The atmospheric light show, nicknamed STEVE, is perhaps even stranger than Skywatchers thought.
STEVE, short for Strong Thermal Emission Velocity Enhancement, is a sky light that appears south of the northern lights (SN: 03/15/18). The main feature of STEVE is a purple band of light formed by a stream of plasma flowing west through the atmosphere – a different phenomenon from the one that creates aurors (SN: 04/30/19). However, STEVE’s purple arch is often accompanied by a “picket fence” of vertical green stripes. This fence resembles the shimmering green curtains of the Aurora Borealis, so scientists thought that at least this part of STEVE could be some kind of aurora.
Recently, studies of the color of the picket fence have cast doubt on its origin. Aurors form when electrons cascade into the atmosphere from the magnetic bubble or magnetosphere surrounding the earth (SN: 02/07/20). These electrons make nitrogen glow blue and oxygen glow green in the air. While STEVE’s green picket fence also contains glowing oxygen, a lack of nitrogen emissions suggests that the fence is not the same type of light show as an aurora.
Now researchers and citizen scientists have identified an even more unusual aspect of STEVE’s picket fence: little green streaks sticking out 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 aurors, researchers reported in December AGU advances.
“It’s really weird and no one really knows what’s going on right now,” says Joshua Semeter, an engineer at Boston University. However, the new observations suggest that these horizontal stripes – and possibly the similarly colored fence – came from a STEVE-specific process.
Semeter and colleagues examined horizontal strips under picket fences in high-resolution images from STEVE taken by citizen scientists. Analysis revealed that the streaks in these images weren’t lines stretching across the sky, but merely appeared this way due to motion blur as spherical drops of glowing gas moved through the atmosphere.
These green spots could be caused by turbulence in the flow of the plasma that creates STEVE’s purple ribbon, says Semeter. Positively charged atoms in the plasma can flow largely unhindered through the atmosphere and form a smooth purple arc. Meanwhile, electrons in the plasma are much lighter and more prone to being triggered by Earth’s magnetic field lines – giving these particles a much more bumpy ride through the air. As a result, these high-energy electrons can get caught in small eddies at the edge of the plasma stream below the purple stripe. There the particles could stimulate oxygen pockets to glow green.
For now, this is just a theory of what could happen. Computer simulations of the plasma flowing through the atmosphere could test whether the idea is correct.
Whatever is going on with STEVE’s horizontal green features, “there is some tantalizing evidence” that they are related to the vertical picket fence, says Semeter. “We have found events where these little feet appear in front of or at the same time as the green column above.” And some horizontal and vertical stripes looked connected. “It appears that the green emission is actually expanding upwards along the magnetic field line,” says Semeter. If so, this could explain why STEVE’s picket fence isn’t quite the same color as typical Aurors.
While these observations suggest the fence might be created by STEVE-specific particle interactions, it’s difficult to be safe just from photos of the ground, says Toshi Nishimura, a space physicist at Boston University who was not involved in the work.
Future satellite observations could confirm whether electrons from the magnetosphere flow into the atmosphere in the area of a STEVE picket fence, he says. If satellites fail to see such electron showers, it will reinforce the idea that the fence is different from normal aurors.