by A very smooth and dull Christmas day aurora observed over the Arctic in 2022 as a result of a ‘rain storm’ electrons straight from the sunsay Japanese and US researchers.
This is the first time such a rare aurora has been seen from the ground, and it came at a time when the winds of the solar wind fell off almost completely, leaving a region of calm across the Earth.
Usually shows the aurora, similar to those seen around the world in May, soft and pulsating, with clearly recognizable shapes in the sky. These aurora displays are powered by electrons from the solar wind — a stream of charged particles flowing from the sun — that are trapped in time. The Earth’s magnetic field called the magnetotail. When space weather becomes extreme, for example when coronary mass ejection (CME) — a large ejection of plasma and magnetic field from the sun — the magnetosphere can be pinched off (don’t worry, it grows back). The electrons trapped there flow down Worldmagnetic field lines to the poles. As they do so, they encounter molecules i The Earth’s Atmospheremeeting them and encouraging them to glow in the the colors of the aurora (blue for nitrogen emission, green or red for oxygen depending on its height).
However, the smooth aurora on 25–26 December 2022 was very different. Imaged by the All-Sky Electron-Coupled Charge-Coupled Dispersion Device (EMCCD) camera in Longyearbyen, Norway, the aurora was a faint featureless glow that spanned 2,485 miles (4,000 kilometers) in size. There was no structure, no pulsing or different brightness. No kind of aurora like it had been seen from Earth before.
To solve the mystery, a team led by Keisuke Hosokawa, from the Center for Space Science and Radio Engineering at the University of Electrocommunications in Tokyo, compared this faint aurora with what the Special Sensor Ultraviolet Scanning Imager (SSUSI ) on the polar orbit. observed by Defense Meteorological Satellite Program (DMSP) satellites. The DMSP is operated by the National Oceanic and Atmospheric Administration and the US Space Force on behalf of the US Department of Defense.
The satellites saw the aurora from above, discovering that it had the hallmarks of a rare type of aurora called a polar rain aurora, which had never before been seen except from space.
The regular solar wind travels about 250 miles (400 km) per second. However, the sun is hot crown It is full of holes, especially at higher solar latitudes from which an extremely ‘fast’ solar wind travels out at up to 500 miles (800 km) per second. Sometimes these coronal holes can be seen at lower latitudes, which is what happened during Christmas 2022 as it coincides with the cessation of the regular solar wind.
At the location of the coronal holes, the Sun’s magnetic field lines are open – they do not bend back on 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 stream.
In the case of the polar aurora, these electrons traveled across space, and the open magnetic field lines connected to the Earth’s magnetic field above the north pole, allowing the electrons to rain directly on the poles rather than being trapped within the magnet.
Normally we don’t notice this happening, because the regular polar wind particles scatter the fast electrons coming from the coronal hole. This time, however, the pressure of the solar wind was reduced to as little as it was, and the fast wind electrons could reach the Earth unimpeded.
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Furthermore, the diameter of this magnetic funnel opening is about 4,600 miles (7,500 km) when projected at The distance of the earth from the sun. That’s why the aurora looked so smooth; the open magnetic flux tubes originating from the sun covered an area wider than the Earth’s north polar cap. Because the electrons were high energy, the auroral emission was green rather than red because it takes more energy to ionize oxygen deeper in the atmosphere.
The clinching evidence was that the DMSP satellites saw only the polar aurora over the Earth’s north magnetic pole, which is tilted towards the sun during the Northern Hemisphere winter.
“When the solar wind disappeared, the DMSP observed an intense flux of electrons with energies >1keV, which made the white rain aurora visible even from the ground as bright green emissions,” Hosokawa’s team said in their published research paper.
White rain itself has previously been studied intensively by particle detectors satellites in orbit, but there are few such studies. These fine auroras are usually not visible to the naked eye on the ground. So, no one knew what the smooth, featureless aurora that turned the sky green over December 2022 was, until now. The full explanation can be found in the June 21 issue of the magazine Scientific Progress.
