A lucky stargazer spotted a rare proton arc aurora in Alberta.

(Theresa Tanner )

Stargazers in northwestern Canadawere amazed to witness a rare purple-hued aurora phenomenon — which is yet to be fully understood by space scientists.

Darlene and Theresa Tanner captured a photograph of the extraordinary purple streak in the sky, colloquially known by aurora chasers as a "proton arc."

"We chase storms in the summer and aurora in the winter, going into our fourth year," Theresa explained to weather.com. "We are out looking at the skies almost always, unless [it is] cloudy. We were out that night trying to find clear skies to photograph the aurora, as we knew there was a solar storm happening. When we finally found a clear patch at 12:19 a.m., that is what we saw."

It is that this unusual purple light display is caused by "massive protons that bombard the Earth's atmosphere following an energetic event on the Sun," according to NASA. But some experts think there might be another cause.

"These arcs have been known since their discovery in 1939 by Lars Vegard, and they have been studied since with rockets and satellites,"Vassilis Angelopoulos, space physicist at the UCLA Institute of Geophysics and Planetary Physics, told weather.com.

"They are usually quite faint," Angelopoulos added."They are displaced from the standard location of the aurora; they appear more towards the equator than the typical auroras. They appear often colorless and dull and faint. Sometimes, though, they are more intense and vivid."

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Elizabeth MacDonald, a heliophysicist at NASA's Goddard Space Flight Center, who helps coordinate the Aurorasaurus project, a citizen science initiative that helps track aurora sightings throughout North America, told weather.com that this type of aurora remains an enigma to much of the space science community.

"The usual green aurora is caused by charged electrons particles following the Earth's magnetic field and raining down on the polar regions to produce light when they hit the upper edges of our atmosphere," she explained."[The exact cause of the proton arc is] a bit of a mystery. ...We understand the big picture, but there's parts of the plasma physics and micro-scale processes that are not fully understood. ... There's all different types and flavors of auroras caused by charged particles, [and] there's a lot going on in space that influence those particles."

MacDonald also notes that theterm "proton arcs" isn't fully accepted by heliophysicists because protons interacting with the atmosphere might not be the root cause.

"Identifying this little purple arc as a proton arc is what’s not fully accepted; the term is typically used for something different, something subvisual that can be picked up on filtered scientific cameras," she said. "The aurora-chasing community uses the term, and it might not be the most-apt term. ... We're looking to verify whether protons cause it. Most auroral specialists we’ve talked to aren’t sure what it is."

Although, she observed that the purple aurora discovery was a "great example of enthusiasts working together with scientists to uncover mysterious phenomena."

"Citizen scientists can get involved with Aurorasaurus, and we’re tremendously grateful for the response and opportunities to collaborate thus far,"MacDonald noted.

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