescope in Ooty, India was designed to study the cosmos—events that took place millions of years ago at distances that confound the human imagination. What researchers didn’t expect was that it would also shed light not just on cosmic history, but on a mystery much closer to home: the massive power hidden in a thundercloud.

Benjamin Franklin was the first to produce a definitive study on a thunderhead's electric charge*. With his famous kite-in-a-thunderstorm experiment, along with many others, he showed that thunderclouds separate electric charge, piling up negative charge at their lower edges and positive charge at the top. (It’s since been found that although this is the case most of the time, occasionally the charges are switched.) This charge imbalance creates an electric potential difference—also known as a voltage—across the cloud vertically, much like a giant battery. In 1929, Scottish physicist Charles Wilson estimated this voltage to be well over one gigavolt, or one billion volts—an astounding figure. While it was initially accepted with enthusiasm, this estimate eventually fell out of favor as field measurements repeatedly struggled to break one-tenth that amount.

That shouldn’t really come as a surprise, though, says Sunil Gupta, corresponding author on a new article in the American Physical Society's journal Physical Review Letters. After all, voltage is usually determined by connecting the positive and negative sides with a terminal and measuring the current that flows across it, powered by the potential difference. “But how do you put a terminal across a two- or three-kilometer high thundercloud?” he asks rhetorically. You simply can’t.

Instead, researchers usually use balloon-borne instruments to measure the local electric field at many points throughout the thundercloud, and from that extrapolate the overall voltage. But balloons are slow and can take hours to transverse a cloud, and thunderstorms have short lifetimes. “They weren’t using the right tool,” Gupta says. In contrast, he and his colleagues think that they’ve found the ideal tool, one that can flit through a thundercloud in a matter of microseconds: high-energy, electron-like particles known as muons.

This story begins the same way so many others do in the scientific community: an unexpected experimental measurement. Although the project would eventually develop into a collaboration among twenty-two researchers at six institutions dotted across India and Japan, it started with one team from the Tata Institute of Fundamental Research in Mumbai, and another from Osaka City University. The two institutes were initially united in a collaborative survey of muons, with the hope that it would provide some insight into the cosmos.

The GRAPES-3 team poses by one of the four telescope halls.
Image credit: GRAPES-3 experiment.
Muons themselves do not come from space, but astronomers find them useful as a proxy for cosmic rays, which do. Cosmic rays are an assortment of high-energy particles—primarily composed of protons and helium nuclei, but also containing representatives of most of the periodic table—that bombard Earth’s atmosphere from outer space.

After being ejected from stars, supernovae, and more exotic objects such as quasars, these particles travel over light years of empty space, practically unimpeded. But when they enter the atmosphere, they're confronted with a wall of air molecules, and quickly interact with these particles' nuclei to produce a whole spectrum of secondary particles, including muons. Except for muons, these secondary particles never reach the ground; they're either too light or too highly charged to carry much momentum in a straight line. Muons, however, have the charge of an electron but more than 200 times its mass, which means they can hurtle down toward Earth at incredible speeds, without being deflected much by the charged particles they pass on the way. “Almost like a shower of particles moving at nearly the speed of light propagating towards Earth,” Gupta explains.

It was this shower of particles that the team originally set out to survey with the GRAPES-3 muon telescope. However, they were surprised to find that during certain times of year the muon intensity tended to dip (or, more rarely, spike) as much as 2% for a brief period. “Now, for most experiments 2% is a very small number… you generally don’t worry about it,” Gupta admits. But outside of these anomalies, the fluctuations in the number of muons measured was comparatively tiny, only about 0.2% off the mean. That means that such an enormous variation is extremely unlikely to be due to chance; in fact, “it will likely not happen even in the lifetime of the universe,” Gupta says. There had to be some other explanation.

