The aurora is seen in Narva-Joesuu in northeastern Estonia on Oct. 7, 2015. (Xinhua/Sergei Stepanov)
HELSINKI, Oct. 4 (Xinhua) -- The sound associated with Northern Lights, or Aurora borealis, is not human imagination, but caused by the electromagnetic phenomenon, albeit at a very low level.
These are conclusions by Finnish acoustics professor Unto Laine. His team has established that the sound originates at the temperature inversion level, only some 70 metres from ground, local media reported on Monday.
Positive electric charges from the ionosphere collide with charges rising from the ground surface in the temperature inversion level. The reaction creates strong sound that may reach the ground, such as banging and rattling.
"The sound is most likely if the weather is totally calm during a strong Northern Lights storm. These optimal conditions are rare, but when they prevail, the creation of sound is almost 100 percent certain," Laine told national broadcaster Yle.
Laine compared the situation to a cinema: The screen is well over 100 kilometers from the ground, but the loudspeakers are very low.
Laine launched his project 17 years ago when he was professor of acoustics at the then Helsinki University of Technology, now part of the merged Aalto University.
He was bothered with the conflict between human observations and prevailing scientific facts. For years his research was met with disbelief.
The results were published at a Northern Lights seminar in Kumo, Finland, recently and earlier this summer at an acoustics research conference in Stockholm, Sweden.
Scientists of early 1900s had confirmed that the actual electromagnetic phenomenon was never lower than 80 kms from earth. As a human ear could not possibly detect anything from that distance, the sound associated with Northern lights were deemed to be caused by the masts of the measurement equipment or were created by the magnetic storm on the ground.
Final dismissal to the existence of such sound was given by the U.S. space research agency NASA in 1960s. For decades thereafter the sound was seen mainly as a product of the human imagination.
At the beginning, Laine had been prepared for the alternative that the sound could be associated with the physiological function of the human ear during a magnetic storm. "I was even told that the human brain could produce sound during a magnetic storm, if the person has sensitive brains," Laine told Yle.
The project started in cooperation with the geophysical observatory in Sodankyla, Finnish Lapland, in 1999 and was initially financed by professor Laine privately as financiers saw no reason to believe any sound could exist.
Northern Lights are a touristic attraction in the arctic region.
Even though the actual lights show brighter in the north, Laine recorded the loudest sound in southern Finland. He gave as the reason that in the north the sound is weaker, but when the storm is energetic enough it reaches southern Finland and the sound is louder there.
