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early a decade ago, Neil Zeller spied a mysterious purple glow in the night sky. Zeller was on a family vacation in British Columbia, Canada. One August 2014 night, he glimpsed something strange while photographing the northern lights. A band of purple light ran east to west across the sky. This was south of the auroras that made up the northern lights. The purplish-white streak looked almost like the vapor trail of an airplane — yet not quite. Intrigued, Zeller swiveled his camera toward the mystery light. Later, he found that other aurora chasers online had seen similar purple streaks. They called them proton arcs. The term referred to known features of the northern and southern lights. But a couple of years later, Zeller and his fellow photographers would learn that they actually had been documenting a natural light show that was yet unknown to science. The revelation took place in a pub in Calgary, Canada. A group of aurora-chasers were hanging out with researchers from the University of Calgary. “I ended up sitting with Dr. Eric Donovan,” Zeller says. “And just in casual conversation, we started a tiny bit of an argument.” Zeller said he’d seen a proton arc. Donovan replied that this was impossible. That type of northern light can’t be seen with the unaided eye, he explained. So to prove what he’d seen, Zeller pulled out a photo of the purple streak. Zeller recalls Donovan saying: “Yeah, I don’t know what that is, but it’s not a proton arc.” In fact, it didn’t look like any aurora Donovan or his colleagues had ever seen. Since no one had a clue what this ribbon of purple could be, aurora chasers started calling it Steve. Scientists later turned that name into an acronym. It’s now short for “Strong Thermal Emission Velocity Enhancement.” But to this day, researchers are still struggling to understand STEVE. One mystery is what makes STEVE’s purple light. Sky detectives also are puzzling over what causes the “picket fence” of green stripes that can sometimes glimmer in the sky below STEVE. And recently, citizen-science photos have raised questions about how STEVE might be related to another non-aurora light show known as a stable auroral red arc. “STEVE and the picket fence are arguably the biggest mystery in space physics right now,” says Claire Gasque. She’s a space physicist at the University of California, Berkeley. Conditions in Earth’s atmosphere where STEVE appears can affect satellite signals, she notes. So explaining what causes STEVE could have uses beyond understanding a pretty sky light.