Hark---the Northern Lights
This summer's bluegrass music festival in Talkeetna, I'm told, was interrupted by a spectacular early-season auroral display. The festival management respectfully turned off the stage lights so all could watch the shimmering sky undistracted.
Perhaps they should have turned off the music, too. With all those attentive listeners and bits of sensitive equipment on hand, folks at the Talkeetna festival might have bagged the first authenticated record of auroral sound.
Many records of auroral sound exist, but none are the sort one can play back. A scan of the appropriate chapter in The Aurora Watcher's Handbook reveals that author Neil Davis has tallied about 300 documented anecdotal reports of people hearing the aurora. (In scientific circles, "anecdotal" covers everything from hearsay to personal eyewitness---or earwitness---accounts by trained observers. Anecdotal evidence counts, but not for much.)
The anecdotes sketch a peculiar picture. In a group of people watching an aurora, only one or two may hear it. Someone may hear only one display out of hundreds seen during a lifetime of watching. People hearing sounds associated with auroras usually describe what they hear in similar terms: a hissing, rustling, or crackling noise, or a combination of those sounds. The sounds most often accompany especially active, vivid auroras, and seem to match the visible activity.
More than 300 fairly similar recorded observations would do for virtual proof of many phenomena, but not for auroral sound. The real problem is that there is no agreed-upon physical mechanism by which auroras could cause sound. The processes generating auroras can't produce sound waves audible at the earth's surface. Even if noises could be generated at the altitude where auroras occur, it would take five minutes for the sound waves to reach the ground. No such delay between visible aurora and audible sound has been reported.
Because of this negative evidence, some scientists suspect that auroral sound is psychological rather than physical. According to this argument, human brains insist that fast-moving lights must be accompanied by sound. In effect, the brain tricks itself into accepting either ambient noise, such as icy twigs crackling in a breeze, or imagined sound as genuine background music for the dancing lights. A nice theory, although unflattering to people who've heard the sounds, but it doesn't explain the many cases where people blindfolded or indoors have heard auroras whoosh and fizz.
Harder to refute are various suggestions linking the unknown of auroral sound with the well-known auroral electric fields. A recent theory came from Australian physicist Colin S. L. Keay, who extrapolated to auroras from his earlier work on inexplicable sounds generated by meteors and by lightning.
Keay was intrigued by the loud click that people sometimes hear at the same time they see a nearby lightning strike, well before the thunder arrives. Some researchers suspect this click is produced by "electrophonic hearing"---the listener's auditory nerves have been stimulated directly by an electric field, and the brain registers the stimulus as a sound. This explanation has been advanced to explain auroral sound also.
Keay disagrees, and thinks radio waves cause both lightning clicks and auroral hisses as well as odd noises sometimes produced by meteorites entering Earth's atmosphere, In his view, very low frequency radio waves generated by lightning (or an aurora or a meteorite) are changed into sound waves by objects surrounding the observer. In laboratory experiments, he has used strong electric fields to produce audible sound from things as diverse as aluminum foil and tree twigs. Just possibly, someone hearing sounds while the aurora plays above has been listening to a radio broadcast playing through a living spruce-twig speaker.
No one knows if Keay's theory is valid; many theories exist, but not one is proven---yet. So far, I share the conclusion of the handbook chapter on auroral sound: "all we can do is watch, listen and wonder. "
