And suddenly a purple arc appeared in the night sky

A strong geomagnetic storm was brewing in the skies above Alberta, Canada, on Sept. 27th.

And suddenly a purple arc appeared in the sky. It was the mysterious aurora known as “Steve”

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Mysterious auroral arc named Steve appears in the sky of Alberta on September 27, 2017. By Alan Dyer

For many years, northern sky watchers have reported this luminous form occasionally dancing among regular auroras. It was widely called a “proton arc” until researchers pointed out that protons probably had nothing to do with it. So members of the Alberta Aurora Chasers group gave it a new name: “Steve.”

The Canadian Prairies seems to be ideal for sighting Steve, as they often get the main aurora to the north, placing Steve overhead or to the south. The Steve arc appeared for only about 20 minutes, starting at 10:45 pm MDT:

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Steve making a strange isolated auroral arc over Alberta on September 27 2017. By Alan Dyer

No one fully understands the underlying physics of the purple ribbon. One of the European Space Agency’s Swarm satellites flew straight through Steve during a previous apparition. Data revealed a relatively hot river of gas, about 25 km wide, flowing rapidly through Earth’s outer atmosphere.

Steve seems to be a thermal emission from hot flowing gas rather than from precipitating electrons, but his origin and nature are still mysterious.

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