What we still don't know....

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Have I mentioned a central, utterly basic, piece of scientific attitude? It's this: There's a lot we still don't understand......

Take this: The Aurora Borealis produces sounds.... maybe. Well...

.....Aurora are charged particles from the Sun crashing into the upper atmosphere of Earth [1]. So it makes sense that they'd produce radio noise. Like this:
 


Video above: The radio song of the aurora - is this related to the sounds recorded by the Aalto University team? Video Courtesy of Minnesota Planetarium

That's not a problem. The problem is that, for a long time, there have been reports that Aurora produce... sound wave sounds. Which is hard to explain: Aurora are between sixty and eighty kilometres up in the sky, where the atmosphere is incredibly thin - and hence sound doesn't travel well. So it's hard to see how they could be heard on the ground.

But the phenomena has a long, long, history of reports, from some quite credible witnesses. For example, Dr. H. D. Curtis, who was in charge of the Labrador Station of the US Lick Observatory, in 1906:

The station was located at Cartwright...and auroral displays were frequent and bright during July and August. On several nights I heard faint swishing, crackling sounds, which I could only attribute to the Aurora. There were times when large, faintly luminous patches or "Curtains" passed rapidly over our camp; these seemed to be close, and not more than a few hundred feet above the ground, though doubtless much higher. The faint hissing and crackling sounds were more in evidence as such luminous patches swept past us...I tried in vain to assign the sounds heard to some reasonable source other than the aurora, but was forced to exclude them as possible sources; besides, what I heard didn't sound like anything from anything I could postulate....,In short, I feel certain that the sounds I heard were caused by the aurora and nothing else. There was, moreover, a certain synchronism between the maxima of these sounds and the sweeping of auroral curtains across the sky. 

[Reproduced from materials provided by the Australian Government Bureau of Meteorology.]

Various explanations - aside from the old fall back of "you're imagining it" - have been put forwards. These range from the Auroral electrical fields being picked up and transduced [2] (by the same principles as a microphone) into sound by everyday objects, to the 'sounds' being a mild form of auditory hallucination [3] bought on by the human brain having a short between its visual and sound centres.

Professor Unto. K. Laine hasn't found an explanation for all this. But he has come a step closer to proving the Auroral sounds exist outside of peoples heads: Over a hundred nights between 2000 and 2012 he and his team from Aalto University [4] have been recording the ambient noise during auroral displays - part of an ongoing attempt [5] to verify or disprove the reports of Auroral sounds.

They've found quietly audible 'clap' sounds, apparently coming from points not too far above the ground - points that are seemingly just open sky! This was worked out by recording the sounds with three different microphones, and measuring when the sound reached each one. The sounds only seem to be heard during an aurora, but not during every aurora. The researchers have also been documenting reports of Auroral  sounds from across Finland, and have built up records of over three hundred cases.....



Video above: Two of the unknown sounds, seemingly related to the aurora. I love it when there's a mystery afoot - as long as the mystery isn't 'where has all my money gone' or 'why am handcuffed naked to a lamppost?' Video courtesy of the Aalto University.

Are they really related to the Aurora? What are they?

We don't know. The researchers are presenting a preliminary report at this years International Congress on Sound and Vibration [6]. They will - as long as they can find funding -  keep investigating though, and we will find out....

List of links:
[1]http://en.wikipedia.org/wiki/Geomagnetic_storm
[2]http://home.pacific.net.au/%7Eddcsk1/solutio2.htm
[3]http://www2.gi.alaska.edu/ScienceForum/ASF6/698.html
[4]http://www.aalto.fi/en/
[5]http://www.acoustics.hut.fi/projects/aurora/
[6]http://www.icsv19.org/index.php