STEVE (Strong Thermal Emission Velocity Enhancement) is a thin ribbon of pinkish-red or mauve-colored light stretching from east to west, farther south than where aurorae usually appear. Even more strange, STEVE is sometimes joined by green vertical columns of light nicknamed the picket fence. According to a new study, published in the journal Geophysical Research Letters, STEVE’s picket fence is caused by a mechanism similar to typical aurora, but the mauve streaks are caused by heating of charged particles higher up in Earth’s atmosphere.
This photograph of STEVE was taken on May 8, 2016, in Keller, WA, the United States. The major structures are two bands of upper atmospheric emissions 100 miles (160 km) above the ground, a mauve arc and green picket fence. The background star constellations include Gemini and Ursa Major. Image credit: Rocky Raybell.
Aurorae are produced by glowing oxygen and nitrogen atoms in Earth’s upper atmosphere, excited by charged particles streaming in from the near-Earth magnetic environment called the magnetosphere.
Researchers didn’t know if STEVE was a kind of aurora, but a 2018 study found its glow is not due to charged particles raining down into Earth’s upper atmosphere.
“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 study co-author Dr. Bea Gallardo-Lacourt, a physicist at the University of Calgary.
“The precipitating electrons that cause the green picket fence are thus aurora, though this occurs outside the auroral zone, so it’s indeed unique.”
An 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. Image credit: Emmanuel Masongsong, UCLA / Yukitoshi Nishimura, BU & UCLA.
In the study, Dr. Gallardo-Lacourt and colleagues wanted to find out what powers STEVE and if it occurs in both the Northern and Southern Hemispheres at the same time.
The team analyzed data from several satellites passing overhead during STEVE events in April 2008 and May 2016 to measure the electric and magnetic fields in Earth’s magnetosphere at the time.
The scientists then coupled the satellite data with photos of STEVE taken by amateur auroral photographers to figure out what causes the unusual glow.
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.
Interestingly, they found the picket fence is powered by energetic electrons streaming from space thousands of miles above Earth. While similar to the process that creates typical aurorae, these electrons impact the atmosphere far 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.
The researchers also found the picket fence occurs in both hemispheres at the same time, supporting the conclusion that its source is high enough above Earth to feed energy to both hemispheres simultaneously.
“Public involvement has been crucial for STEVE research by providing ground-based images and precise time and location data,” said study lead author Dr. Toshi Nishimura, a physicist at Boston University.
“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.”
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Y. Nishimura et al. Magnetospheric signatures of STEVE: Implication for the magnetospheric energy source and inter-hemispheric conjugacy. Geophysical Research Letters, published online April 16, 2019; doi: 10.1029/2019GL082460
