Astronomers Discover Mysterious Purple 'Aurora' 

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A group of amateur astronomers has helped discover a new feature of the Northern Lights using a photograph posted on Facebook.

When the mysterious purple streak was first discovered in British Columbia, it appeared to be the first of its kind and, as the Boaty McBoatface debacle proved, when you leave naming something to the public, they do so in style. In this instance, the newly-discovered natural phenomenon was named Steve.

Since its discovery, over 70 pictures of Steve have been taken around the world. Now, a European Space Agency (Esa) mission has helped explain how Steve is created.

Images of the mysterious purple 'aurora' light Steve

The Alberta Aurora Chasers is a Facebook group for members of the public interested in taking and sharing pictures of the Northern Lights, or Aurora Borealis. The Northern Lights are caused by charged particles from Earth raining down on the upper atmosphere. They excite gases and cause them to glow red, green and purple.

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Professor Eric Donovan, from the University of Calgary, came across a photograph in the Facebook group that included a strange purple streak he had never seen before. Some members of the group thought the streak was a proton arc, but Donovan knew this could not be the case and, along with colleagues, turned to data from the Swarm mission.

Swarm is Esa’s first constellation of Earth observation satellites designed to measure the magnetic signals from Earth.

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“As the satellite flew straight though Steve, data from the electric field instrument showed very clear changes,” explained Donovan. Steve turned out to be a hot stream of gas.

“The temperature 300km above Earth’s surface jumped by 3000°C and the data revealed a 25 km-wide ribbon of gas flowing westwards at about 6 km/s compared to a speed of about 10 m/s either side of the ribbon.”

Swarm is ESA’s first constellation of Earth observation satellites designed to measure the magnetic signals from Earth

He continued that Steve is actually "remarkably common" but simply hadn’t noticed it before.

“It’s thanks to ground-based observations, satellites, today’s explosion of access to data and an army of citizen scientists joining forces to document it,” said Roger Haagmans, Esa’s Swarm mission scientist.

"It is amazing how a beautiful natural phenomenon, seen by observant citizens, can trigger scientists\' curiosity.”

However, Haagmans added that there is still a lot we need to learn about Steve. For example, it is not created by the interaction of solar particles with the Earth\'s magnetic field, meaning it is not classified as an Aurora and requires further investigation.

For centuries, a ‘proton arc’ has been a common term for mysterious purple streaks spotted in the sky alongside the aurora, but scientists have been saying for years that a ‘proton arc’ is unlikely to be related to protons.

"Ordinary auroras we see from the ground and space are caused by electrons precipitating down into the atmosphere,” Dennis Gallagher of the Nasa Marshall Space Flight Centre said last year. "Protons can cause auroras, too, but they are different. For one thing, proton auroras are brightest in the UV part of the spectrum, invisible to the human eye."

There is some visible light from proton auroras, but these are broad and spread out, not tight and filamentary like the streaks seen in the photographs.

Exactly why these structures are called proton arcs is unknown.