Introduction to RF Radiation Safety / 6 / 6.2 / 6.2.2

6.2.2 Terrestrial RF Sources

In addition to radiation arriving from space, natural terrestrial RF sources include thermal emission from the Earth and electromagnetic fields generated by lightning.

With a representative surface temperature of about 300 K, the Earth emits thermal radiation primarily in the infrared. A miniscule fraction, only about 0.0006%, of this emission lies between 30 kHz and 300 GHz, corresponding to an RF power density of a few milliwatts per square meter, some thousand times larger than the RF power density from extraterrestrial sources.

The ionosphere reflects or strongly attenuates much extraterrestrial RF radiation below approximately 30 MHz, although its behavior varies with frequency, time, location, and atmospheric conditions. Lightning is an important terrestrial source in this frequency region and the electromagnetic fields generated are impulsive and depend on the type of discharge and the distance from it. A representative electric-field pulse with an amplitude of 4 V m^–1 measured 200 km from a discharge corresponds, under plane-wave conditions, to a peak power density of approximately 42 mW m⁻², again some thousand times more than the extraterrestrial sources. However, because this value represents a short-lived peak, its associated energy and duration must also be considered when assessing exposure.