The northern and southern lights have dazzled sky watchers for millennia with their eerie, greenish glows. Now, a new purplish aurora is joining the show, stretching from east to west at lower latitudes. Discovered by citizen scientists, and substantiated by professional scientists using photographs and satellite data, the new type of aurora is similar to a previously measured—but never seen—atmospheric phenomenon. Its nickname: Steve.
Citizen scientists first began posting about Steve on social media several years ago. Across New Zealand, Canada, the United States, and the United Kingdom, they reported an unusual sight in the night sky: a purplish line that arced across the heavens for about an hour at a time, visible at lower latitudes than classical aurorae, mostly in the spring and fall. (You can see video of the aurora above.) “It’s similar to a contrail but doesn’t disperse,” says Notanee Bourassa, an aurora photographer in Saskatchewan province in Canada.
Traditional aurorae are often green, because oxygen atoms present in Earth’s atmosphere emit that color light when they’re bombarded by charged particles trapped in Earth’s magnetic field. They also appear as a diffuse glow—rather than a distinct line—on the northern or southern horizon. Without a scientific theory to explain the new sight, a group of citizen scientists led by aurora enthusiast Chris Ratzlaff of Canada’s Alberta province playfully dubbed it Steve, after a line in the 2006 children’s movie Over the Hedge.
Aurorae have been studied for decades, but people may have missed Steve because their cameras weren’t sensitive enough, says Elizabeth MacDonald, a space physicist at NASA Goddard Space Flight Center in Greenbelt, Maryland, and leader of the new research. MacDonald and her team have used data from a European satellite called Swarm-A to study Steve in its native environment, about 200 kilometers up in the atmosphere. Swarm-A’s instruments revealed that the charged particles in Steve had a temperature of about 6000°C, “impressively hot” compared with the nearby atmosphere, MacDonald says. And those ions were flowing from east to west at nearly 6 kilometers per second, propelled by electric and magnetic fields in the atmosphere, the team reports today in Science Advances. All of this heating and motion probably contributes to Steve’s purplish appearance, says MacDonald, but the team hasn’t yet determined the precise wavelengths of light responsible. Citizen scientists are planning to make those measurements, says Ratzlaff, which will help reveal which atoms or molecules are lighting up.
Surprisingly, the researchers found that Steve’s physical properties are consistent with an atmospheric event called “subauroral ion drift,” a rapid flow of charged particles through Earth’s atmosphere, that has never been observed visually. That is, says MacDonald, until now. “We can now connect this phenomenon that we measured with satellites many times with a rare physical feature.” Mapping Steve’s position in the sky should help researchers determine the structure and dynamics of Earth’s magnetic field.
In homage to the original nickname bestowed by citizen scientists, MacDonald and her team have suggested a new scientific term for the phenomenon: Strong Thermal Emission Velocity Enhancement (STEVE). To John Bonnell, a physicist at the University of California, Berkeley, who was not involved in the research, this investigation is a “great example of what can happen when a critical mass of motivated amateur observers, professional space physicists, and space- and ground-based instrumentation come together.”
Professional and citizen scientists alike are looking forward to spotting STEVE in the coming months. It hasn’t been seen much this winter in the Northern Hemisphere, MacDonald says. “[But] people are looking for it right now.”