Radio Waves From the Aurora
The aurora does more than provide a light show on clear nights; it's the world's most powerful radio station as well. Most of the energy is beamed outward to the stars, but some of it can be listened to with a simple wire loop, about 10 meters across and in a vertical plane, attached through an amplifier to a pair of headphones. In the early morning hours you'll hear the chorus, or dawn chorus; in the evening, auroral hiss is more likely.
Dawn chorus is in the frequency range our ears respond to, about 400 to 1000 hertz, or cycles per second. It's made up of radio waves rather than sound waves, which is why you need the wire loop and headphones to hear it. These waves come in a series of short, rising tones, like a tree full of birds on a spring morning.
Chorus is actually generated by fast-moving electrons which are trapped by the geomagnetic field over the equator, near the orbits of the geostationary satellites that transmit our telephone calls and bring us live television. These electrons are guided poleward and toward the ground by the earth's magnetic field. The motion of the electrons generates radio waves, which cause the electrons to change their spiraling motion so they are moving more nearly parallel to the magnetic field. This change in angle allows the electrons to get deeper into the atmosphere than normal, producing patches of aurora that turn on and off at about ten second intervals. Each time a patch turns on, a burst of chorus accompanies it.
Most of us are more likely to be watching the aurora in the evening, when the aurora normally occurs in discrete bands. The electron beams that produce these bands produce radio emissions as well, but at higher frequencies than chorus -- from two thousand to a hundred thousand hertz. 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. It has a frequency above the range we could hear as sound, about two hundred thousand hertz, and a wavelength of around a kilometer -- thus its name, auroral kilometric radiation. It's a good thing it doesn't reach the ground, because it's in the same frequency range as radio navigational aids such as LORAN. This emission makes Earth one of the noisiest planets in the solar system. Intensities reach levels of a billion watts. (For comparison, the most powerful radio stations in Alaska radiate at fifty thousand 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. Hiss, however, is generated by electrons moving downward almost along the magnetic field lines. Electrons moving more sideways than downward are likely to be reflected back upward, and it is these electrons spiraling back upward that produce the powerful kilometric radiation.
