Natural VLF Radio Emissions
Our planet generates a profusion of noise across the radio spectrum. Thunderstorms
are the main source of Earth's natural radio emissions. Violent currents of waterlogged air
inside these clouds of life-giving moisture produce static electricity, better known as lightning.
Lightning and thunderstorms are well-known yet not quite fully understood. Lightning is a
constant activity on earth; striking approximately 16 times every second.
That's about 1,382,400 bolts every day! Each and every strike is a powerful event. The sound
of thunder created with each electrical discharge is only part of the story.
Click here to hear a recording
of thunder (830Kb)
A lightning strike can
noise on many frequencies, however, most of the effective radiated energy is concentrated
in the very low frequency range. Lightning strikes are also known as "triggers"; most
VLF radio emissions are the direct results from a single strike or a grouping of
strikes over a certain period of time. Other sources of radio noise are beyond
the atmosphere and into outer space. Listed below are terms used to identify a
few of the many different types of VLF radio emissions along with audio wave file examples.
Sferics is from the shortened term atmospherics; the static interference (radio noise)
received from a lightning strike or strikes. Most medium and high band (AM broadcast
through Short-wave) radio receivers can intercept sferics, which sounds like crunches or
crackles, much like the sound of biting into a potato chip or chips. In close proximity
(0 to 50 miles) sferics have very high signal strength. Distant strikes (more then 500 miles
away) have very little influence on broadcast band radio reception, however, a receiver
tuned to VLF frequencies can pick up sferics originating from many thousands of miles away.
Recordings of Sferics:
Tweeks are sferics with a twist. Sferics are generally heard directly,
in other words, the radio signal takes the shortest route to the receiving station. Tweeks
on the other hand are strikes that are reflected. The earth's Ionosphere has the effect of
stretching or delaying part of the signal from the originating strike or trigger. The amount
of delay is just enough to notice; only milliseconds. The effect of this delay produces a
musical note-like sound called a tweek. Tweeks are generally heard in the evening hours
when conditions are right for Ionospheric propagation.
Recordings of Tweeks:
Whistlers are similar to tweeks in that they are stretched out, however the
delay is seconds. They are the remnants of a trigger (strike) after passing through earth's
magnetic field; the Magnetosphere. There are many different types of whistlers. It is said
by natural radio listeners that no two whistlers are alike. Because of the fluid-like qualities
of the Magnetosphere, none are exactly identical.
Types of Whistlers:
The strength and location of the trigger, the location of the listener, and the
characteristics of the Magnetosphere at that moment determine what kind of whistler
will be heard. According to "The Beginners Guide to Natural VLF Radio Phenomena"
(Michael Mideke) whistlers are categorized into hops. A hop refers to the electrical
signal (a trigger or strike) that has traveled through the Magnetosphere. A single trip or
hop through the magnetosphere produces a quick, high-pitched whistler. One-hop whistlers
originate from triggers located on the opposite side of the planet from the listener. Double
or multiple hop whistlers are seconds in length of time and can come from
the same or opposing side of earth. Most whistlers (99.9 percent) are similar in that they are falling
tones, however, the similarity stops there. They can be sharp, pure notes, or wispy; like
the sound of wind blowing through a forest of trees. They also can be a mixture of both
types, which makes whistlers so interestingly strange.
Recordings of Whistlers:
Chorus is an emission that sounds like wispy or whoosh-like whistlers.
It sometimes resembles upside-down whistlers that never seem to stop. At other
times, it can sound like an imaginary beachfront in the distance. Chorus can be a rare
event; it is a real treat to VLF radio enthusiasts to hear it live.
Recordings of Chorus: