Why auroras have different colours

Why May 2024's aurora appeared a magenta colour over Japan?

Around the world, the historic geomagnetic superstorm of late spring 2024 inspired millions of non-scientists around the world—many armed with highly sensitive smartphone cameras—to take a fantastic, unprecedented number of images of the aurora it produced.

In Japan, this widespread popular uptake of what is now quite advanced imaging technology (even if it is kept in everyone's pocket) proved to be a tremendous boon for atmospheric physicists and other scientists specializing in "space weather." It allowed them to discover why the Northern Lights over Japan appeared as a mysterious magenta color this time instead of the typical red that is observed when aurorae are visible over that country.

In early May this year, one of the most extreme geomagnetic storms in the history of recording such events hit the Earth's atmosphere. This great "storm" in space, composed of ionized particles, is what produces the aurora borealis, or Northern Lights, in the northern hemisphere and the aurora australis, or Southern Lights, in the southern hemisphere.

This time, however, the storm was so strong—the ninth most severe storm in the 110-year history of Japan's Kakioka Magnetic Observatory, one of the oldest geomagnetic stations in the world—that the polar lights could be photographed at much lower latitudes than normal.

In Japan, space weather researchers took advantage of ordinary people taking pictures of the aurora with their smartphones to organize one of the densest citizen science observation efforts anywhere, despite being a low-latitude country where the aurora was somewhat fainter than in places like Canada or northern Europe.

The different colors of an aurora come from the emission of light from different atoms and molecules in the atmosphere when they are bombarded by the particles from space. The dramatic green hue seen in many photographs of the polar lights comes from atomic oxygen (single atoms of oxygen rather than molecular oxygen, or two oxygen atoms bound together) at the lower altitudes within the atmosphere that are visible to people. (The human eye is also just very sensitive to this colour). At even lower altitudes, where atomic oxygen is less common, blue is more visible, and this comes from the greater presence of nitrogen.

At the very highest altitudes in the atmosphere, however, there is a lower concentration of atoms of any kind. The fewer collisions there result in a perception by humans of the excited atomic oxygen atoms as the color red. This is why the upper parts of the aurora curtains can appear as green fading into a scarlet hue.
At low latitudes, as in Japan, normally there is no green at all, only red because only the upper part of the aurora can be seen above the horizon.

"Yet this time, weirdly, the images revealed a very clear and dominant magenta hue to the aurora 'curtains' over Japan, not red.
To solve the mystery, the researchers quickly took to social media to encourage people to observe and report their sightings of the auroras, as well as to input data into a questionnaire asking about observation locations, time, elevation angles and other details, allowing researchers to analyze the auroras' characteristics in unprecedented detail.

The effort resulted in an impressive 775 grassroots submissions, which the researchers then combined with satellite observations and advanced modeling techniques to explore the conditions that had led to the magenta aurora.

The elevation data from these citizen scientists proved to be particularly useful. The researchers used elevation angles to calculate the position of the aurora over time, and found that it was often a surprisingly high altitude of roughly 1,000 km above sea level—which should thus drive a red appearance. But on top of this, the time and season of year meant the atmosphere was more "preheated" ahead of the aurora, in turn driving an upwelling of ionized molecular nitrogen—what is usually responsible for a blue hue.
"Blue plus red makes us see magenta.
And the magenta was made all the more visible and vibrant by the sheer volume of solar activity, even though, ironically, the preheating would also have worked to reduce the peak brightness of the aurora."
Better understanding of magnetic storms goes beyond explaining why humans see the pretty colors of aurorae; these storms can have profound, negative impacts on satellite operations, GPS systems, power grids and even the safety of passengers and crews aboard high-altitude flights.

 a) Aurora photos taken from Aomori, b) from Hokkaido, c) from Chubu, and d) from Tohoku, Japan. Credit: a) 🄫KAGAYA b) ~d) @Courtesy of a citizen scientists

Ryuho Kataoka et al, Extended magenta aurora as revealed by citizen science, Scientific Reports (2024). DOI: 10.1038/s41598-024-75184-9