Recently, the scientific community stumbled upon a rare atmospheric phenomenon called Strong Thermal Emission Velocity Enhancement (STEVE) that has been well documented by amateur photographers for decades. When researchers first looked at images of STEVE, they realized the lights were slightly different than light from typical aurorae but were not sure what underlying mechanism was causing them. In a new study, University of Calgary physicist Bea Gallardo-Lacourt and co-authors analyzed a recent STEVE event to see whether it was produced in a similar manner as the aurora. The results, published in the journal Geophysical Research Letters, suggest STEVE is produced by a different atmospheric process than the aurora, making it an entirely new type of optical phenomenon.
Alberta Aurora Chasers capture STEVE on the evening of April 10, 2018 in Prince George, British Columbia, Canada. Fellow Aurora Chaser Robert Downie kneels in the foreground while photographer Ryan Sault captures the narrow ribbon of white-purple hues overhead. The vibrant green aurora is seen in the distant north, located to the right in the photo. Image credit: Ryan Sault.
Aurorae are produced when electrons and protons from Earth’s magnetosphere, the region around our planet dominated by its magnetic field, rain down into the ionosphere, a region of charged particles in the upper atmosphere. When these electrons and protons become excited, they emit light of varying colors, most often green, red and blue.
Amateur auroral photographers from the Alberta Aurora Chasers brought STEVE to scientists’ attention in 2016. They had occasionally noticed bright, thin streams of white and purple light running east to west in the Canadian night sky when they photographed the aurora.
Aurorae are visible every night if viewing conditions are right, but the thin light ribbons of STEVE were only visible a few times per year. The light from STEVE was also showing up closer to the equator than the aurora, which can only be seen at high latitudes.
The photographers first thought the light ribbons were created by excited protons, but protons can only be photographed with special equipment. The light protons produce falls out of the range of wavelengths picked up by normal cameras.
Researchers then started using data from satellites and images from ground-based observatories to try to understand what was causing the unusual light streaks.
The first scientific study published on STEVE found a stream of fast-moving ions and super-hot electrons passing through the ionosphere right where this phenomenon was observed. The study authors suspected these particles were connected to STEVE somehow but were unsure whether they were responsible for producing it.
After that first study, scientists wanted to find out if STEVE’s light is produced by particles raining down into the ionosphere, as typically happens with the aurora, or by some other process.
In the new study, Dr. Gallardo-Lacourt and colleagues analyzed a STEVE event that happened over eastern Canada on March 28, 2008, using images from ground-based cameras that record aurorae over North America.
The team coupled the images with data from NOAA’s Polar Orbiting Environmental Satellite 17 (POES-17), which happened to pass directly over the ground-based cameras during the STEVE event. The satellite is equipped with an instrument that can measure charged particles precipitating into the ionosphere.
The study’s results suggest STEVE is an entirely new phenomenon distinct from typical aurorae.
“POES-17 detected no charged particles raining down to the ionosphere during the STEVE event, which means it is likely produced by an entirely different mechanism,” the researchers said.
“STEVE is a new kind of optical phenomenon we call ‘skyglow’.”
“Our next step is to see whether the streams of fast ions and hot electrons in the ionosphere are creating STEVE’s light, or if the light is produced higher up in the atmosphere.”
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B. Gallardo-Lacourt et al. On the Origin of STEVE: Particle Precipitation or Ionospheric Skyglow? Geophysical Research Letters, published online August 20, 2018; doi: 10.1029/2018GL078509
