In 2016, Steve appeared in the Canadian skies. Steve was majestic: a glowing purple ribbon with green “picket fence” rays protruding from it. It resembled an aurora only in that it was a glowing band in the sky, but Steve’s properties were not consistent with those of an aurora. Steve appeared too far south, above areas that never see the northern lights. Steve stood vertically and with colors different from an aurora. Steve was a mystery. Now, three years later, researchers say they’ve finally solved that mystery.
According to a study published in the journal Geophysical Letters on April 16, 2019, researchers have concluded that the Steve anomaly is caused by the friction between hot plasma and magnetic waves some 15,000 miles above Earth’s surface. Furthermore, it seems that the green “picket fence” rays and the towering purple ribbons are two different phenomena caused by two different processes. In a statement, Bea Gallardo-Lacourt, a space physicist at the University of Calgary and co-author of the study, said:
“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. The precipitating electrons that cause the green picket fence are thus aurora, though this occurs outside the auroral zone, so it’s indeed unique.”
Which is a fancy way of saying “it’s a different thing.”
Steve was named after the animated film “Over the Hedge,” in which a group of forest animals see the titular hedge for the first time, and not knowing what to call it, name it Steve. It was later turned into the “backronym” STEVE (strong thermal emission velocity enhancement) which just sounds like technobabble. Credit: Ryan Sault.
In the new study, scientists used satellite data combined with pictures of Steve taken by amateur astronomers all over the globe. They wanted to know what powers Steve and if it occurs in both the Northern and Southern Hemispheres at the same time. Researchers analyzed the measurements of the electric and magnetic field taken by satellites passing through the Steve events and then compared that data to the pictures of Steve taken by amateur astronomers.
They found that the purple ribbons are caused by rivers of charged particles colliding with strong magnetic waves in Earth’s Ionosphere. According to the American Geophysical Union, the process that causes the towering purple ribbons are similar to those that cause incandescent light bulbs to light up. The friction between between the rivers of hot plasma and the the magnetic waves high up in the Ionosphere causes the charged plasma particles to heat up and emit light. A beautiful soft purple light named Steve.
A Steve event that shows the green “picket fence” rays streaming off the side. Credit: Rocky Raybell.
The green picket fence, on the other hand, is caused by a similar phenomenon as traditional auroras. High-frequency waves moving from the Magnetosphere to the Ionosphere can energize electrons and knock them out of the Magnetosphere, creating the striped picket fence pattern. However, this display still occurs in areas much further towards the equator than traditional auroras.
The researchers say that public interest and involvement in the project was key to to its success. Toshi Nishimura, Lead author of the study, says:
“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.”
Although Steve’s cause has been identified, there’s still a bit of mystery remaining. Scientists still do not know why Steve appears as far south as it does. Despite all the time and effort put in to figuring it out, Steve hasn’t let us in on all his secrets quite yet.
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