The part of the electromagnetic spectrum termed as VLF (Very Low Frequency) spans the frequency region 3-30 kHz and has free-space wavelength of 100-10 km. Ground-based observations of this frequency spectrum are dominated by “sferics”, the strong impulses radiated by lightning discharges (seen as vertical lines in the figure). In the region above 10 kHz man-made signals from communication and navigation transmitters can be observed (as horizontal lines) and local electrical noise is also present below ~5 kHz.
VLF waves propagate in the so-called Earth-ionosphere waveguide, reflecting from the conducting surface of the Earth and the lower part of the ionosphere (the so-called D-region) at ~70-90 km heights. They have a low attenuation rate and can therefore be received at thousands of km-s from their source. They can also penetrate sea water. These properties led to the construction of VLF transmitters all over the globe, which are used for communication and navigation purposes. The signals of such man-made transmitters can be recorded using VLF receivers and can also be used for scientific purposes.
As VLF waves propagate in the Earth-ionosphere waveguide, they are affected by the variations of atmospheric electrical conductivity on the boundaries, which results in changes in the observed amplitude and phase of the VLF transmitter signals.
If you would like to learn more on how these perturbations can be used in sprite research click here:
During Eurosprite2005 VLF data is recorded at various sites all over Europe, with receivers operating continuously during nighttime.