This graph indicates the percent change in observed muon intensity over time for a small region of the sky. The red bars indicate the standard error of roughly 0.2%. It was the enormous dip of 2% (starting at 10:42) that caught the researcher's attention.
Image credit: Hariharan et al.
After a little head-scratching, they realized that those times corresponded to peak thunderstorm season in their location—and, in fact, the changes to muon intensity always occurred in conjunction with a storm! Gupta is quick to point out that they weren’t the first to observe such a phenomenon: “The thunderstorm connection was well known,” he says, “but the direct relationship between the two was not established.” In other words, although thunderstorms and muons were linked in research literature, it wasn’t clear why thunderstorms would have such a large effect on the number of muons reaching the ground—or what it could tell us.

GRAPES-3, however, has something that other muon telescopes don’t: the ability to tell with high accuracy the muon’s direction of travel based on their angle of impact. “We gained direction,” Gupta says of the telescope’s unique construction, “and that turned out to be a critical difference in studying this phenomenon.” It’s the same difference, he says, as the one between an optical telescope and a solar panel; both register photons hitting their surfaces, but only one can reconstruct an image of the sky. Similarly, the team could use GRAPES-3 data to reconstruct a map of muon intensities across a thundercloud, something that hadn’t been done before.

The reconstructed field of view as seen by GRAPES-3. Each pixel is colored according to the muon variation observed; note the maximum variation of -2% in the lower right-hand corner. The approximate shape of the thundercloud is traced out by the dark line.
Image credit: Hariharan et al.
These muon maps gave the research team a more complete picture of the thunderstorm’s anatomy, allowing them to accurately compare muon intensities under a thundercloud to those in clear skies. This was key, because muons have a special property: as charged particles, their energy changes when they pass through an electric potential like that of a thundercloud. And since the GRAPES-3 receiver has a threshold energy of about 1 GeV, that change in energy can affect whether a muon is detected at all. It seems like a perfect explanation for the observed drop in muon intensity, since some muons presumably lost enough energy passing through the thundercloud that they no longer registered.

Well, it’s almost a perfect explanation. You see, muons come in two flavors: positively charged and negatively charged. Assuming the typical thundercloud charge distribution (positive on top, negative on bottom), a negatively charged particles with an energy of, say, 1.3 GeV would lose enough energy that it would fall below the threshold—but on the other hand, a positively charged muon of 0.7 GeV could gain enough energy to be detected, resulting in a zero net change.

“This is where nature comes to our rescue,” Gupta says with a chuckle. This conundrum is very real, but only assuming an equal number of positive and negative muons (and an even distribution of energies). However, since many of the cosmic rays that produce the muons in the first place are the positively-charged protons, there is actually an imbalance in muon charges that results in a 10-20% excess of positive muons. Without this convenient fact, the researchers could never have measured a difference in muon intensity.

Yet they did, and from those measurements they managed to extrapolate a function that maps an observed change in muon intensity to the voltage that would have caused it. After analyzing the numerous thunderstorms caught by GRAPES-3, they were delighted to find that one thundercloud was particularly powerful, clocking in at 1.3 gigavolts—right on target with Wilson’s prediction from nearly a century ago! To put that in perspective, after making some reasonable assumptions about the cloud’s size and shape, the researchers estimate that it contained over 720 gigajoules of power. “That’s a massive amount of power." says Gupta, "If you could tap this power… it is enough to sustain New York City for 26 minutes. It’s really unbelievable.”

The team has already measured several more thunderstorms with similar voltages, indicating that their 1.3 gigavolt monster was not an anomaly. “It’s not a one-off thing,” Gupta says of the remarkable potential. Unfortunately, it’s unlikely that we’ll ever be able to harness the remarkable power held in thunderstorms, but Gupta is already hoping that this study will help explain other phenomena—like the mysterious high-energy gamma rays that populate the atmosphere. “The story has just begun,” he promises.

—Eleanor Hook

*In fact, Gupta spent considerable time poring over Franklin’s original writings from 1751. “I found it very hard to read the papers of Franklin,” he admitted, mainly for stylistic reasons, “but the writing is absolutely precise.”
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