The ghostly sounds of Auroras can be heard even when they are invisible

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

Almost imperceptibly, the sounds had only been heard during the wildest aurora displays, described as rushing sounds, like a waterfall in the distance, or popping and crackling like faint noise.

However, new evidence suggests the sounds are happening high up in the atmosphere even when we can’t hear them — perhaps even if we can’t see the Northern Lights at all.

Acoustic engineer Unto Laine of Aalto University in Finland has managed to record these strange popping sounds in the air, on a night when no curtains of light appear.

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

“This cancels the argument that aurora is extremely rare and that the aurora borealis should be exceptionally bright and vibrant,” Laine says

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

The researchers also identified where the sounds were coming from in the atmosphere — at an elevation of about 70 to 100 meters (230 to 330 feet), which is surprisingly low.

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

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

in 2016, Laine and his colleagues revealed that they had thought what is causing the noises? some people have reported hearing.

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

Opposing electric charges can be built up in these two layers; when geomagnetic disturbances, perhaps caused by the aurora, propagate through the atmosphere, it can cause an electrical discharge between the layers, causing the noise.

The new recordings were made in an attempt to further investigate the phenomenon. Near the village of Fiskars, the team set up their recording equipment to listen for popping, crackling sounds coming from the atmosphere.

The observations were then compared with measurements of geomagnetic activity by the Finnish Meteorological Institute. In all, the team collected a library of hundreds of candidate sounds, the 60 most strongly associated with changes in Earth’s magnetic field.

“Using the geomagnetic data, measured independently, it is possible to predict when aurora will appear in my images with 90 percent accuracy,” Laine says

The work suggests there is likely a causal relationship between the aurora 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 aurora displays; however, since both are produced by geomagnetic activity, they can appear together.

The new work shows that they don’t have to coincide. Many aurorae have been observed in the absence of aurora; now aurora has been observed in the absence of aurora.

“That was the biggest surprise!” Laine says

“The sounds are much more common than anyone 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.”

However, we can continue to use the term “auroral sound,” because of the historically observed connection between the two, Laine says.

The research was presented at the UROREGIO/BNAM2022 Joint Acoustics Conference and is available at: Research Gate

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