The atmospheric light spectacle nicknamed STEVE may be even rarer than sky observers thought.
STEVE, short for Fort Strong Emission Velocity Enhancement, is a glow from the sky that appears south of the northern lights (SN: 15/03/18). The main feature of STEVE is a mauve light band formed by a plasma current flowing westward through the atmosphere, a phenomenon different from that which gives rise to auroras (SN: 30/04/19). But STEVE’s purple bow is usually accompanied by a “picket fence” of vertical green stripes. That fence looks similar enough to the bright green curtains seen in the northern lights that scientists thought at least this part of STEVE could be some kind of aurora.
Recently, studies on the color of the picket have questioned its origins. Auroras form when the electrons in the magnetic bubble, or magnetosphere, cascade around the Earth into the atmosphere (SN: 2/7/20). Those electrons make the nitrogen in the air glow blue and the oxygen glow green. Although STEVE’s green fence also contains bright oxygen, a low nitrogen emission hints that the fence is not the same kind of light show as an aurora.
Now, citizen researchers and scientists have identified an even more unusual aspect of the STEVE picket: small green stripes protruding like feet from the bottom of some of its vertical stripes. The structure of these horizontal stripes cannot be formed by the electron showers responsible for the auroras, researchers report in AGU Advances in December.
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“It’s really weird and no one really knows what’s going on,” says Joshua Semeter, an engineer at Boston University. But new observations suggest that these horizontal stripes – and perhaps about similar colors – arise from some specific STEVE process.
Semeter and colleagues examined horizontal stripes under picket fences in high-resolution STEVE images taken by citizen scientists. The analysis suggested that the streaks in these images were not actually lines stretched across the sky, but only appeared that way due to motion blur, while spherical patches of bright gas moved through the atmosphere.
J. Semeter et al / AGU Advances 2020
These green spots can arise from the turbulence in the plasma torrent that creates the purple band of STEVE, says Semeter. Plasma-positively charged atoms can precipitate through the atmosphere without impediment, forming a smooth purple arc. Meanwhile, the electrons in the plasma are much lighter and are more likely to be hit by the lines of the Earth's magnetic field, which gives these particles a much more rugged ride through the air. As a result, these high-energy electrons can become entangled in small vortices at the edge of the plasma stream, below the purple stripe. There, the particles could excite oxygen bags to glow green.
For now, this is just a theory of what could happen. Plasma computer simulations flowing through the atmosphere could check if the idea is correct.
Whatever’s going on with STEVE’s green horizontal features, “there’s some tempting evidence” that they’re related to the vertical fence, Semeter says. "We find events in which these little feet appear before or at the same time as the green column above it." And some horizontal and vertical stripes seemed to be connected. “It looks like the green emission is expanding upward along the magnetic field line,” Semeter says. If so, this could explain why STEVE fencing does not have the same color as typical auroras.
While these observations hint that the fence may arise from STEVE-specific particle interactions, it’s hard to be sure it’s only based on photographs of the ground, says Toshi Nishimura, a Boston University space physicist who didn’t participate in the work.
Future satellite observations could confirm whether electrons from the magnetosphere are pouring into the atmosphere in the region of a STEVE picket fence, he said. If satellites do not see these electron showers, this will reinforce the idea that the fence is different from normal auroras.