On a summer night in Colorado, a Hampton University scientist and his collaborator glimpsed one of Mother Nature’s weirdest, rarest fireworks shows.
About three times a day, a glowing electric donut appears in the outer reaches of the sky, just at the edge of space. Then, in less than the time it takes to blink an eye, it’s gone.
Known by atmospheric scientists as elves (Emissions of Light and Very-low-frequency perturbations due to Electromagnetic pulse sources), the donuts are an unusual kind of lightning. They tend to crop up in the wake of more conventional, cloud-to-ground lightning strikes. Those near-ground strikes can send the electrons in the upper atmosphere, the so-called ionosphere, into a frenzy. When the energetic electrons collide with nitrogen molecules in the air, the molecules glow red.
The resulting light show, though incredibly brief, can stretch for hundreds of miles across the sky. Elves were first spotted in 1990, in video footage from the space shuttle Discovery. Since then, they’ve remained about as elusive as the mythical creatures from which they take their name.
In late spring of 2013, Jia Yue, a professor in Hampton University’s department of atmospheric and planetary sciences, and his collaborator, Walter Lyons, set out looking for elves. The team set up shop at the Yucca Ridge Field Station, just outside Fort Collins, Colorado. From that perch, about a mile above sea level, they could see for hundreds of miles across the surrounding Great Plains. On June 12, while their cameras were pointed toward Rapid City, South Dakota, they captured footage of an elve with a most unusual trait: It had stripes. Naturally, they dubbed it a tiger elve.
The striped elve didn’t come as a complete surprise. Harvey Rowland and colleagues at the Naval Research Lab predicted their existence nearly two decades ago. At that time it was known that the atmosphere sometimes hosts gravity waves, alternating rings of thin and thick air that fan out in the sky like ripples in a pond. Since an elve’s light is the result of electrons jostling nitrogen molecules in the air, Rowland and company figured that when elves cross paths with a gravity wave, they should glow brightest in the rings where the air is thickest—after all, that’s where most of the nitrogen molecules are.
Spotting a striped elve and pinning down the origin of its stripes, however, is easier said than done. Because they exist for just a fraction of a millisecond, elves can’t be seen with the human eye or with conventional cameras. And high-speed cameras fast enough to glimpse the elusive creatures typically don’t yield the kind of high-resolution images that are needed to tease out the presence of a gravity wave. Yue and Lyons’ solution was to film the event simultaneously with two cameras—a high-speed Phantom camera, which spotted the elve, and a customized Canon digital single-lens reflex (DSLR) camera, which imaged the gravity waves in the elve’s afterglow.
In the DSLR image above, you can just make out the gravity-wave ripples near the top of the frame. By comparing that image and others like it with high speed video of the elve, the researchers were able to confirm that the elve’s stripes were caused by gravity waves that had rippled outward from a storm in western Nebraska.
Elves are just one in a host of exotic lightning phenomena that occur in the ionosphere. Collectively, scientists call them transient luminous events, or TLEs. And it’s quite possible that all TLEs—sprites, gnomes, trolls, jets—interact with gravity waves in some way. In fact, one group has already seen evidence of gravity-wave-affected sprites. But whereas scientists have spent centuries unravelling the mysteries behind conventional lightning, they’ve only known about TLEs for about a couple of decades. Surely there’s plenty left to learn about the fireworks show at the edge of the sky.