Aurora’s ghostly sounds can be heard even when invisible

(ORDO NEWS) — If you’re up high and listening carefully at night when the northern sky dances with blazing sheets of green light – the awe-inspiring earth phenomenon aurora borealis – you might hear ghostly sounds.

Almost imperceptible, these sounds were heard only during the most violent manifestations of the aurora and were described as the sound of the surf, like a waterfall in the distance, or the beeping and crackling, like faint static.

However, new evidence suggests that these sounds occur high 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 from Aalto University in Finland was able to record these strange popping sounds in the sky on a night when none of the light curtains appeared.

He presented his results at the EUROREGIO/BNAM2022 Joint Acoustics Conference in Denmark this month.

“This defeats the argument that auroral sounds are extremely rare and that aurora borealis must be exceptionally bright and vibrant,” says Lane.

Auroral sounds have long been something of a mystery. Reports of them have been described for at least a century, but it was not until 2012 that recordings made by Lane and his colleagues finally confirmed that the sounds were real.

The researchers also determined where the sounds in the atmosphere were coming from – at an altitude of approximately 70-100 meters (230-330 feet), which is surprisingly low.

Auroras are created when solar wind particles collide with Earth‘s magnetosphere, then accelerate along magnetic field lines to high latitudes, where they enter the upper atmosphere.

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

In 2016, Lane and colleagues reported that they had figured out the cause of the sounds some people hear.

On particularly cold, clear and calm nights, a layer of warmer air forms over a shallow layer of cold air in the lower atmosphere.

Opposite electric charges can accumulate in these two layers; when geomagnetic disturbances, possibly caused by aurora, propagate downward through the atmosphere, this can cause an electrical discharge between the layers, which is what causes the noise.

New recordings have been made to further study this phenomenon. Near the village of Fiskars, the group set up recording equipment to listen to the beeping and crackling sounds coming from the atmosphere.

Then the results of observations were compared with the data of measurements of geomagnetic activity carried out by the Finnish Meteorological Institute. In total, the team assembled a library of hundreds of candidate sounds, of which the 60 strongest were associated with changes in the Earth’s magnetic field.

“Using independently measured geomagnetic data, I can predict with 90 percent accuracy when auroral sounds will appear in my records,” says Lane.

The work suggests that there is likely a causal relationship 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 auroral manifestations; however, since both are produced by geomagnetic activity, they can appear together.

The new work shows that they do not have to match. Many auroras have been observed in the absence of auroral sounds; now auroral sound is observed in the absence of auroral light.

“That was the biggest surprise!” Lane says.

“Sounds are much more common than everyone thought, but when people hear them without visible aurora, they think it’s just ice cracking or maybe a dog or some other animal.”

However, we may continue to use the term “auroral sound” because of the historical connection between the two, Lane says.

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