6.2 NATURAL SOURCES OF EMISSIONS

Natural RF emissions have existed since long before the appearance of human technology and therefore provide a useful baseline against which artificial sources can be compared. Natural emissions originate from astronomical objects, atmospheric electrical activity, and the thermal radiation emitted by the Earth itself. Although natural RF fields span an enormous range of frequencies, their power densities are generally very small compared with those produced by modern human-made systems. Nevertheless, understanding these natural sources provides useful perspective when considering the magnitude of exposures arising from human-made sources.

6.2.1 Extraterrestrial RF Sources

Extraterrestrial RF sources include emissions from the Sun and radiation originating from beyond the Solar System:

The cosmic microwave background (CMB), the remnant thermal radiation associated with the early Universe, closely follows a blackbody spectrum at a temperature of approximately 2.725 K. When spectral intensity is expressed per unit frequency, it reaches its maximum near 160.2 GHz. Integration over the frequency range 30 kHz–300 GHz gives an incident power density of approximately 2.5 μW m⁻² before allowance is made for the shielding and absorption produced by the Earth’s atmosphere.
The Sun radiates across much of the electromagnetic spectrum, including X-rays, ultraviolet radiation, visible light, infrared radiation, and radio waves. At the mean Earth–Sun distance, the total solar irradiance above the atmosphere is approximately 1,361 W m⁻². Only a minute proportion of this energy falls within the frequency range 30 kHz–300 GHz, amounting to approximately 5 μW m⁻².

These extraterrestrial RF power densities are at the boundary of the Earth’s atmosphere and both contributions are substantially reduced by the atmosphere before reaching the Earth’s surface. The ionosphere reflects much of the radiation below approximately 30 MHz, while atmospheric oxygen and water vapor absorb energy at a number of higher-frequency bands. ICNIRP consequently estimates the combined natural RF power density at the Earth’s surface from the Sun and the sky, including the CMB, to be approximately 3 μW m⁻².