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When a group of amateur aurora hunters discovered what they believed to be a new form of aurora early last year, the science community was stumped. What could possibly cause this narrow band of dancing pinkish-red light arching into the sky accompanied by a green “picket fence” of vertical light – was it a sort of previously unknown aurora or something else?
At the time, sky-watchers named the atmospheric event Strong Thermal Emission Velocity Enhancement, or STEVE for short.
STEVE differed from the typical swirling green and blue auroras that dance across northern and southern skies not only in its different color varieties and visual patterns but in its location, which is further south than most auroras are observed. These sky glow events occur when glowing oxygen and nitrogen atoms in the upper atmosphere are excited by charged particles that stream through the magnetosphere. A 2018 study found that STEVE was not the result of charged particles. However, it also appeared STEVE can show up during the kind of solar-induced magnetic storm around Earth that normally produces the brightest auroras.
“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 co-author of the study Bea Gallardo-Lacourt in a statement.
Publishing their work in Geophysical Research Letters, scientists set out to find the energy source of these sorts of lights and measure the electric and magnetic fields in the magnetosphere that occur during these events. Past studies were limited to observations by ground-based imaging and low-altitude satellites, so the team looked at data from several satellite passings during STEVE events in April 2008 and May 2016 in order to measure the electric and magnetic fields coupled with satellite data and photos of STEVE from the ground.
They found that the reddish arc and green picket fence are two different phenomena created by two very different processes and are “connected to fast plasma flows, sharp plasma boundaries and intense waves 25,000 kilometers (15,000 miles) up in space.” Both colored lights are associated with sub-auroral ion drifts, electron heating, and plasma waves.
STEVE is a flowing “river” of charged particles into Earth’s ionosphere that collide and create friction that heats particles, causing them to give off the mauve light that aligns from east to west. It works in a similar way to lightbulbs, where electricity heats a tungsten filament until it’s hot enough to glow.
Though plasma heating from the fast flows and waves is believed to drive the mauve colored arc, it doesn’t explain the picket fence. This green phenomenon forms at lower altitudes than the mauve arc, powered by energetic electrons streaming in from space thousands of kilometers above Earth. Though it’s similar to normal auroras, it impacts the atmosphere further south than traditional auroral latitudes. Satellite data showed that as high-frequency waves move from the magnetosphere to the ionosphere, they energize electrons and throw them out of the magnetosphere to create the picket fence.
Altogether, the researchers say their work provides a way to study the invisible, complex world that makes up our magnetosphere and could help us better understand how particle flows develop in the ionosphere above our planet, which can impact GPS signals, radio communications, and other mechanisms that rely on satellite data.
