Bill Tayler «Inspire» 1989 - 2005.
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.
Sferics, short for 'atmospherics,' are the impulsive signals emitted by lightning. The frequency range is from a few hertz to millions of hertz. The part of this range that we can hear (the audio range) has frequencies up to about 15 thousand hertz (15 kHz). The spectrograms of sferics are characterized by vertical lines on the frequency-time graph indicating the simultaneous arrival of all of the audio frequencies. The sound of sferics consists of sharp crackling noises like twigs snapping or sizzling noises like bacon frying. Sferics are caused by lightning strokes within a couple of thousand kilometers of the receiver. The VLF radio signal is ducted to the receiver between the surface of the earth and layers in the ionosphere which reflect radio waves. This path is called the earth-ionosphere waveguide.
Inspire - Earth electromagnetic field sound
Under the right conditions, the VLF signal travels out away from earth and returns by traveling along a magnetic field line. During this long path, dispersion is much greater than with tweeks. While tweeks might disperse a few hundred kilohertz over a few thousandths of a second, whistler show a dispersion of a second or more over several thousand kilohertz. The sound of a whistler is a musical descending tone that lasts for a second or more. On the spectrogram, whistlers appear as long sweeping arcs showing the sequential arrival of the frequencies. It is important to remember that all of the frequencies start out at the same time (a sferic), but the path taken by a whistler is so long that the dispersion of the frequencies is quite pronounced.
A pure note whistler has traveled along a signal magnetic field line. It is heard as a clear whistling sound and appears on the spectrogram as a strong single curve. The following shows two whistlers: one at :09 seconds and a stronger one at :13 seconds. The horizontal dashes are OMEGA signals. Several other faint whislters are audible in the sound sample.
Two-hop whistlers originate near the receiver site. The signal that travels along the magnetic field line bounces off the ionosphere in the other magnetic hemisphere and returns to be heard as a whistler near where the original lightning stroke occurred. Two-hop whistlers can be identified by the presence of a strong "local" sferic between one and two seconds before the whistler is heard. Remember, local lightning is within about 2000 kilometers of the observing site.
Occasionally, especially in the quiet times of the morning, chorus can be heard. Chorus sounds like many birds calling in turn. Chorus seems to be the result of many brief, short-path whistler-like emissions occurring at almost the same time.
Seconds after Tue Apr 25 20:29:00 2006 UTC togaspot.elte.hu
25 Apr 2006 18:19 GMT weather.com
Seconds after Tue Apr 25 20:37:00 2006 UTC togasuva.usp.ac.fj
25 Apr 2006 20:09 GMT weather.com