A remarkably smooth and confusing Christmas day Aurora The 2022 Arctic meteorological observation was the result of a ‘rain storm’ of electrons directly from the sunsay researchers from Japan and the US.
It is the first time that a rare aurora of this kind has been observed from the ground, and it happened at a time when the winds of the solar wind had almost completely disappeared, leaving a calm region around the Earth.
Normally the Northern Lights appear as we see them everywhere in the world in Maymoving and pulsating, with distinct shapes in the sky. These auroral displays are driven by electrons from the solar wind — a stream of charged particles streaming from the sun — that get caught in an extension of The Earth’s magnetic field called the magnetotail. When space weather becomes extreme, such as when a coronal mass ejection (CME) — a large ejection of plasma and magnetic field from the sun — is released, the magnetic tail can be pinched off (don’t worry, it grows back). The electrons trapped there flow down Soilmagnetic field lines toward the poles. As they do so, they encounter molecules in the earth’s atmospherecollide with them and cause them to glow in the colors of the northern lights (blue for nitrogen emissions, green or red for oxygen emissions, depending on altitude).
The soft aurora of December 25-26, 2022, however, was very different. Captured by an All-Sky Electron Multiplying Charge-Coupled Device (EMCCD) camera in Longyearbyen, Norway, the aurora was a faint, featureless glow that spanned 2,485 miles (4,000 kilometers). It had no structure, no pulsing or varying brightness. It was the first aurora to be seen from Earth.
To solve the mystery, a team led by Keisuke Hosokawa of the Center for Space Science and Radio Engineering at the University of Electro-Communications in Tokyo compared this dull aurora with what was seen by the Special Sensor Ultraviolet Scanning Imager (SSUSI) on the polar-orbiting satellites of the Defense Meteorological Satellite Program (DMSP). The DMSP is operated by the National Oceanic and Atmospheric Administration and the U.S. Space Force on behalf of the United States Department of Defense.
The satellites saw the aurora from above and found that it had all the hallmarks of a rare type of aurora, called polar rain aurora, which had previously only been observed from space.
The normal solar wind travels about 250 miles (400 km) per second. However, the sun is hot corona is full of holes, especially at higher solar latitudes from which an exceptionally ‘fast’ solar wind streams outwards, reaching speeds of up to 800 km per second. Sometimes these coronal holes can appear at lower latitudes, and that is what happened during Christmas 2022, when it coincided with a shutdown of the normal solar wind.
At the location of coronal holes, the Sun’s magnetic field lines are open — they do not extend back toward the Sun’s surface, the photosphere. As the open magnetic field lines extend out into space, the coronal hole forms the base of a magnetic funnel from which high-energy electrons flow.
In the case of the polar rain, these electrons traveled through space and the open magnetic field lines came into contact with the Earth’s magnetic field above the north pole. This allowed the electrons to rain directly onto the poles instead of being caught in the magnetotail.
Normally we don’t notice this, because the normal polar wind particles scatter the fast-wind electrons coming out of the coronal hole. This time, however, the solar wind pressure had decreased to the point where it was negligible, and the fast-wind electrons could reach Earth unhindered.
Furthermore, the diameter of this magnetic funnel opening is approximately 4,600 miles (7,500 km) when projected onto Distance from Earth to Sun. That’s why the aurora appeared so smooth; the open magnetic flux tubes emanating from the sun covered a larger area than the Earth’s north polar cap. Because the electrons were high-energy, the aurora emission was pure green instead of red, since it takes more energy to ionize oxygen deeper in the atmosphere.
The conclusive evidence was that the DMSP satellites saw polar rains only over the Earth’s magnetic north pole, which is tilted toward the sun during the Northern Hemisphere winter.
“When the solar wind disappeared, an intense flux of electrons with energies >1 keV was observed by the DMSP, making the aurora visible even from the ground as bright green emissions,” Hosokawa’s team wrote in their published research paper.
The polar rain itself has previously been studied in depth by particle detectors on satellites orbiting Earth, but such studies are rare. These smooth auroras are not normally visible to the naked eye on the ground. As such, no one knew what the smooth, featureless aurora was that turned the sky green during Christmas 2022, until now. The full explanation can be found in the June 21 issue of the journal Scientific progress.