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Natural radio is a term given to very low frequency (VLF), ultra low frequency (ULF), and extremely low frequency (ELF) radio waves emitted by charged particles in Earth's atmosphere and magnetosphere. These emissions can have frequencies in the range of tens to thousands of hertz, as opposed to the megahertz and gigahertz frequencies we normally associate with man-made broadcasts. Because of its low frequencies, natural radio can be translated directly into sound, revealing its complex and beautiful nature. Natural radio signals are being generated all the time, but go unnoticed because they are difficult to detect. They are frequently overwhelmed by man-made VLF noise sources, primarily the noise from alternating current power lines.

Accelerating charges generate electromagnetic radiation. Charges may accelerate rapidly, as in an electrical spark, generating a broadband wash of noise. Or they may spiral around a magnetic field (due to the Lorentz force) and emit radiation at discrete frequencies. The charged particles responsible for natural radio have two sources: atmospheric electrical discharges (lightning), and the solar wind. They can generate a wide variety of natural radio signals, including whistlers, sferics, and the dawn chorus.

Whistlers and sferics are caused by terrestrial lightning. Whistlers consist of a short burst of sound during which the pitch drops rapidly. These are probably caused by electrons from a strong lightning burst moving into space, getting tangled in the Earth's magnetic field, and emitting radiation as they spiral around the field lines. Sferics (atmospherics) are broad-band bursts of static emitted by lightning strikes within a few thousand miles of the listener. They can be detected at large distances because, like ham, shortwave and AM radio, the frequencies are low enough to be reflected by the Earth's ionosphere.

The dawn chorus or auroral chorus is generated by the solar wind. Charged particles in the solar wind can interact with both the atmosphere and the magnetosphere of the Earth. In the case of the atmosphere, these particles can generate an aurora by striking atoms in the upper atmosphere, causing the atoms to fluoresce. They can also generate VLF radiation when they encounter the Earth's magnetic field. These sounds are the most interesting of all, since they tend to last the longest, and exhibit a wide range of behavior. The chorus could be composed of a series of wavering tones, groups of whistlers, or could even sound like the croaking of frogs, depending upon the strength and morphology of the solar wind. They are called the dawn chorus because they are most prominent when the Sun is first coming into view relative to the observer, facing the oncoming stream of the solar wind.

Detection of natural radio signals requires special electronic equipment capable of detecting VLF signals. They are rarely sold commercially, but the circuit diagram for a receiver can be found by digging through the first website below. It also requires that you are far away from all sources of manmade interference, particularly power lines.

Sources: Stephen McGreevy's website at http://www.auroralchorus.com and the NASA INSPIRE program at http://image.gsfc.nasa.gov/poetry/inspire/.

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