Looking out of his backyard in the Canadian town of Strathmore, amateur astronomer and photographer Alan Dyer saw an unfamiliar sight dancing in the night sky: rippling arcs of reddish-pink aurora borealis underlined with another band of aurora in the classic green color. It later became apparent that the same phenomenon had also been snapped in the Finnish towns of Ikaalinen and Orimattila.
Not only did the scene make for some stunning images, the strange observation is now helping to unfold the mystery of how double red and green auroras are formed. Together with satellite observations, Dyer’s photographs and the other images appear to show that the two phenomena may be intimately linked.
In a paper published in the Journal of Geophysical Research: Space Physics, physicists found that the green proton aurora transitioned into the stable auroral red (SAR) arc, and this was initiated by proton precipitation, which had been suggested but never successfully demonstrated before.
It’s known how greenish yellow auroras are formed. They're the product of solar wind, charged particles burped out by the Sun that flow toward Earth. When these energetic electrons and protons crash into Earth's magnetosphere, they release energy and excite gases in our upper atmosphere. The excitement causes ionization of the atmospheric molecules and the release of light photons. Different elements can produce different colors.
The formation of stable auroral red arcs, however, is less clear. The researchers argue red auroras differ from another recently discovered aurora called STEVE because they lack the purple-mauve tinges, as well as other defining features.
However, citizen scientists – contributing significantly to our understanding of auroras in recent years – recently documented a red arc evolving into a STEVE for the first time.
In the new study, Dyer and an international team of physicists and other citizen scientists suggest that the rosy red band might be caused by electrons.
As per their observations and the footage captured of the phenomenon, the green aurora quickly diffused and decayed, but the red arc continued to remain more stable, drifting towards the equator during the substorm recovery phase
Proton rain triggers the classic green aurora. The proton aurora then transitions into a stable auroral red arc when secondary electrons enter the picture by raining down through the atmosphere. Red light has lower frequency, so it requires less energy to produce, hence the red aurora is produced by the electrons that are lower in energy compared to protons. However, both are the product of the same gusts of solar wind blowing into and across Earth’s upper atmosphere.
This is just one theory, the researchers say, but yet another discovery that shows just how complex the simple beauty of the Northern Lights really can be.