Feast your ears on these curious musical tones:
Those little glissandos you’re hearing are known as whistlers. They’re natural radio signals created in Earth’s atmosphere which occur just at the right frequencies to be converted directly into sounds we can hear. Both the sounds themselves and the process of how they’re made are really quite beautiful.
For a start, whistlers don’t start out as whistlers. They start out much sharper and cracklier.
This staccato selection of snaps, crackles and pops are known as sferics. Shorthand for atmospherics, these sounds come from radio waves, most often generated by lightning. Because of the way these waves propagate through the lower levels of Earth’s atmosphere, you can hear them hundreds of kilometres away from the storm where they originated, still just as fresh and crackly as they were when the lightning bolts made them.
Interesting things start to happen when sferics travel a long distance before they’re heard. These sounds are known as tweeks.
Sferics travel in all directions, so logically some of them travel upwards. When they do, they rapidly come into contact with Earth’s ionosphere, high in the atmosphere. This guides radio waves very similarly to the way optic fibres guide light, curving their path before sending them back downwards towards Earth. The ionosphere is what’s termed a ‘dispersive medium’ – different radio frequencies travel through it at different speeds. As a result the sferics start to have a boingy chirpy sound to them as their wavelengths become spread out, becoming what are known as tweeks.
But for a sferic to transmute into a whistler, it has to travel quite a long way!
Sometimes the radio waves that make up a sferic can escape from the ionosphere… But not quite escape from Earth. Earth’s magnetic field lines radiate outwards from our planet for thousands of kilometres. Satellites and space stations in orbit are still well within Earth’s magnetic field – and those magnetic field lines carry a collection of stray electrons and ions, spiralling around them on a little adventure out into space and back again★. This tenuous collection of particles can also guide electromagnetic waves, so those waves which originated near Earth’s surface during lightning strikes travel in a huge loop through space before ending up back at Earth’s surface on the opposite side of the world to where they started. The result of them travelling so far through a medium like this means they become highly dispersed, giving the characteristic falling tones of whistlers. All of this, from lightning strikes somewhere in Earth’s atmosphere…
The same kind of process will happen anywhere that radio frequencies travel through any dispersive media, and interstellar space with a plethora of stray electrons ambling about makes a very good dispersive medium, as Carl Sagan explains in this excerpt from his novel, Contact:
❝ [Ellie] heard a glissando down the radio frequencies, a “whistler” due to the scattering of radio waves by electrons in the tenuous interstellar gas between the radio source and the Earth. The more pronounced the glissando, the more electrons were in the way, and the further the source was from the Earth. She had done this so often that she was able, just from hearing a radio whistler for the first time, to make an accurate judgment of its distance. This one, she estimated, was about a thousand light-years away–far beyond the local neighborhood of stars, but still well within the great Milky Way Galaxy. ❞
– Carl Sagan, Contact
★ An electron’s tale, by Bilbo Baggins…?