Sounds of the aurora can be heard even if they are invisible

(ORDO NEWS) — Sounds can be heard as the northern sky dances with bright streaks of green light—Earth‘s spectacular aurora borealis.

Almost imperceptible sounds were heard only during the strongest manifestations of the aurora, described as rushing sounds, like a waterfall in the distance, or crackling, like faint static.

However, new evidence suggests that 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 managed to record these strange popping sounds in the sky at night when there were no light curtains.

He presented his findings at the EUROREGIO/BNAM2022 joint acoustic conference in Denmark this month.

The sounds of the northern lights have long been a mystery. Reports have been described for at least more than a century, but it wasn’t until 2012 that recordings made by Line and colleagues conclusively confirmed that the sounds were real.

The researchers also determined where the sounds in the atmosphere come from – at an altitude of 70 to 100 meters, which is surprisingly low.

Auroras occur when solar wind particles collide with Earth’s magnetosphere and then accelerate along magnetic field lines to high latitudes, where they rain down into the upper atmosphere.

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

In 2016, Lane and his colleagues said they had figured out what caused the sounds some people heard.

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

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

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

The observations were then compared with measurements of geomagnetic activity made by the Finnish Meteorological Institute. Overall, 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.

The work suggests that there is likely a causal relationship between 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 auroras; however, since both are caused 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 aurora sounds; now auroral sound was observed in the absence of auroral light.

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

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