Clean view of STEVE and picket fence captured from Ontario, Canada

A clean view of STEVE and picket fence captured from Old Woman Bay, Ontario, Canada on September 4, 2022.

STEVE is short for “Strong Thermal Emission Velocity Enhancement.”

Typical auroras, the northern and southern lights, are usually seen as swirling green ribbons spreading across the sky. But STEVE is a thin ribbon of pinkish-red or mauve-colored light stretching from east to west, farther south than where auroras usually appear. Even more strange, STEVE is sometimes joined by green vertical columns of light nicknamed the “picket fence.”1

Authors of a new study published in AGU’s journal Geophysical Research Letters analyzed satellite data and ground images of STEVE events and conclude that the reddish arc and green picket fence are two distinct phenomena arising from different processes.

The picket fence is caused by a mechanism similar to typical auroras, but STEVE’s mauve streaks are caused by the heating of charged particles higher up in the atmosphere, similar to what causes light bulbs to glow.

“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 Bea Gallardo-Lacourt, a space physicist at the University of Calgary and co-author of the new study.

“The precipitating electrons that cause the green picket fence are thus aurora, though this occurs outside the auroral zone, so it’s indeed unique.”

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