A study traces centuries of observation and confirms: the Northern Lights really do emit a particular sound that is audible from Earth
The Northern Lights are a phenomenon that has fascinated scientists around the world for centuries: they were recorded by the first technical instruments as early as 1859 on the sidelines of the Carrington Event, "the Great Aurora" that was even visible from Rome, the result of the largest solar storm ever recorded on Earth.
Between the different studies that try to unveil the deepest nature of the fascinating phenomenon, one of the most complex is the one concerning the sound emitted by the aurora borealis.
An ancient study but just at the beginning
That the aurora borealis emit a particular sound is a widespread opinion since the beginning of the century. On May 20, 1933, the Shetland News published a letter entitled "Listening to the Aurora" in which a Whalsay resident, Peter Hutchison, recounted the peculiar experience.
"I remember one clear, frosty night about thirty years ago, the 'beautiful dancers' (that's what we called the auroras) came with their yellowish flashes across the northeastern sky," Hutchison recalled, "making a sound like rubbed wooden boards - a dull noise we could all hear."
Since then, numerous citizens have reported hearing the sounds of the northern lights, but instances of scientists hearing them directly are still quite rare, which is why most accounts cannot yet be accepted by the scientific community.
A very recent study published in the Journal of Royal Society signed by Dr. Fiona Amery, of the Department of Philosophy of Science at the University of Cambridge, finally puts in order the many and confusing news available.
The historical reconstruction of the listening of the aurora borealis aims first of all to discriminate the observations classifiable as objective from those illusory or imaginary.
Only in 2016 it was in fact confirmed, data in hand, that the aurora borealis are actually able to produce sounds. In 2017 came the mathematical model able to explain the sound emission and only a couple of years later, by the same scientist - Unto K. Laine of Finland's Aalto University - the scientific explanation of the phenomenon.
Boreal crackling and the Earth's natural resonance
Just to start clarifying, so begins Fiona Amery's study, "the credibility of reports is intimately linked to measurements of the altitude of auroras."
One of the certainties of scientists about the sound of the northern lights, in fact, is that related to the audibility of the phenomenon: a sound emitted from more than 80 meters away can not be perceived by the human ear.
So can be discarded all the "sound" testimonies about aurora borealis formed at higher distances: most of the times, the phenomenon occurs at a distance of about 100 meters from the Earth, so the field is narrow.
And yet some scientists have dedicated their lives to the subject, as is the case of Carl Størmer, one of the most influential scholars of aurora borealis in the world, who recently joined his testimony "sound" to that of ordinary citizens, legitimizing not a little the issue.
Another scientist who has dedicated an important part of his life to the study of the phenomenon is the already mentioned professor emeritus Unto K. Laine, from whom also comes the first scientific hypothesis on the origin of the sound of auroras.
When an aurora borealis occurs, according to the 2019 study, "the spectrum of the temporal envelope of the crackling contains frequencies peculiar to Schumann resonances," a group of particularly low frequencies peculiar to the electromagnetic field of the Earth.
The special resonance cavity formed by the area between the Earth's surface and the ionosphere can be naturally excited by phenomena such as electrical discharges from lightning, and also by northern lights.
The sound of the aurora borealis has been shown to be generated by such particular resonances, specifically between 70 and 80 meters away, where the accumulated electrical charges originate the auroral corona and the crackling sounds that the people of the north have been talking about, almost unheard, for centuries.
To listen to the aurora borealis, however, will not be easy: the sound is emitted only in 5% of auroras, and only at particular latitudes and observation distances.