More than just a typical aurora, researchers have now figured what powers this stunning lightshow and where it comes from.
The recently-discovered atmospheric glow known as STEVE took the sky-gazing world by storm when it first appeared. While looking like a family member of the aurora borealis clan we've come to know and love, STEVE was different. Typical auroras are usually seen as swirling green ribbons spreading across the sky; but Steve is a thin ribbon of pinkish-red light snaking from east to west, and also farther south than where auroras usually appear. Even odder, Steve is sometimes accompanied by green vertical shafts of light lovingly known now as the "picket fence."
Scientists have mused about the strange nature of STEVE (which stands for Strong Thermal Emission Velocity Enhancement), and weren't sure if it was a kind of aurora at all. "Auroras are produced by glowing oxygen and nitrogen atoms in Earth's upper atmosphere," explains the American Geophysical Union, "excited by charged particles streaming in from the near-Earth magnetic environment called the magnetosphere."Shedding some light on the mystery, a 2018 study found that STEVE's unique spectacle was not due to charged particles raining down into Earth's upper atmosphere. Rather, the authors explained it more as a "sky-glow" that is distinct from the aurora – yet they were unsure exactly what was causing it.
But now a new study from the American Geophysical Union (AGU) has some answers about what makes STEVE tick. They have found from where in space STEVE hails, and the two mechanisms that cause it.
Authors of the new study looked at satellite data and ground images of our mystery glow and concluded that the reddish arc and the picket fence are two distinct phenomena born from two different processes. "The picket fence is caused by a mechanism similar to typical auroras, but STEVE's mauve streaks are caused by heating of charged particles higher up in the atmosphere, similar to what causes light bulbs to glow," notes AGU.
"Aurora is defined by particle precipitation, electrons and protons actually falling into our atmosphere, whereas the STEVE atmospheric glow comes from heating without particle precipitation," said Bea Gallardo-Lacourt, a space physicist at the University of Calgary and co-author of the new study. "The precipitating electrons that cause the green picket fence are thus aurora, though this occurs outside the auroral zone, so it's indeed unique."
To see what fuels STEVE and if it occurs in both the Northern and Southern Hemispheres at the same time, the researchers used data from satellites that had passed over STEVE to measure the electric and magnetic fields in the magnetosphere at the time. They then compiled that data with photos of STEVE taken by amateur auroral photographers to figure out what causes the phenomenon.
AGU explains, "They found that during STEVE, a flowing 'river' of charged particles in Earth's ionosphere collide, creating friction that heats the particles and causes them to emit mauve light. Incandescent light bulbs work in much the same way, where electricity heats a filament of tungsten until it's hot enough to glow."
Image above: Artist’s rendition of the magnetosphere during the STEVE occurrence, depicting the plasma region which falls into the auroral zone (green), the plasmasphere (blue) and the boundary between them called the plasmapause (red). The THEMIS and SWARM satellites (left and top) observed waves (red squiggles) that power the STEVE atmospheric glow and picket fence (inset), while the DMSP satellite (bottom) detected electron precipitation and a conjugate glowing arc in the southern hemisphere.
As for the origin of the picket fence, the scientists concluded that it is powered by energetic electrons streaming from space thousands of kilometers above Earth. They explain that while similar to the process that forms typical auroras, the picket fence electrons play with the atmosphere farther south of usual auroral latitudes: "The satellite data showed high-frequency waves moving from Earth's magnetosphere to its ionosphere can energize electrons and knock them out of the magnetosphere to create the striped picket fence display." Also supporting this was that the picket fence happens in both hemispheres simultaneously, further suggesting that the source is high enough above Earth to deliver energy to both hemispheres at the same time.
There is so much to love about all of this, not the least of which is that such as extraordinary event has such an ironically banal name. (Sorry, Steves of the world – I love the name! It's just doesn't have the same majestic ring as an ancient deity.) And how wonderful that the sky keeps delivering to us such astonishing surprises. But one of the best things here is that the public's involvement was crucial in the sharing of images from the ground, with exact time and location data, according to Toshi Nishimura, a space physicist at Boston University and lead author of the new study.
"As commercial cameras become more sensitive and increased excitement about the aurora spreads via social media, citizen scientists can act as a 'mobile sensor network,' and we are grateful to them for giving us data to analyze," Nishimura said.
Anything that gets people out into nature and gazing up at the sky in wonder is a great thing in my opinion. If they help unravel the deep mysteries of an extraordinary celestial phenomenon along the way? All the better.
For more, see the study in the AGU journal, Geophysical Research Letters.