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 propagate 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

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:

• Sample 1 (650Kb)
• Sample 2 (650Kb)
• Sample 3 (1.3Mb) Stereo
• Sample 4 (650Kb) Recording of sferics from an AM broadcast receiver tuned to 530 kHz
  

Tweeks

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:

• Sample 1 (650Kb)
• Sample 2 (650Kb)
• Sample 3 (650Kb)
• Sample 4 (1.3Mb) Stereo
• Sample 5 (650Kb) Narrowband filter

Whistlers

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:

• Sample 1 (650Kb)
• Sample 2 (650Kb)
• Sample 3 (650Kb)
• Sample 4 (1.3Mb) stereo
• Sample 5 (650Kb)

Chorus

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:

• Sample 1 (650Kb)
• Sample 2 (650Kb)
• Sample 3 (650Kb)

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