On Christmas Day 2022, a highly peculiar aurora was seen over an enormous section of the Arctic sky, the likes of which have never previously been observed from the ground. Known as a polar rain aurora, this exceptional light show was produced by a waterfall of electrons that emanated from the sun before cascading peacefully over the North Pole.
Typically, auroras consist of dynamic, shifting lights that change shape and color as they dance across the sky. This phenomenon is caused by charged solar particles interacting with the Earth’s magnetic field, before being channeled towards the poles.
However, the spectacle observed on the night of December 25 2022 was completely different. For one thing, it was massive, covering an area of around 4,000 kilometers (2,500 miles). More interestingly, though, it was completely uniform and unchanging, consisting of a smooth, featureless green glow that just hung in the sky without morphing or rearranging itself into any shapes.
To find out what happened, the authors of a new study examined data from the US Defense Department’s polar-orbiting satellites. In doing so, they discovered that the solar wind had dropped to virtually zero at the time of the aurora, which also coincided with the opening of a so-called coronal hole on the sun’s surface.
As a consequence, high-energy electrons were able to flow out of this coronal hole without becoming scattered by the solar wind. These particles – which would normally be blown from pillar to post, creating fast-moving aurorae – were therefore able to gently rain down over the North Pole in a steady stream, creating a completely flat aurora.
“By combining ground-based and satellite observations, we proved that this unique aurora was produced by suprathermal electrons streaming directly from the Sun, which is known as “polar rain.”,” write the researchers. Until now, this phenomenon had only ever been observed from space, but had never been seen from the ground.
The study authors also calculated that the spread of electrons streaming out of the coronal hole would have covered some 7,500 kilometers (4,600 miles) of the sky, which explains why this edition of the Northern Lights was so large.
“This incredibly smooth and gigantic form is distinctively different from that of a typical polar cap aurora,” explain the researchers. “Thus, it cannot be categorized as any previously identified class of aurorae visible at polar cap latitudes.”
The study has been published in the journal Science Advances.
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