If you are up high and listen carefully on nights when the northern sky dances with blazing sheets of green light, you might hear ghosts.
The sounds had only been heard during the craziest displays, which were described as rushing sounds, like a waterfall at a distance, or popping and crackling like faint static.
New evidence suggests that the sounds can be heard high up in the atmosphere even when we can't see the northern lights.
The sound of popping sounds in the sky can be heard on a night when there is no light in the sky.
He presented his findings at a conference.
The argument that the sounds of the Aurora are rare and that the Aurora should be bright and lively is no longer valid.
Auroral sounds have been a mystery for a long time. It wasn't until 2012 that recordings made by Laine and his colleagues confirmed that the sounds were real.
The researchers found out where the sounds were coming from at an altitude of about 70 to 100 meters, which is surprisingly low.
When particles from the solar wind collide with the Earth's magnetosphere, they travel along the lines of the magnetic field to high latitudes, where they rain down into the upper atmosphere.
There, they interact with atmospheric particles to produce the light that dances across the sky.
The sounds some people have reported hearing were found to be caused by something.
A layer of warmer air forms above a shallow layer of cold air at the bottom of the atmosphere on calm nights.
The noise can be caused by an electrical discharge between the layers when the atmosphere is disturbed by the Aurora.
The new recordings were made to investigate the phenomenon further. Near the village of Fiskars, the team set up their recording equipment to listen for popping, crackling sounds.
The observations were compared against the data from the Finnish Meteorological Institute. The team collected a library of hundreds of candidate sounds, of which the 60 strongest were linked with changes in Earth's magnetic field.
It is possible to predict when the Auroral sounds will happen with 90 percent accuracy using the geomagnetic data.
The work suggests that there is a link between the sounds of the sun and the activity of the earth.
The processes that produce the sounds are different from the processes that produce the displays.
The new work shows that they don't have to coincide. In the absence of the light of the Aurora, an Aurora has been observed.
That was the biggest surprise, says Laine.
The sounds are more common than people think, but when people hear them, they think it is ice cracking or a dog.
We may continue to use the term "auroral sound" because of the historical link between the two.
The research is available via Researchgate.
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