In 2016, a kaleidoscopic ribbon of light fluttered across the skies above Alberta, Canada. Manifesting as a purple band that stretched east to west, occasionally studded with a picket-fence-like row of vertical green stripes, the phenomenon seemed reminiscent of an aurora, but had a wonky shape and was spotted at unusually low latitudes.
Perplexed, a group of citizen scientists dubbed the strange, shimmering arc “Steve.” Reports of the light show and its intermittent encores quickly went viral—but a full explanation for Steve continues to elude scientists worldwide.
Now, a team of researchers might finally be pulling back the curtain on a sliver of Steve’s enigmatic backstory—and it appears to be living up to the hype: Steve has not one origin, but two. Their research, published last month in the journal Geophysical Research Letters, suggests that while Steve’s temperamental emerald spikes are aurora-esque, the sky glow’s trademark mauve streak is not, hailing instead from the shimmer of heated particles in the atmosphere.
Questions remain about how and why the two structures intertwine, but for now, “we’re moving, little by little, closer to understanding Steve,” study author Bea Gallardo-Lacourt, an astrophysicist at the University of Calgary, told Robin George Andrews at National Geographic.
Visible in locations as far-flung as Alaska and New Zealand, Steve is by no means rare. But it wasn’t until a few years ago that the event had its own name. A few years ago, a particularly dazzling display prompted a photographer to suggest “Steve”—an homage to the 2006 animated film Over the Hedge, in which that name is given to a shrub the main characters can’t identify.
Steve soon caught the attention of astronomers, who bemusedly reverse-engineered the playful moniker into an acronym: Strong Thermal Emission Velocity Enhancement. But despite a surge in scientific interest, researchers remained baffled by its cause.
To unravel the mystery, a team of astrophysicists led by Yukitoshi Nishimura of Boston University analyzed data from satellites that had picked up on three Steve events. Combined with photographs taken by citizen scientists, measurements of the electrical and magnetic activity in Earth’s atmosphere told the researchers that Steve was happening in the ionosphere, a region a few hundred miles above Earth’s surface where energy from the Sun’s radiation fractures molecules into charged particles.
The ionosphere is also where typical aurorae occur, sparkling out of the skies when electrons from space bombard molecules in the Earth’s ionosphere, unleashing bursts of colorful light. That seems to be what’s responsible for the greener half of Steve’s palette—but because Steve can be seen so close to the equator, where garden-variety aurora don’t usually appear, it’s clear “something special is happening,” Gallardo-Larcourt told Maria Temming at Science News.
Steve’s violet ripples raise questions as well. Unlike their chartreuse companions, the purple parts of the sky show don’t have their roots in aurorae. Instead, they appear to be the result of energy being transferred from a westward-flowing stream of plasma to other particles in the atmosphere. A byproduct of this reaction is heat that’s emitted as light, giving Steve its luminous lavender luster. “Different [chemical] species in the atmosphere create different colors,” Nishimura told Temming, but the researchers haven’t yet identified the molecular culprits behind this particular sky glow.
Regardless of their source, perturbations to the ionosphere are important to keep tabs on. The same jets of plasma that seem to be generating Steve’s purple patina can also punch low-density “holes” in the ionosphere, creating roadblocks for human-made radio waves that frequent this layer of Earth’s atmosphere. But Steve’s sheen could have the potential to act as a spotlight, allowing researchers to visualize these disruptive gaps, Nishimura told Monica Young at Sky & Telescope.
Steve’s newly identified double act means that its components should be studied separately going forward, Nishimura told Andrews. Two-faced or not, it’s clear that this celestial light show has its own way to glow.