At the boundary between the teeming atmosphere of Earth and the emptiness of space, random noise can suddenly, briefly become pure musical tones. Or, the reverse -- the strains of radio music rising up from the planet can be crumbled to bits of random noise.
It is the action of the magnetosphere, studied for decades by Robert Helliwell and his colleagues at Stanford University. Their latest results explain how the magnetosphere can translate sounds, from random to sensible or the reverse.
"It means the medium is nonlinear," he said. "If you excite it in one way, you get something different in return."
The magnetosphere extends from 5,000 miles up to 40,000 miles up, and is shaped by lines of magnetic force that bend outward from the poles of the planet. The medium of magnetosphere is plasma -- highly charged subatomic particles called ions -- that cluster into broad bands along the lines of the Earth's magnetic field.
When radio waves hit the magnetosphere, the energy of the radio wave jostles the ions and can get them to throw off electrons and bursts of energy. These bursts of energy can amplify the radio signal a thousandfold and can break up coherent radio waves into many waves of higher and lower frequencies, then break the waves again until the coherent signal becomes a smear of noise.
In the magnetosphere, Stanford reports, the crack of lightning spreads out into a drawn-out whistle. The whistle continues, dropping in pitch, for several seconds.
Also heard in the magnetosphere is the "chorus." The sounds were named by researchers who were reminded of birds chirping at dawn in the English countryside. With the "chorus," a resonance can be set up between the energetic ions and radio "wavelets" of certain frequencies. Thus the wavelets can gain energy from the particles and rise above the general hiss to make a magnetospheric music.
Says Helliwell, "You can send up a broken, incoherent [set of] radio waves that sounds like noise, and when it goes through the magnetosphere it sounds musical."