Adequate signal to noise ratio can be obtained with a simple e-field probe (short whip) or a magnetic loop antenna. Even though the wave length is measured in miles, you don't need a big antenna. Instead of connecting a microphone you simply connect an antenna. Essentially, all you need is a high gain audio amplifier.

The equipment required for reception of these very emissions can be very simple. There have also been manmade emissions from navigational beacon transmissions, land based scientific studies, and from the space shuttle during project Inspire. These are auroras which produce a swishing sound, geomagnetic storms in which the Earth's magnetic field quivers like a bowl of Jell-O, and several other sources that are all related to effects from the sun. There are other sources that generate their own types of emissions. The vertical response just before the 1 second marker is probably the lightning strike that created the whistler. This is known as a whistle train.Д«elow is a spectrogram of a very strong whistler. If the path is efficient enough, this energy can bounce back and forth between the two hemispheres several times each time creating multiple whistlers, each with a longer duration. This creates a very nice descending whistle. Again, due to the varying velocity factor with wave length in a plasma, what starts off as an emission at all frequencies at the same time gets filtered in time by frequency at the other end of the path. When lightning strikes in one hemisphere, energy somehow (probably via Sprites and Blue Jets) gets coupled into these plasma belts and is propagated to the opposite hemisphere. There are also plasma trapped in the magnetic field lines that extend from one hemisphere to the other and can extend out several Earth radii. This can make for some interesting sound effects that can very greatly with the state of the ionosphere. This is because at the low frequency end of this static burst, the emission is slurred in time. What happens is that what started out as a burst of static, energy at all frequencies, now has a noticeable chirp. Right near the cutoff frequency of the Earth - ionosphere waveguide, say 3 kHz, a very noticeable delay is perceived in the lowest frequency. Since the ionosphere is essentially a plasma, it has an interesting effect on distant surface waves. This is only perceivable for frequencies in the VLF where the wave lengths can vary by miles by only a small in frequency. One fact to note is that shorter wave lengths (higher frequencies) will travel through a plasma faster than a longer wave length. Since neither the Earth nor the ionosphere are perfect conductors there will always be exceptions to this. Therefore, you will notice that for distance strikes, frequencies below about 3 kHz will be greatly attenuated.

These frequencies usually appear to be about 3 kHz and above. The Earth and it's ionosphere create a sort of spherical waveguide that will efficiently pass frequencies that have a wave length that is short enough to fit between the two. Distant strikes will sound much different for several reasons. Nearby strikes will sound like any ordinary static crash on your car's AM radio. Most of the emissions are heard via traveling waves over the surface of the Earth and at these frequencies they can be heard for thousands of miles. The wave lengths is the VLF part of the spectrum are the most interesting. When lightning strikes the Earth electromagnetic energy is generated literally from DC to daylight. I could write a book on all of the different types of emissions to be heard here but for now I'll give just a few examples. I've recorded strong whistlers while monitoring from my city back yard. Don't be too discouraged though if you can't get too far away. I've had very quite reception in places such as Mesa Verde National Park in Colorado. 5 miles from the power grid are preferred but very difficult to obtain. One must get away from man's power grid because harmonics of the 60Hz power system extend well into the kHz region. These emissions are generated by many different mechanisms and are often beautiful to listen to. In the VLF portion of the spectrum it is easy to listen to the emissions created by the Earth itself. Officially the three frequency ranges are designated as follows: To be a little more precise, I'm referring to natural radio emissions in the VLF (Very Low Frequency) range and miscellaneous radio transmissions in the LF (Low Frequency) to MF (Medium Frequency) ranges. Over the years I have enjoyed monitoring the long wave part of the electromagnetic spectrum.

Inspire vlf receiver schematic