An Eerie Celestial Display
In late 2022, the Arctic sky was graced by an ethereal aurora, captivating observers with its unusual appearance. Japanese space physicist Keisuke Hosokawa and his colleagues have now identified the phenomenon as a rare polar rain aurora.
Confirming the Extravagance
As reported in National Geographic, researchers have provided scientific confirmation of the exceptional aurora that adorned the Arctic nearly two years ago. The findings are detailed in the journal Science Advances.
Unprecedented Characteristics
Unlike the familiar Northern Lights with their dynamic, serpent-like structures, this aurora spread across the sky in an almost uniform "blanket" of green. "It's very smooth in shape, and the structure is just a diffuse patch of green. It's like a big, green pancake," described Hosokawa.
The Science Behind the Spectacle
Auroras are the result of solar electrons being trapped and accelerated by Earth's magnetic field. These electrons escape from the Sun's corona, its outermost atmosphere, through a phenomenon known as the solar wind.
Typically, solar wind electrons lack sufficient energy to produce visible auroras when they reach Earth. However, when captured and energized by the planet's magnetic field, the electrons interact with atoms in our atmosphere, giving rise to the auroral display. These light shows are most commonly seen around Earth's poles, but rarely over the polar caps.
In contrast, polar rain auroras are caused directly by solar wind electrons, occurring during rare instances when there is little or no solar wind.
"The 2022 event was a beautiful example of a polar cap that was seemingly filled with electrons coming directly from the solar corona," explained David Knudsen, a physicist at the University of Calgary who was not involved in the study. "This is an extremely unusual occurrence."
Rare Terrestrial Observation
Polar rain auroras have been previously recorded in satellite data, but never before from ground-based cameras on Earth.
Hosokawa routinely reviews aurora camera footage, but it was only after a holiday that he stumbled upon the 2022 event. Reviewing the backlog of data, he discovered the remarkable aurora – a belated Christmas gift.
"When I saw that strange aurora, I immediately realized that this is something special, and I have to do something about it. I started to look at satellite data taken at the same time, and I found the signatures of polar rain," said Hosokawa.
A Sunless Day
Not only was this aurora a direct result of relatively weak electrons traveling straight from the Sun, but it also coincided with a rare 28-hour period of near-zero solar wind, except for the polar rain electrons.
The solar wind continuously streams into the solar system from the Sun's corona, so its complete absence is highly unusual. Hosokawa described the experience as "once-in-two-decades": the only other recorded observation of a polar rain aurora was in 2004, and that event was observed solely from space.
The lack of solar wind rendered the polar rain aurora discovered by Hosokawa particularly bright and therefore more visible from the ground. The dual observation from Earth and space is crucial for understanding both the fine details and large-scale patterns of the aurora.
As the electrons producing polar rain auroras come directly from the Sun, they act as a shadow or tracer of the environment from which they originated.
Hosokawa and his colleagues hope to use these findings to unravel the relationship between the electrons arriving at Earth and where within the Sun's atmosphere they originate.
"The really exciting thing about this paper is that it shows there are still very basic things waiting to be discovered," said Larry Paxton, co-author and astrophysicist at the Johns Hopkins University Applied Physics Laboratory in Baltimore, Maryland. "We now have satellite and ground-based observing systems that are allowing us to see, for the first time, a new way that the Sun connects to the Earth."