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New observations of STEVE, the mauve celestial ribbon that aurora chasers have seen fluttering in the sky, have helped scientists determine the energy source of this unique phenomenon.
A few years ago aurora chasers kept finding a mauve arc crossing the sky, sometimes accompanied by green stripes. The phenomenon lacked both an explanation and a name, so they dubbed it Steve. But even as their discovery went viral, its origin continued to stump amateurs and professionals alike.
As scientists began to investigate the pinkish celestial ribbon from within, using satellite data, they managed to turn the name into a scientific description: Strong Thermal Emission Velocity Enhancement (STEVE). They soon found that the ribbon itself isn’t an aurora after all but rather the warm glow from fast-flowing plasma above Earth’s atmosphere. These regions appear following frequent space weather squalls called substorms, but still STEVE’s origin remained unclear.
Now, Toshi Nishimura (Boston University) and colleagues report on the energy source that fuels STEVE in the Geophysical Research Letters, following an in-depth probe of the regions just outside of Earth’s atmosphere.
Nishimura’s team worked with data from three satellite constellations orbiting Earth at different levels above the atmosphere. The three Swarm satellites returned data from 400 to 500 kilometers (250 to 310 miles) above Earth’s surface, probing the middle part of theionosphere. This region is where the Sun’s radiation breaks apart atmospheric molecules into charged particles. Meanwhile the Defense Meteorological Satellite Program (DMSP) examines the sparser ionosphere up at 800 km. And far above the ionosphere, the Time History of Events and Macroscale Interactions (THEMIS) satellites explore themagnetosphere, where Earth’s magnetic field dominates particles’ behavior. The THEMIS probes travel as far as 80,000 km from Earth in their elliptical orbits.
But Nishimura’s team needed more than satellite data; the researchers also needed to know what was happening on the ground. The continued involvement of amateur astronomers was essential to the project, Nishimura explains: “There is no scientific camera in the Seattle area because that's not the place to do auroral research, but [astrophotographer’s] beautiful photographs acted as mobile sensors for scientists to spot STEVE.”
Aurora chasers, such as study coauthor Rocky Raybell, notified researchers when they spotted (and photographed) STEVE. Then the researchers used satellite data to measure the electric and magnetic fields associated with the phenomenon, as well as particle energies and densities in the ionosphere and farther out in the magnetosphere.
From this full spectrum of data, Nishimura’s team has pieced together STEVE’s story: First, a magnetic disturbance in the tail of Earth’s magnetosphere pinched off a pocket of energetic electrons and pushed it inward toward Earth. THEMIS noted the substorm in this study from an altitude of 22,000 km.
Generally, when electrons rain down on Earth, they crash into the ionosphere to produce nighttime auroras, and this is exactly what happened to make the green stripes, or “picket fence,” associated with STEVE. So the green picket fence is aurora. But somehow the electrons that create it are finding a unique pathway, making the picket fence visible at more southern latitudes than ordinary aurora.
The team also confirms that, unlike the green picket fence, STEVE’s pink ribbon is not aurora - that is, it doesn’t come from the electrons that are raining down into the ionosphere. It does have the same energy source, though. The same substorm that pushes electrons toward Earth is the source of an energetic electric field, which propagates inward along with the electrons and accelerates charged particles along the way. These heated particles glow, creating STEVE’s pink ribbon.
That glow is a handy thing, as it associates STEVE with fast-flowing plasma streams that create low-density “holes” in the ionosphere. These holes delay and disrupt space-ground communications, such as GPS signals. “Those holes are difficult to spot because we can't see them,” Nishimura explains, “but STEVE allows us to visualize where the holes are and how they evolve dynamically.”
So, aurora chasers, keep chasing STEVE (and reporting your observations at Aurorasaurus) - you’re catching a unique sky phenomenon that affects our understanding of space weather and satellite communications alike.
Monica Young, a professional astronomer by training, is news editor of Sky & Telescope.