The ghostly sounds of aurora can be heard, even when invisible

If you’re high up and listen closely to the nights when the northern sky dances with blazing sheets of green light – Earth’s spectacular aurora borealis – you might hear ghostly sounds.

Almost imperceptible, the sounds had only been heard during the craziest Northern Lights, described as rushing sounds, like a waterfall falling from a distance, or popping and crackling sounds like faint clutter.

New evidence, however, suggests the sounds are happening high up in the atmosphere even when we can’t hear them – even, perhaps, when we can’t see the Northern Lights at all.

Acoustic engineer Unto Laine of Aalto University in Finland managed to record these strange crackling noises in the sky on a night when no curtains of light appeared.

He presented his findings to the Joint EUROREGIO/BNAM2022 conference on acoustics in Denmark this month.

“This negates the argument that auroral sounds are extremely rare and that the Northern Lights should be exceptionally bright and vivid,” Wool says.

Auroral sounds have long been a mystery. Reports had been described for more than a centuryat least, but it was not until 2012 that the recordings, made by Laine and his colleagues, finally confirmed that the sounds were real.

The researchers also pinpointed where the sounds are coming from in the atmosphere – at an altitude of around 70 to 100 meters (230 to 330 feet), which is surprisingly low.

Auroras are caused when particles from the solar wind collide with the Earth’s magnetosphere, and are then accelerated along the lines of the magnetic field to high latitudes, where they rain in the upper atmosphere.

There, they interact with atmospheric particles to produce the twinkling lights that dance across the sky.

In 2016, Laine and her colleagues revealed that they understood what causes the sounds some people have reported hearing.

On particularly cold, clear, and calm nights, a layer of warmer air forms above a shallow layer of cold air at the bottom of the atmosphere.

Opposite electric charges can accumulate in these two layers; when geomagnetic disturbances, possibly triggered by the aurora borealis, propagate through the atmosphere, they can cause an electrical discharge between the layers, which causes the noise.

The new recordings were made in an effort to further investigate the phenomenon. Near the village of Fiskars, the team set up their recording equipment to listen to the crackles and crackles emanating from the atmosphere.

The observations were then cross-referenced with measurements of geomagnetic activity made by the Finnish Meteorological Institute. In total, the team assembled a library of hundreds of candidate sounds, the loudest 60 of which were related to changes in the Earth’s magnetic field.

“Using geomagnetic data, which has been independently measured, it is possible to predict when auroral sounds will occur in my recordings with 90% accuracy,” Wool says.

The work suggests that there is likely a causal link between auroral sounds and geomagnetic activity, with different types of activity producing different sounds.

The processes that produce these sounds are also different from the processes that produce the auroral displays; however, as both are produced by geomagnetic activity, they can appear together.

The new work shows that they don’t have to coincide. Many auroras have been observed in the absence of auroral sounds; now auroral sound was observed in the absence of auroral light.

“That was the biggest surprise!” Wool says.

“The sounds are much more common than previously thought, but when people hear them with no visible aurora, they think it’s just ice cracking or maybe a dog or some other animal.”

We may, however, continue to use the term “auroral sound,” due to the perceived historical connection between the two, Laine says.

The research was presented at the joint UROREGIO/BNAM2022 acoustics conference and is available via Research portal.

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