This range, when turned into sound by the headphones, sounds like hissing. The radio signal is produced by interaction of the auroral electron beam with electrons in the upper ionosphere well above the aurora, at a height of around 3000 kilometers. The radio waves are emitted to the sides of the beam, rather than along it, so they are more likely to reach the earth when the auroral bands are on the horizon than when they are overhead. The most powerful of the auroral radio emissions never reaches the ground. This emission makes Earth one of the noisiest planets in the solar system.
Intensities reach levels of a billion watts.
Satellites first discovered auroral kilometric radiation in 1974, but it wasn't understood until 1979. Then C. S. Wu, at the University of Maryland, and L. C. Lee, at the Geophysical Institute, University of Alaska, took account of the fact that some of the electrons were moving at a fair fraction of the speed of light. This meant that Einstein's theory of relativity had to be included in their equations. When they did this, the equations accounted for the kilometric radiation. Like hiss, auroral kilometric radiation is generated by fast-moving auroral electrons at about three thousand kilometers above the earth's surface.