Citizen Science #28 by Jamie Zvirzdin

Solving the Mystery of Celestial Elves, Trolls and Sprites

Imagine you’re driving home from a party out in the countryside, and the sky is dark and foreboding. Through a break in the clouds, you see something strange: a tentacled, reddish flicker in the upper sky, too high and fast for any fireworks, too ghostly and structured to be lightning. It vanishes before you can point it out to anyone.

You swear it was not a trick of fatigue or your imagination, and you are right—it was a sprite.

Sprites, TROLLs, ELVES—these are not fairytale beings but real and eerie atmospheric light events tied to thunderstorms, each with names as otherworldly as their appearances. Like the mysterious light-tinged footprints Nancy Drew finds in a moonlit garden, these atmospheric phenomena whisper of hidden processes and rare clues in the otherwise well-lit mystery of lightning.

We tend to think of lightning as a downward phenomenon, a jagged bolt cracking from cloud to ground. But the truth is more dynamic. Sometimes lightning launches upward into the upper atmosphere, triggering spectacular events above the storm. Sprites, which appear as orange-red tentacled flashes, form high in the mesosphere, about 50 miles above ground. A TROLL is a faint red, threadlike streak, short for Transient Red Optical Luminous Lineament, and they often drip down as the lowest tendrils from strong sprites. And ELVES—short for the hilarious mouthful “Emissions of Light and Very low frequency perturbations due to Electromagnetic pulse Sources”—are even higher and far faster than sprites. Unlike your cheery-creepy Elf on the Shelf at Christmastime, these ELVES expand like glowing, instantaneous monster halos high up in the ionosphere.

And then there are even flashier lights in our sky with less flashy names: blue jets, which erupt upward from thundercloud tops like short fountains; gigantic jets, which span from clouds to the lower ionosphere; and Terrestrial Gamma-ray Flashes (TGFs), which fire off particle radiation at nearly the speed of light—some downward, some up. Our scintillation detectors at the Telescope Array in western Utah, which were designed to detect ultrahigh-energy cosmic rays, also caught the particle byproducts of these otherwise invisible downward-facing gamma-ray showers in 2018.

But the unraveling of this mystery story continues. Professor Rasha Abbasi and her group at Loyola University Chicago have been investigating optical emission signatures—brief flashes of light we can detect—that are associated with these TGFs.

“Recently, these optical emissions have become a focal point in both space-based and ground-based studies,” Dr. Abbasi said. “They offer valuable insight into the conditions under which TGFs form during thunderstorms.”

A few months ago, Dr. Abbasi’s team and my group, the Telescope Array Collaboration, published a paper in the “Journal of Geophysical Research: Atmospheres.” We reported something never seen before: the time-resolved spectra from the optical leaders—the very beginning threads—of a lightning flash coinciding with a downward TGF.

What we found surprised us. Using a high-speed camera and a slitless spectrograph (a device that captures the color fingerprint of every flash in view), we caught the optical emission signatures of both ionized and neutral atoms (nitrogen, oxygen, hydrogen) lighting up the sky. The leader glow began before the TGF, then faded, then reignited in a pulse of ion emissions—the light given off by energized atoms.

As Dr. Ny Kieu from Loyola University Chicago group, the excellent lead author of our paper, said, “Together with the high electric field in front of the leader tip, this process produces a large number of runaway electrons, which in turn generates Terrestrial Gamma-ray Flashes. These TGFs are among the most energetic forms of natural radiation observed in Earth’s atmosphere.”

I think of tracking down invisible particles and their glowing byproducts like trying to solve a mystery in the dark, but for a split second someone shines a colored flashlight on the scene—and you see red. You see blue. You see fingerprints. You don’t get the whole picture, but you get a lead. In our case, a leader of lightning.

These clues are vital, not just for understanding lightning but also for better interpreting the strange flashes we catch from space. Satellites like ASIM and Fermi have been documenting these odd lights for decades, but the precise whens and hows of their formation remain unclear. Our ground-based observations now add missing pieces to that puzzle: It turns out that optical emissions don’t just happen after the gamma-ray event, as satellites have shown—they can happen before and during, too. That changes the game.

Mysteries in physics rarely arrive with tidy answers. But they often start with a flash—an odd bit of light in the corner of your eye, a signal that something isn’t fully known. And they grow clearer the more eyes and instruments we bring to bear on them. Rather than assuming that these odd flickerings in the sky are either aliens or a government conspiracy, reach first for science. Those lights might be the result of some truly amazing and humbling physics at work. This is why we need to keep supporting scientists, engineers, and quiet sky-watchers—the detectives of the natural world—who build and use these tools to help us find evidence we’ve overlooked.

So the next time you’re outside on a warm night, thunder rumbling in the distance, look up. Just for a second. There might be something rare and red flickering above the clouds—a sprite or some other physical phenomenon. We may not know all their names yet, or the full story behind them, but we’re finally beginning to watch and listen. Let’s not turn away now.

Jamie Zvirzdin researches cosmic rays with the Telescope Array Project, teaches science writing at Johns Hopkins University and is the author of “Subatomic Writing.”