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 Lightning discharges inject electromagnetic energy into Earth's magnetic field. This energy travels along a magnetic field line to distant locations on Earth where it can be picked up on special radio receivers. As the high and low-frequency waves from the lightning travel through the clouds of trapped particles, the high frequency waves travel faster than the low-frequency waves, so that they arrive back on Earth sooner. This causes the lightning burst to be turned into a whistle with a falling pitch!
During World War I, radio operators used low-frequency receivers on the battlefield, and often heard what sounded like bombs falling out of the sky!
Because these radio waves can travel along Earth's magnetic field like water down a soda straw, sometimes these magnetic field lines can be so long that a several second delay can be detected between the lightning stroke and the whistler. Some people used to worry that this violated Einstein's theory of relativity, because it seemed to suggest that light could travel much slower than 300,000 km/sec in the vacuum of space. Even more bizarre explanations, including UFOs and aliens, were offered by non-scientists to explain these 'anomalous propagation delays'. In fact, they weren't anomalous at all, just another sign that even the vacuum of space is not a perfect one!
Stephen McGreevy (www.spaceweathersounds.com) and other radio-amateurs, have used sensitive, very low-frequency (VLF) radio receivers to pick up and record these unusual sounds in the radio band below 50 kHz. Even NASA satellites visiting Jupiter can hear these sounds. No matter what the scientific explanation, you will agree that they are very beautiful - and still make the hairs on the back of your neck stand up. Note - do not listen to them in the dark!!
OK...here are a few examples: |
