STEVE may not be who you think he is. Or, rather, what you think it is.
Not too long ago, professional aficionados of the night sky were thrilled to take a closer look at something amateur night sky watchers had been seeing for decades. This curious luminous strand, one that danced a little differently from the rest of the aurorae, was unofficially named the Strong Thermal Emission Velocity Enhancement (STEVE) in a paper back in March of this year.
It was not definitively termed an aurora by the team at the time, but it seemed probable it was a new type of aurora. According to new research, featuring some of the authors of that same paper, STEVE looks to be an imposter. It’s in fact not clear what STEVE is at all, which means there’s a mystery up there that we aren’t close to solving just yet.
The Northern (borealis) or Southern (australis) Lights were once often thought of as ancestral spirits dancing in the sky, or perhaps a bridge to a deific dimension. Today, we know that they form when particles launched from the Sun are dragged down by our planet’s magnetic field lines, which are concentrated around the magnetic poles.
As they come up against our atmosphere, they collide with atoms within it. They become energetically excited, which kicks up their electrons to higher-energy shells. As energy is lost, their electrons fall down to lower energy levels, and they emit light: green for oxygen, reds and blues for nitrogen.
If you are standing in the Northern Hemisphere, somewhere considerably dark and without moonlight, you may have spotted a purple sliver of light just south of the main event – north of it, if you’re in the Southern Hemisphere. It certainly looked like part of the aurora, but its segregation struck skygazers as curious.
It was given the backronym STEVE. This hinted at what it might be, but without giving the impression that its physical properties were entirely understood.
Passing it on to several NASA aurora researchers, it was found that STEVE was associated with a distinct stream of energetic particles, one known as a subauroral ion drift, or SAID. SAIDs were known about before, and were seen to occur more commonly in spring and fall, which matches when STEVE was most commonly observed.
It seemed, then, that STEVE was a visualization of a SAID, possibly making it an aurora. This wasn’t a smoking gun, though – it was correlative evidence. It was just associated with the aurora, with the direct cause-and-effect relationship related to STEVE remaining enigmatic.
This new Geophysical Research Letters study analyzed a 2008 STEVE event using a network of ground- and satellite-based energetic particle detectors. They found that no charged particles were pouring into the ionosphere, the sliver of atmosphere in which the aurorae are forged, during that particular event.
The team conclude, then, that it was “clearly distinct from the aurora” and instead refer to it as a skyglow, a diffuse light source in the sky whose origins remain unknown – perhaps a bit odd, as STEVE tends to have a filamentous structure.
STEVE may not be guilty of fraud just yet. This analysis is based off a single STEVE event, so it’d be interesting to see how others compare, as the researchers themselves emphasize in their study.
The paper also includes some interesting caveats. Protons and high-energy electrons weren’t observed, and although an electron energy discharge was seen, it was too low-energy to be responsible for auroral generation. The authors do note, however, that they “cannot rule out the possibility that STEVE might be associated with low‐energy proton precipitation” – a process that can generate aurorae.
They cite a few other possibilities for STEVE’s genesis, but they can’t yet tell where it is even appearing: the aurora-filled ionosphere, or elsewhere. The team certainly don’t think it’s an aurora, but they can’t be absolutely sure just yet. As before, no one knows why it's purple.
STEVE is certainly something, but right now, it’s an astrophysical question mark.