Tuesday night's possibility for a minor display of northern lights, which will almost certainly be blanked by a bright moon, brings to mind a recent research paper on a newly reported aurora-related phenomenon. I've written before about STEVE (Strong Thermal Emission Velocity Enhancement), a narrow, purplish band of light resembling a jet contrail that crosses the sky from northwest to southeast.
Sometimes it's accompanied by a neat series of vertical green columns that look like a picket fence. Because of its resemblance to the aurora and occasional appearance during displays a lot of people assumed the same origin for both. But recent studies have shown quite the opposite.
Auroras form when Earth's magnetic field guides atomic particles in the solar wind into the upper atmosphere where they collide and energize zillions of oxygen and nitrogen atoms. When the atoms return to their "rest state" a fraction of a second later, they release tiny bursts of green and red light that make and color the aurora.
While STEVE is driven by auroral storms it's not an aurora in the usual sense because nothing is raining down into the atmosphere to create it. Instead, it's a fast-moving stream of extremely hot atomic particles (5,430° F / 3,000° C) about 16 miles (25 km) wide and from 80 to 168 miles (130-270 km) high that flows westward at a speed in excess of 13,300 miles per hour (6 km/ sec).
While uncommon, STEVE typically shows up during auroral displays and lasts from 20 minutes to an hour. If you're graced by an appearance, watch closely, and you'll see flowing movements and subtle pulsations within the stream — a truly eerie sight!
In a recent paper titled "The Mysterious Green Streaks Below Steve," Joshua Semeter of Boston University and a team of researchers examined yet another STEVE phenomenon not reported on before. Dr. Tony Phillips at Spaceweather.com calls them "little green cannonballs of light." In photos they show up as short, green streaks of light at the base of the picket fence.
They look like dashes instead of dots in the photos because the spheres or whatever they are are moving during the time exposures and trace out trails the same way airplanes and satellites do when the camera shutter is left open. Each is about 1,100 feet (350 meters) across and 62-68 miles (100-110 km) high.
Their green color matches that of the picket fence, and both arise from excited oxygen atoms which subsequently release tiny jolts of green light. But instead of solar particles hitting atoms as in the aurora, it appears that fast-moving electrons, the same particles responsible for electric currents and making light bulbs shine, makes these globes glow.
Semeter hypothesizes that STEVE's powerful electrical fields induce turbulent flows of extremely hot electrons that directly excite oxygen atoms. Although the green resembles that of the aurora it's purer because only oxygen in a pancake-thin layer of air produces the light. Auroral colors are more blended because they arise from different atoms at a variety of altitudes.
Much like a river with its turbulent flows and eddies, energy from STEVE likely creates these short-lived balls of glowing oxygen that last just 20-30 seconds. The next time you find yourself by a stream imagine another one overhead, flowing with super-speedy ions spinning off whorls of ghostly green light along its banks.
"Astro" Bob King is a freelance writer for the Duluth News Tribune. Read more of his work at duluthnewstribune.com/astrobob.