A rare aurora borealis covered the skies of the Arctic at the end of 2022, and a Japanese space physicist along with colleagues has identified this phenomenon as a rare polar rain aurora.
According to National Geographic, researchers have provided scientific confirmations of this unusual aurora phenomenon that occurred in the Arctic nearly two years ago.
Rare aurora in the Arctic sky at the end of 2022. (Source: Getty Images/National Geographic).
On a Christmas morning on Svalbard Island, Norway, in the Arctic, a fisheye lens (a special lens with a wide field of view and significant curvature) pointed up at the bright blue night sky.
Unlike the typical northern lights, where thin, snake-like structures weave beneath constellations, this aurora spread across the sky in “a blanket” of nearly uniform blue.
“This aurora has a very smooth shape, and its structure is just a patch of diffused green,” it looks like a large green pancake,” said Keisuke Hosokawa, a space physicist at the University of Electro-Communications in Tokyo, Japan. Hosokawa had never seen anything like it before.
The strange aurora covered the Arctic sky on December 25 and 26, 2022. Currently, in the journal Science Advances, Hosokawa and colleagues have confirmed this phenomenon as a rare polar rain aurora.
Rain in the Arctic
The aurora is an “outcome” of electrons from the sun being captured and accelerated by the Earth’s magnetic field. The electrons “flow out” from the sun’s corona, its outer atmosphere, also known as solar wind.
Due to the variety of high-energy particles in the solar wind, solar wind electrons are often not energetic enough to create visible auroras when they reach Earth.
However, once they are captured and stimulated by the planet’s magnetic field, the electrons will interact with atoms in our atmosphere and create the aurora. These “light performances” appear around the Earth’s poles but rarely occur at the polar caps.
In contrast, polar rain auroras are directly caused by solar wind electrons, in rare instances with little or no solar wind.
The 2022 event “is a compelling counterexample, in which a polar cap seemed to be filled with electrons coming directly from the sun’s corona,” said David Knudsen, a physicist at the University of Calgary in Canada, who did not participate in the research.
“This is an extremely unusual phenomenon,” he said.
Polar rain auroras have been recorded in satellite data before, but they have never been documented from ground-based cameras on Earth.
A all-sky camera in Longyearbyen, Norway, captured images of the green aurora rain covering the sky above the Arctic. (Source: National Geographic).
Hosokawa routinely checks aurora cameras weekly but only stumbled upon the 2022 aurora after a holiday. While reviewing the backlog of data, he found the rare aurora phenomenon – like a late Christmas gift.
“When I saw that strange aurora, I realized this was special and I needed to do something. I started to look at satellite data collected at the same time, and I saw signs of polar rain,” Hosokawa said.
A Day Without Solar Wind
Auroras are not just a direct result of relatively weak electrons moving straight from the sun; they are also a rare example of a nearly 28-hour period with almost no solar wind, except for electrons from polar rain.
The solar wind continuously blows into the Solar System from the sun’s corona, so it is very unusual to see it completely disappear. Hosokawa described this as a “once-in-20-years experience”: The only previously recorded polar rain aurora was in 2004, and that phenomenon was only observed from space.
The absence of solar wind made the polar rain aurora that Hosokawa discovered exceptionally bright and therefore easier to see from the ground. Dual observations from both ground and space are essential for understanding both the small details and large-scale patterns of auroras.
Since the electrons creating polar rain auroras come directly from the sun, they act like a shadow or imprint of the environment from which they originate.
Hosokawa and his colleagues hope to use this data to understand the relationship between the electrons arriving on Earth and where they come from in the sun’s atmosphere.
“The really exciting point of this paper is that it shows there are still fundamental things to explore,” said Larry Paxton, co-author and astrophysicist at the Johns Hopkins Applied Physics Laboratory in Baltimore, Maryland.
“Now we have a satellite system and ground-based observatories that allow us to see in a new way how the sun connects with Earth.”
