The source of most natural radio signals is lightning. When a lightning bolt strikes, a massive amount of charge is moved and this acts in much the same way as moving charge in a radio transmitting antenna. The frequencies emitted by lightning range from 0 Hz to over 100 kHz with all frequencies being emitted simultaneously with the visible flash of lightning.

The lightning signal, when received and amplified, sounds like a dry crackling sound like the popping of a campfire. These sharp, popping emissions are called "sferics", from atmospherics. Figure 1 shows a frequency-time spectrogram of several sferics. The vertical lines in the spectrogram illustrate that a sferic consists of a wide range of frequencies received simultaneously resulting in the dry, popping sound referred to as static.

Figure 1: SFERICS




Sferics

Vertical lines indicate the presence of all frequencies. The design frequency range of the receiver is 0-10 kHz. Horizontal lines appearing between 10 kHz and 15 kHz are signals from the Russian ALPHA navigation system. Even though they are higher frequency than the design of the receiver they are still sometimes detected because of their high signal strength. In this 10-second interval there are over 20 strong sferics and numerous weaker ones. This would be considered quiet conditions.

Sferics from lightning as much as 2000 to 3000 kilometers away can be detected by the VLF receiver even though an event that far away is occurring well over the visual horizon. This is because VLF radio waves commonly reflect off a layer of charged particles (electrons) in the ionosphere redirecting the signal back toward the surface of the earth. The signal can then bounce off the surface of the earth, and again off the ionosphere, with this process repeating as the signal is carried around the earth. This process can sometimes carry the VLF wave more than half way around the earth.

When the sferic signal travels a long distance it undergoes dispersion. That is, the higher frequencies travel slightly faster than the lower frequencies and therefore arrive at the receiver slightly ahead of the lower frequencies. The lower frequencies lag only a few hundredths of a second, but that makes a big difference in how the signal sounds and also leaves a characteristic imprint on the spectrogram. The sound heard is modified by dispersion from the dry popping sferic so that it has a pronounced ringing nature. This is called a "tweek". Figure 2 shows the spectrogram of several tweeks.

Figure 2: TWEEKS

A whistler appears at 5 seconds and a stronger one appears at 13 seconds. The sweeping curve is the signature of a whistler. The dispersion of the signal is almost 3 seconds. The horizontal dashes just above 10 kHz are from the now-inactive OMEGA navigation system.

The dispersion of frequencies in whistlers (more than 1 second and as much as 3-4 seconds) is much greater than that of tweeks, indicating that the signal has traveled much farther than just half way around the earth. In order to account for the huge distance necessary to produce the large dispersion, the signal must travel out away from the earth and loop back following a magnetic field line. This is the link between natural radio and space physics.

Studying whistlers may lead to an improved understanding of the nature and properties of the magnetosphere.

~ Bill Pine, Co-Founder Emeritus, The INSPIRE Project, Inc.