If you happen to see a vertical ribbon of dancing, flickering purple light playing across the sky of the Northern Hemisphere, fear not. It's just Steve.
That's right — Steve. The hilarious name comes from the Alberta Aurora Chasers, a group of aurora enthusiasts who discovered the atmospheric phenomenon in 2016. Unlike your standard aurora displays, which look like gently wafting curtains, Steve is more of a narrow arc of light.
The members settled on the unusual name in honor of the 2006 animated film "Over the Hedge," in which some woodland creatures name an unknown object "Steve" to make it appear less frightening. (Scientists later turned the name into an acronym, standing for "Strong Thermal Emission Velocity Enhancement.")
Steve may appear similar to other auroras because it lights up the night sky when the sun's charged particles interact with Earth's magnetic fields. But Steve is definitely in a class of his own — especially with the spectacular show of dancing purple lights.
What makes Steve unique?
First and foremost, Steve actually isn't an aurora. Although researchers speculated for years that Steve was like other auroras due to its location and movements, a 2018 study negated that idea. When Steve appeared in March 2018, NOAA's Polar Orbiting Environmental Satellite 17 measured for charged particles in the sky around Steve. No charged particles were detected. Therefore, the process for creating Steve isn't the same for auroras.
"Our results verify that this STEVE event is clearly distinct from the aurora since it is characterized by the absence of particle precipitation," said the study's author Bea Gallardo-Lacourt. "Interestingly, its skyglow could be generated by a new and fundamentally different mechanism in the ionosphere."
Steve and the 'picket fence'
In an April 2019 study, researchers shed more light on Steve's identity, locating its source region in space and outlining mechanisms that cause it. Although Steve is sometimes accompanied by a stripy series of green "picket fence" of auroras, Steve itself is caused by heating of charged particles higher in the atmosphere, akin to the process that illuminates incandescent light bulbs.
The study's authors found that during Steve, charged particles collide with each other as they flow like a river through Earth's ionosphere, creating friction that heats the particles until they emit mauve light. Incandescent light bulbs work in a comparable way, using electricity to heat up a tungsten filament and make it glow.
Steve's green picket fence, on the other hand, is caused by energetic electrons falling from space. This is more similar to the way typical auroras develop, although it happens much farther south of the latitudes where auroras usually form. High-frequency waves move from Earth's magnetosphere to its ionosphere, the satellite data showed, energizing electrons and knocking them out of the magnetosphere to create the fence-like pattern of lights. The picket fence also occurs in both of Earth's hemispheres simultaneously, the study found, suggesting its source is high enough above the planet to feed both hemispheres at the same time.
The photographer notes Steve began as a fountain of white light in the west and stretched across the sky meeting a band of light from the east directly above at about 90 degrees, drifting south. (Photo: Rocky Raybell/Flickr)
Where and when to see Steve
Steve travels along the sub auroral zone (lower latitudes closer to the equator) while auroras are found at higher latitudes — thus giving it its unique purple hues. "Steve might be the only visual clue that exists to show a chemical or physical connection between the higher latitude auroral zone and lower latitude sub auroral zone," NASA's Liz MacDonald said in 2018.
On average, Steve can be seen about 20 km vertically (north-south direction) and 2,100 km horizontally (east-west direction), according to another study conducted in 2018 by Gallardo-Lacourt and her team. They also discovered Steve only lasts about an hour and usually only occurs after substorms — a disturbance in the magnetosphere when energy from Earth's "tail" enters the ionosphere.
The purple lights are made up of "a fast moving stream of extremely hot particles called a sub auroral ion drift, or SAID." "People have studied a lot of SAIDs, but we never knew it had a visible light. Now our cameras are sensitive enough to pick it up and people's eyes and intellect were critical in noticing its importance," said NASA's Eric Donovan.
To investigate the phenomena, Donovan combed over data captured by a trio of ESA satellites called Swarm. Located in two different polar orbits, the three satellites are constantly recording measurements of the strength, direction and variations of the Earth's magnetic field. To Donovan's delight, one of the satellites recently passed through a visit by Steve and captured its unique characteristics.
"The temperature 300 km above Earth’s surface jumped by 3000°C and the data revealed a 25 km-wide (15.5-mile) ribbon of gas flowing westwards at about 6 km/s compared to a speed of about 10 m/s either side of the ribbon," he said in an ESA press statement.
As you can see in the photos below, Steve isn't at all frightening; it's just plain beautiful.
Steve was spotted on Sept. 7, 2017 over Owen Sound, Ontario, Canada. (Photo: Northern Lights Graffiti/Flickr)
This composite image, made from 11 photos stitched together, shows Steve and the Milky Way at Childs Lake in Manitoba, Canada. (Photo: Krista Trinder/NASA)
NASA is asking for help with Steve. If you think you've spotted Steve, you can submit your photos and video to aurorasaurus.org or download the app. NASA also has tips on how you can know if you've seen Steve.
Editor's note: This article has been updated since it was originally published in April 2017.