On Christmas Day 2022, the skies over the Arctic presented an unusual spectacle, an aurora unlike any that had been seen from the ground before. It is Known as a polar rain aurora, this phenomenon spread an unchanging ethereal green glow over a vast expanse of the northern polar cap covering approximately 4,000 kilometers (2,500 miles).
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On December 25, 2022, a type of aurora was seen over the Arctic. This polar rain aurora was distinct for its coverage and unchanging nature.
Unlike typical auroras that shift and dance across the sky, this polar rain aurora was a featureless, smooth green glow, spreading uniformly without any pulsations or alterations in brightness.
This event is spectacular because it was the first time a polar rain aurora was observed from the ground.
Auroras occur when charged particles from the solar wind interact with Earth’s magnetic field which channels these particles towards the poles.
The interaction between solar particles and the Earth’s magnetosphere creates shifting light displays often seen as vibrant ribbons of color that move across the sky.
The magnetotail, an extension of the Earth’s magnetic field plays a crucial role by trapping electrons that eventually collide with atmospheric molecules causing them to emit light.
On the night of the aurora, data from the US Defense Department’s polar-orbiting satellites showed an unusual drop in the solar wind reducing it to nearly zero.
The drop in solar wind coincided with the formation of a coronal hole on the sun’s surface. This hole allowed high-energy electrons to flow out unimpeded creating the conditions for a polar rain aurora.
With the solar wind absent these high-energy electrons rained down gently over the North Pole forming the smooth aurora observed.
Researchers from the Center for Space Science and Radio Engineering at the University of Electro-Communications in Tokyo and other institutions analyzed data from polar-orbiting satellites to understand the phenomenon.
The aurora was captured by an All-Sky Electron Multiplying Charge-Coupled Device (EMCCD) camera in Longyearbyen, Norway confirming its characteristics.
The study revealed that the aurora covered an area approximately 7,500 kilometers (4,600 miles) wide.
Unlike the typical auroras which display a variety of colors and shapes due to fluctuating solar winds, the polar rain aurora remained a static green glow throughout the event.
Auroras such as the Northern Lights are generally formed by electrons from the solar wind interacting with Earth’s magnetosphere.
These electrons are trapped in the magnetotail, an extension of the Earth’s magnetic field and eventually collide with atmospheric molecules emitting light in various colors like blue for nitrogen and green or red for oxygen depending on the altitude.
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Traditional auroras are characterized by their dynamic pulsating patterns and shapes. They are a direct result of the continuous bombardment of charged particles and their complex interactions with Earth’s magnetic field.
The rare aurora was captured by an All-Sky Electron Multiplying Charge-Coupled Device (EMCCD) camera in Longyearbyen, Norway.
This one was a smooth featureless green glow spanning approximately 2,485 miles (4,000 kilometers). It displayed no structural variation or changes in brightness.
The phenomenon was mysterious as it didn’t fit the usual characteristics of auroral displays making researchers to investigate more.
Led by Keisuke Hosokawa from the Center for Space Science and Radio Engineering at the University of Electro-Communications in Tokyo, the team sought to unveil the mystery behind the smooth aurora.
Researchers compared ground-based observations with satellite data from the Special Sensor Ultraviolet Scanning Imager (SSUSI) aboard the Defense Meteorological Satellite Program (DMSP) satellites.
The satellite data revealed that the aurora was a rare type known as a polar rain aurora, which had previously only been observed from space.
The solar wind usually travels at speeds of about 250 miles (400 km) per second. During this event, high-speed solar wind originating from coronal holes reached speeds of up to 500 miles (800 km) per second.
Coronal holes are regions on the Sun where the magnetic field lines are open allowing high-energy particles to escape into space. These are typically located at higher solar latitudes but can occasionally appear at lower latitudes.
Over Christmas 2022, the regular solar wind had diminished creating a calm space environment around Earth and allowing the high-energy electrons from the coronal holes to stream directly towards the planet.
The open magnetic field lines from the coronal holes connected with Earth’s magnetic field above the north pole creating a funnel that allowed high-energy electrons to rain directly onto the polar regions.
The smooth nature of the aurora was due to the coverage of the magnetic flux tubes which extended beyond Earth’s north polar cap.
The high-energy electrons caused a green glow as they interacted with oxygen molecules in the atmosphere.
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