Japanese researchers locate blue auroras

As two Japanese researchers report in the journal "Earth, Planets and Space", this blue glow occurred at an altitude of 400 to 900 kilometers - which contradicts previous explanations for the phenomenon.

At the beginning of May, a whole series of huge eruptions occurred on the sun, during which high-energy electrically charged particles were ejected into space. On the night of May 10-11, these particles reached the Earth and caused a geomagnetic storm, as a result of which northern lights were visible far to the south.

The Earth's magnetic field deflects the particles coming from the sun, preventing them from penetrating directly into the atmosphere. However, some of the deflected particles - mainly electrons - flow along the magnetic field lines on spiral paths down to the poles.

When the electrons hit atoms and molecules in the atmosphere, they cause them to glow: Oxygen atoms at an altitude of around 300 kilometers glow red, while those further down, at an altitude of around 100 kilometers, glow green. And below the green range, blue aurora can often be seen, which comes from excited nitrogen atoms.

When auroras are visible far to the south, as in May, they usually appear reddish, as the lower green and blue parts are already below the horizon. This makes reports of blue auroras high in the sky from mid-latitudes all the more unusual.

With the help of smartphone photos

Scientists Sota Nanjo from the University of Electrocommunication in Tokyo and Kazuo Shiokawa from Nagoya University in Aichi have now succeeded in investigating this mysterious blue glow in more detail. They were helped by images taken by amateur researchers with their smartphones.

In particular, Nanjo and Shiokawa were able to localize the blue aurora with a series of images taken by two amateur researchers from different locations: It was located at an altitude of 400 to 900 kilometers and stretched over a length of 1200 kilometers. It showed three striped structures that were oriented along the course of the Earth's magnetic field.

Until now, there have been two different explanations for the blue aurora at high altitude, but neither of them can be reconciled with the measurements taken by Nanjo and Shiokawa. It was speculated that neutral atoms could penetrate the atmosphere from space and be stimulated to glow there. "But it is unlikely that neutral atoms produce auroras with structures that are oriented to the magnetic field, as we have observed," says Shiokawa.

The second explanation is based on the scattering of sunlight by nitrogen in the high atmosphere. However, the blue aurora studied by Nanjo and Shiokawa was in the shadow of the Earth, so could not have been caused by sunlight. The blue aurora therefore requires "an alternative explanation", the scientists state.

So far, there has been no explanation for the existence of nitrogen at such high altitudes. "Our results point to a previously unknown process that transports nitrogen to high altitudes," explains Shiokawa. But how the nitrogen molecules get there is still "shrouded in mystery", the researcher continues. The two scientists are now hoping for more strong auroras and many images from amateur researchers in order to understand the mysterious process.