Mystery deepens for glowing arcs in the sky known as 'STEVE'
Thursday, August 23, 2018, 7:05 PM - A mystery of the night sky just got weirder and more mysterious. The purple and green glowing arcs, which some call 'STEVE', are apparently not auroras, but something entirely different - and unknown!
Aurora chasers have been observing a strange phenomena in the sky for years.
Thin glowing arcs of purple and green light have been seen and photographed, stretching across the sky, typically far south from any other aurora activity that may be occurring at the time.
In this photo, captured by Alberta aurora photographers Theresa and Darlene Tanner on July 17, 2018, shows the diffuse green electron aurora off to the lower left, and two swaths of a different phenomenon on the right, which has been called 'proton arc', 'sub-auroral arc', or more recently, 'STEVE'. Credit: Team Tanner Photography
Originally thought to be an aurora in its own right, a new study of these glowing arcs - which go by the name 'STEVE' - is now saying that they aren't any kind of known aurora, at all.
"Our main conclusion is that STEVE is not an aurora," said Bea Gallardo-Lacourt, the University of Calgary space physicist who led the study, now published in the journal Geophysical Research Letters, according to the American Geophysical Union (AGU). "So right now, we know very little about it."
"And that's the cool thing," she added, "because this has been known by photographers for decades. But for the scientists, it's completely unknown."
By examining a specific instance of this phenomenon, from March 28, 2008, Gallardo-Lacourt and her colleagues matched up images from the University of Calgary's THEMIS array of all-sky cameras, with particle data gathered at the same time, from NOAA’s Polar Orbiting Environmental Satellite 17 (POES-17).
Watch Below: The THEMIS network of all-sky cameras captured this amazing display of aurora on April 20, 2018.
Although the timing of the THEMIS photos of STEVE matched up with the timing of the data taken by POES-17, the satellite showed that there were no charged particles raining down from space at the location that STEVE was occurring.
"With Steve what's happening is we can't find evidence of that particle precipitation, so it seems like the energy that's causing the light is coming from somewhere else," Eric Donovan, a coauthor on Gallardo-Lacourt's study from the University of Calgary's Auroral Imaging Group, told CBC News.
One particular detail to note, however, is that these arcs of light have never been seen to appear on their own. According to aurora photographer Darlene Tanner, in the years she has been observing this phenomenon, they only appear when there is some other aurora activity happening. So, although they do not appear to be caused by the same particle sources that generate the aurora, they do seem to be closely linked to the auroras.
The next step for the research group is to examine more instances of STEVE, to determine its exact origin.
Does it originate from the magnetosphere, as other auroras do, but due to some as-of-yet-undiscovered process, or is it being caused by particles emitting from much closer to the ground - perhaps from the ionosphere, below where the POES-17 satellite would have detected them?
The auroras that make up the Northern Lights and Southern Lights - the Aurora Borealis and Aurora Australis, respectively - are caused by charged particles arriving at Earth on the solar wind, becoming trapped in the planet's geomagnetic field, and then streaming down into the upper layers of the atmosphere. There, these high-energy particles smack into molecules of oxygen and nitrogen, and transfer over some of their energy. When the molecules dump that excess energy, they do so as flashes of light.
This colourful display of the Northern Lights was captured by the Tanners from Central Alberta, on May 28, 2017. Credit: Team Tanner Photography
If the particles are tiny negatively-charged electrons, we see colourful bands and swirls of pink, green, red, blue and purple. If the particles are more massive, positively-charged protons, however, we don't tend to see any aurora, unless we have a special camera. This is because the light the air molecules emit when a 'proton aurora' forms is in the ultraviolet spectrum, which is invisible to the human eye.
According to space weather forecaster Dr. Tamitha Skov, when aurora photographers spotted thin bright arcs of purple and green stretching across the sky, unlike anything they'd seen from the typical auroras, they called them 'proton arcs'.
While the members of the community were aware that these were not actually proton auroras, since they were readily visible, Skov said that the arcs do share certain characteristics of timing and location with the invisible proton aurora. Thus, the name 'proton arc' - rather than attempting to identify them as a type of proton aurora - was simply meant to be a 'placeholder' within the community, until such time as the phenomenon could be properly identified.
Another name that some in the aurora chaser community have been using is 'sub-auroral arc'. This is seen as a more accurate description - an arc of light that is seen in the 'sub-auroral' region, ie: well-south of where the aurora is occurring - without associating it with any particular auroral phenomenon.
In late 2016, members of the Alberta Aurora Chasers met with Donovan at an annual conference, and they showed him images of these long, thin, purple arcs in the sky.
As other scientists had done, in the past, he concluded that they were not proton auroras. Since nobody in the group knew exactly what they were, they gave the phenomena a humorous nickname - Steve.
According to the Aurorasaurus blog:
Why Steve? Well, this is a reference to the popular children’s movie Over the Hedge where one of the characters isn’t sure what he is looking at and randomly names it Steve.
Since then, the name has become something of an unwieldy (but possibly accurate) 'backronym' - STEVE, or Sudden Thermal Emission Velocity Enhancement.