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/.