A First in Astronomical Observation

On Christmas Day 2022, a remarkable and unprecedented type of aurora was observed over the Arctic, offering a unique insight into the interactions between the sun and Earth’s magnetic field.

This rare phenomenon, known as a polar rain aurora, was visible from the ground for the first time, presenting a smooth and expansive green glow across the sky.

Discovery and Characteristics

The polar rain aurora displayed a uniform and unchanging green light, spanning an impressive 2,485 miles (4,000 kilometers) over the North Pole. Unlike typical auroras, which are dynamic and shift in patterns and colors, this aurora remained featureless and steady, creating a serene yet eerie spectacle. Normally, 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. It was massive, covering an area of around 4,000 kilometers (2,500 miles), but more interestingly, 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.

Researchers from the University of Electro-Communications in Tokyo and the US Defense Meteorological Satellite Program (DMSP) satellites conducted a detailed study to understand this phenomenon.

The aurora was imaged by an All-Sky Electron Multiplying Charge-Coupled Device (EMCCD) camera in Longyearbyen, Norway. The satellite data revealed that this aurora was caused by a ‘rainstorm’ of high-energy electrons streaming directly from the sun, a phenomenon previously only observed from space.

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,” explained the researchers. “Thus, it cannot be categorized as any previously identified class of aurorae visible at polar cap latitudes.”

Cause of the Polar Rain Aurora

The smooth aurora occurred when the solar wind—a stream of charged particles from the sun—dropped to nearly zero, creating a calm space environment around Earth. This rare event coincided with the formation of a coronal hole on the sun’s surface, a region where the sun’s magnetic field lines open up and allow high-energy electrons to escape directly into space.

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.

These electrons traveled along the open magnetic field lines, connecting with Earth’s magnetic field above the North Pole. Without the scattering effect of the solar wind, the electrons rained down directly onto the polar cap, creating the extensive and uniform auroral display.

Keisuke Hosokawa, lead researcher of the study, explained, “When the solar wind disappeared, an intense flux of electrons with an energy of >1keV was observed by the DMSP, which made the polar rain aurora visible even from the ground as bright greenish emissions.”

In the case of the polar rain aurora, these electrons traveled across space, and the open magnetic field lines connected with Earth’s magnetic field above the North Pole, allowing the electrons to rain directly onto the poles rather than getting trapped inside the magnetotail.

Normally we don’t notice this happening, because the regular polar wind particles scatter the fast-wind electrons emanating from the coronal hole. On this occasion, however, the pressure of the solar wind had decreased to the extent it was negligible, and the fast-wind electrons could reach Earth unhindered.

Furthermore, the diameter of this magnetic funnel opening is about 4,600 miles (7,500 km) when projected at Earth’s distance from the sun. That’s why the aurora seemed so smooth; the open magnetic flux tubes emanating from the sun covered a wider area than Earth’s north polar cap.

Because the electrons were high energy, the auroral emission was purely green rather than red because it takes more energy to ionize oxygen deeper in the atmosphere. The clinching evidence was that the DMSP satellites only saw the polar rain aurora over Earth’s north magnetic pole, which is tilted towards the sun during Northern Hemisphere winter.

Implications and Future Observations

The observation of the polar rain aurora from the ground provides valuable insights into the behavior of high-energy electrons and their interactions with Earth’s magnetic field. This phenomenon highlights the complex dynamics of space weather and the importance of continuous monitoring and research. Until now, this phenomenon had only ever been observed from space, but had never been seen from the ground.

The findings were published in the journal Science Advances, offering a comprehensive explanation of the conditions that led to this rare auroral display. The study emphasizes the need for further observations and research to understand the full implications of such events on Earth’s space environment.

By combining ground-based and satellite observations, the researchers proved that this unique aurora was produced by suprathermal electrons streaming directly from the Sun, which is known as “polar rain.” The polar rain itself has previously been studied in-depth by particle detectors on satellites in orbit, but such studies are few and far between.

These smooth auroras are not normally visible to the naked eye on the ground. As such, nobody knew what the smooth, featureless aurora that turned the sky green over Christmas of 2022 was, until now.