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Japanese scientists from the University of Electro-Communications in Tokyo have determined the cause of a phenomenon observed over the Arctic in late 2022. There they noticed a very strange aurora, never before recorded on Earth. The study was published in the scientific journal Science Advances (SciAdv).
Normally, auroras move and pulsate, forming clearly visible shapes in the sky. They are fueled by electrons from the solar wind that are trapped in an expansion of the Earth’s magnetic field called the magnetotail. However, the smooth auroras of December 25-26, 2022, were very different.
It was a faint, featureless streak of light, four thousand kilometers long. It had no structure, did not pulse, and did not change in brightness. No aurora like it had ever been observed from Earth before. To solve the mystery, the team compared the ghostly aurora with data from a scanning ultraviolet sensor on a U.S. Department of Defense polar-orbiting climate satellite.
Satellites monitoring the aurora from above found it had all the hallmarks of a rare type of aurora that had previously only been visible from space.
Normal solar wind travels through space at about 400 kilometers per second. However, the sun’s hot corona is full of holes, especially at higher latitudes on the sun, where particularly “fast” solar wind bursts out, moving at speeds of up to 800 kilometers per second.
Sometimes these coronal holes appear at low latitudes, which is what happened on Christmas Day 2022, coinciding with a stop in the normal solar wind. At the location of a coronal hole, the solar magnetic field lines are open – they are not closed off at the solar surface (the photosphere).
When open magnetic field lines extend into space, coronal holes form the bottom of a magnetic funnel from which energetic electrons escape. In the case of polar rain, these electrons travel through space and open magnetic field lines connected to Earth’s magnetic field above the North Pole, allowing the electrons to fall directly to the poles rather than being trapped within the magnetotail.
We normally don’t notice this happening because solar wind particles scatter the fast electrons.
However, in this case, the pressure of the solar wind is reduced to a negligible level and fast electrons can reach the Earth unhindered.
The researchers say the aurora appears pure green rather than red because the electrons have high energy.
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