A Cosmic Menagerie
Ken Tapping, 6th November, 2013
We decided to set up our experiment a few hundred metres along a logging road, near the Barron River, in the Algonquin Provincial Park, Ontario. We laid a big loop of cable among the trees and connected it to the equipment in the car. This consisted of a very sensitive audio amplifier, similar to the ones we use in home entertainment systems, and a tape recorder. Everything was battery powered, and we had carefully chosen a location a long way from power lines. This was necessary to avoid what we wanted to record being lost in the deafening hum power lines produce. Once things were set up, we started to listen, waiting to hear something worth recording.
When World War I bogged down into bloody trench warfare, the combatants set up complex networks of phone communication to coordinate their planning and activities. Then someone discovered that if you laid a wire on the ground more or less parallel with the other guy's phone line and connected that wire to an amplifier, you could listen in on the other guy's communications.
In addition to the noises associated with phone conversations, listeners on occasion heard strange whistles, starting high and descending in tone. Since this is what artillery shells often sounded like, they thought at first that is what they were hearing. However, eventually it was realized they were something else entirely. These noises were coming from the Earth's magnetic field.
Those falling whistles, now called "whistlers" are produced when the magnetic field is "twanged" by lightning, usually in the other hemisphere of the Earth. The twang turns into a whistle because the high frequencies move along the magnetic field faster than the lower frequencies. So, after the waves had followed the magnetic field thousands of kilometres up into space and back down to Earth in the other hemisphere, the twang became an eerie, falling tone. Moreover, as more observations were made, other interesting waves were discovered. The solar wind rubbing over the surface of the Earth's magnetic field produces wavering tones like the song of a hump-back whale, or what we get when rubbing the rim of a wine glass. Then there are all sorts of hisses and clicks, and something called "chorus", which sounds like a tree full of alien birds at dawn. Space scientists have been studying these waves for years, using them to find out about conditions in the space around the Earth that is hard to access.
These waves seem to turn up in any magnetic fields that have particles trapped in them and get disturbed in some way. Spacecraft passing close to Jupiter and Saturn have detected whistlers and other waves in their magnetic fields. They are present in the solar magnetic fields and probably in the magnetic field of the Milky Way. There must be especially interesting waves in the magnetic fields of black holes or neutron stars.
That brings us back to our experiment. To maximize its sensitivity we had to monitor as big a bundle of magnetic field as possible. So we picked a location where the magnetic field is more concentrated and diving down into the ground at a steep angle. Then we made our coil as large as possible, about 15 metres in diameter. Any waves running down the magnetic field lines passing through the coil produced electrical currents which we amplified, listened to and recorded.
I'll never forget that night. The trees were black against an unbelievably starry sky. Outside the car all was quiet, while inside we heard the most amazing sounds from the Earth's magnetic field. It sounded like feeding time at some invisible, alien cosmic zoo. Trying to understand those fascinating sounds is even more magical.
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