THE MYSTERY OF SPIRAL AURORAS
NOAA - Dec 2022
On Dec. 11th, the sky over Tromso, Norway, was blank and featureless. "Suddenly, auroras came out of nowhere, forming an extraordinary spiral," reports Marek Rybar. The giant swirl was so large, he had to stitch together 4 photos to frame the entire display:
Compared to other forms, spiral auroras are rare, and sky watchers are always delighted to see them. But what causes them? Researchers have been debating the issue for more than 50 years.
A few things are known. Spiral auroras rotate clockwise in the northern hemisphere, and counterclockwise in the southern hemisphere. They can be as small as 15 km, or as large as 1300 km. And they're usually green.
Over the years, two completely different theories have vied to explain the vortices. Theory #1 (circa 1976) says that magnetic fields in Earth's upper atmosphere are sometimes twisted by powerful Birkeland currents. Auroras follow magnetic field lines, so a twisted field would naturally create a twisted aurora. Theory #2 (circa 1996) counters that spiral auroras are more likely Kelvin–Helmholtz instabilities in Earth's ionosphere. This is the same type of instability that makes beautiful spiral clouds in the lower atmosphere, so it might work in the aurora zone as well.
Now there's a new idea just published in Sept. 2022: Theory #3 says that spiral auroras are created by a "tearing mode instability." This is a type of plasma instability that can occur when magnetic fields re-connnect and explode. Here are some simulated spiral auroras based on this model:
Source: "Auroral Spiral Structure Formation Through Magnetic Reconnection in the Auroral Acceleration Region" by K. Huang et al (Geophysical Research Letters)
The model produces spirals of about the right size, and they rotate in the correct direction. If it is correct, explosive magnetic reconnection powers the auroras while the tearing instability twists the lights into a spiral. A movie of the tearing mode in action (unaffiliated with the research) lends credence to their conclusions.
