An unusually quiet and uneventful Christmas Day aurora observed over the Arctic in 2022 was the result of a ‘rainstorm’ of electrons directly from SUNsay Japanese and US-based researchers.
It is the first time a rare aurora of this kind has been seen from land and comes at a time when landslides the solar wind had fallen almost completely, leaving a region of calm around the Earth.
Normally the aurora appears, like those seen around the world in May, move and pulsate, with distinct shapes clearly visible in the sky. These aural displays are powered by electrons from the solar wind—a stream of charged particles flowing from the sun—that are trapped in a stretch of Earth’s magnetic field called magnetotail. When space weather becomes extreme, such as when a extraction of the coronal mass (CME) – a large ejection of plasma and magnetic field from the sun – is released, the magnetic tail can be captured (don’t worry, it grows back). Electrons trapped there flow down earthPole magnetic field lines. As they do so, they encounter molecules in the the Earth’s atmospherebumping into them and making them shine in the colors of the aurora (blue for nitrogen emission, green or red for oxygen depending on its altitude).
However, the quiet aurora of December 25–26, 2022 was very different. Photographed by the All-Sky Electron Multiplexed-Charge Camera (EMCCD) at Longyearbyen in Norway, the aurora was a faint, featureless glow stretching 2,485 miles (4,000 kilometers). There was no texture, no shimmer or different shine. No kind of aurora like it had ever been seen from Earth before.
To solve the mystery, a team led by Keisuke Hosokawa, of the Center for Space Science and Radio Engineering at Tokyo University of Electro-Communications, compared these soft auroras with those detected by the Polar Orbiting Special Ultraviolet Scanning Sensor (SSUSI). . the Defense Meteorological Satellite Program (DMSP) satellites saw. DMSP is operated by the National Oceanic and Atmospheric Administration and the US Space Force on behalf of the US Department of Defense.
Satellites viewed the aurora from above, revealing that it had all the hallmarks of a rare type of aurora called a polar rain aurora that had only ever been seen from space before.
The regular solar wind travels about 250 miles (400 km) per second. However, the sun is hot SEK it is full of holes, especially at higher solar latitudes from which flows an extremely ‘fast’ solar wind moving at up to 500 miles (800 km) per second. Sometimes these coronal holes can appear at lower latitudes, and this is what happened during Christmas 2022, as it coincides with a disruption of the regular solar wind.
At the location of coronal holes, the Sun’s magnetic field lines are open – they do not return to the Sun’s surface, the photosphere. As open magnetic field lines extend into space, the coronal hole forms the base of a magnetic funnel from which high-energy electrons flow.
In the case of the aurora borealis, these electrons traveled through space, and the open magnetic field lines connected to the Earth’s magnetic field above the north pole, allowing the electrons to fall directly onto the pole instead of being trapped inside the magnetotail.
Normally, we don’t see this happen because the regular particles of the polar wind scatter the fast wind electrons coming out of the coronal hole. However, in this case, the pressure of the solar wind was reduced to the extent that it was negligible, and the electrons of the fast wind could reach the Earth unhindered.
Furthermore, the diameter of this magnetic funnel opening is about 4,600 miles (7,500 km) when projected onto Earth’s distance from the sun. That’s why the aurora seemed so calm; open tubes of magnetic flux emanating from the sun covered an area wider than the Earth’s north polar cap. Because the electrons were of high energy, the auroral emission was simply green and not red, because it takes more energy to ionize oxygen deeper in the atmosphere.
The main evidence was that the DMSP satellites saw only the aurora borealis over the Earth’s north magnetic pole, which is tilted toward the sun during the Northern Hemisphere winter.
“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 green emissions,” Hosokawa’s team said in their published research paper.
Polar rain itself has previously been studied in depth by particle detectors the satellites in orbit, but such studies are few. These smooth auroras are normally not visible to the naked eye on earth. As such, no one knew what the smooth, featureless aurora that turned the sky green during Christmas 2022 was until now. The full explanation can be found in the June 21 edition of the magazine Advances in science.
