The popping and crackling sounds associated with Aurora borealis (or the Northern Lights) are born when the related geomagnetic storm activates the charges that have accumulated in the atmosphere’s inversion layer causing them to discharge, according to researchers at Aalto University, Finland.
Aalto University scientists find explanation for auroral sounds. Image credit: Unto Laine / Aalto University.
In 2012, Prof. Unto K. Laine of Aalto University and co-authors proved that the source of auroral sounds is located close to the ground at an altitude of approximately 230 feet (70 m).
Now, by combining their measurements with the temperature profiles measured by the Finnish Meteorological Institute, the researchers have found an explanation for the mechanism that creates the sounds.
“Temperatures generally drops the higher the altitude. However, when temperatures are well below 32 degrees Fahrenheit (0 degrees Celsius) and, generally in clear and calm weather conditions during the evening and night, the cold is near the surface and the air is warmer higher up,” Prof. Laine said.
“This warm air does not mix, instead rising up towards a colder layer carrying negative charges from the ground.”
“The inversion layer forms a kind of lid hindering the vertical movements of the charges. The colder air above it is charged positively.”
“Finally, a geomagnetic storm causes the accumulated charges to discharge with sparks that create measurable magnetic pulses and sounds.”
The team carried out the recordings in March 2013, when Southern Finland was treated to exceptionally splendid Northern Lights.
“I recorded hundreds of sound events in Fiskars where the temperature was at minus 4 degrees Fahrenheit (minus 20 degrees Celsius),” Prof. Laine explained.
“I selected the 60 loudest events, the sources of which were directly above the recording microphone array. The magnetic pulses that preceded these sounds proved that their sources were at an altitude of approximately 246 feet (75 m).”
On the same night, the Finnish Meteorological Institute carried out its own measurements, which proved that the inversion layer was located at the same altitude where these noises were born.
The correlation between the strength of the magnetic pulses and the loudness of the sounds was also strong.
The inversion layer hypothesis, according to the scientists, also gives a credible explanation for why auroral sounds have only been heard in calm weather conditions.
“Even a small wind can prevent the birth of an inversion layer, which means no sounds will be created,” Prof. Laine said.
He added: “hypothesis does not rule out other mechanisms. However, this provides the first explanation for all three mysteries related to auroral sounds.”
“In addition to the mechanism behind the sound, this helps us understand how we can hear the sound when the auroral light source is at a distance of 50-62 miles (80-100 km).
“The inversion layer hypothesis also provides answers to how it is possible that sound events occur almost simultaneously with visual observations: from an altitude of 246 feet the sound reaches the human ear in just 0.2 seconds.”
The team’s results were presented at the 2016 Baltic-Nordic Acoustic Meeting in Stockholm, Sweden, on June 22.
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Unto K. Laine et al. Auroral Acoustics project – a progress report with a new hypothesis. BNAM 2016
