The Very Low Frequency (VLF) band is located in the frequency range between 3KHz and 30KHz. This band has the unique characteristic of having a portion fall within the audio frequency range of our ears. What's more, the bulk of a lightning strikes energy is deposited between 2KHz and 10KHz. These two points form the basis for everything which follows.
So how does one go about building a VLF receiver anyway? You might be surprised to find out that it is not very difficult to build one. In it's simplest form (and not so simple forms) a VLF receiver is nothing more than an audio amplifier attached to an antenna. One of the most popular uses for a VLF receiver is for listening to lightning strikes from around the world, and the interesting effects that this activity has on our atmosphere.
The Source of Spherics
The snap, crackle, and pop sounds from lightning activity which one can hear on a VLF receiver, or AM and short wave radio for that matter are called atmospherics or spherics for short. These are the most common sounds heard on the VLF band and can be heard 24 hours a day. Spherics show up as wide band bursts when plotted on a spectrogram. This is because a lightning strike is not a narrow band event as is say a military VLF transmitter like NAA in Cutler Maine at 24KHz. These types of wide band signals are characteristic of natural EMF activity from Earth as well as our solar system and beyond. So it shouldn't surprise anyone that another name for this hobby is "natural radio".
I have been interested in natural radio for a good ten years now. Over the years I have built a number of VLF receivers of the E-field and loop antenna type. As a matter of fact the Schumann resonance receiver which I use, found here, can easily be modified to receive the entire VLF band. I'm currently using a homemade receiver for this project which I detail below. Another option is the VLF-3 from the Inspire Project. The Inspire Project is a NASA sponsored (among others) project involved with interactive NASA space physics ionosphere radio experiments. They are doing very interesting stuff. The VLF-3 kit which they provide is of very good quality and easy enough for just about anyone to build in a few quiet nights in the radio shack. I recommend this receiver for anyone interested in getting into this hobby. That is if you don't want to build your own receiver.
Ok, you might be asking yourself what's the big deal about popping sounds from lightning in the VLF band anyway. Yes I agree, after a short while spherics can get very boring. Thankfully, there is more to natural radio than just spherics. During dusk, dawn and the intervening night hours the ionosphere goes through a transformation that has a profound effect on spheric activity. At dusk the D layer (lowest layer) of the ionosphere fades away leaving the higher E and F layer only. This is due to the lack of ionizing radiation from the Sun during the night hours. This phenomena can easily be seen on a SID receiver plot as a sharp rise in the received signal strength of a monitored VLF transmitter at dusk and the subsequent signal drop at dawn. During the daylight hours spherics can travel upwards of 2000 to 3000 kilometers from the source of the lightning strike. These spherics reach the receiver via the wave guide created by the D layer and the Earth's surface. At night though it's presumably the much higher E and F layers that are responsible for the sky wave component of a VLF signal. This allows VLF signals to travel considerably further at night.