Terrestrial Natural and Man-made EM Noise

By Cesidio Bianchi and Antonio Meloni

2. Introduction

Natural electromagnetic noise sources exist since the origin of the Universe, therefore life on Earth has developed and evolved in a ‘sea’ of electromagnetic noise. Natural radio noise, for example, has been always present and maybe, in a small part, accustomed the evolution of living system. Understanding how the background noise is generated and distributed and how it interacts with living systems can contribute to the general knowledge of the secrecies of life on Earth. In the present work we consider the different frequency ranges, starting from the lowest frequencies that the ionospheric and magnetospheric cavities can sustain, up to the galactic and man-made noise in the micro-wave band. Depending on frequency, the natural noise inside the magnetospheric cavity, originates from the impinging particles and waves that interact at various level with the magnetosphere. Inside the ionospheric cavity atmospheric noise originates from the energy of the lightning discharges that produce several interesting propagating phenomena.
Human technologies implanted for power transmission and communications are the well known non natural cause of man made noise. The man-made noise, that is mainly due to the communication and broadcasting systems, electric energy transport systems, automotive ignition, industrial thermal processes and instruments for scientific/medical appliances, is distributed, albeit not uniformly, in all bands. In VLF-HF band atmospheric noise is still larger than man-made noise in the order of tenth of dB in rural areas and simply overcomes man-made noise in business areas. At higher frequency radio noise originated in the atmosphere becomes less important and cosmic noise is prevalent up to the millimetric wavelength. In general man-made noise is concentrated in certain particular frequencies in correspondence to technological applications.
Both contributions, natural and man-made, affect in some way the living systems. Exposure of biological system in a electromagnetic field induce electrical currents and fields and surface charges at the interface of electrically different media and, as frequency increases, the thermal effects are much more evident; this behavior is described by Maxwell’s equations. Furthermore, if the electric properties of the biological systems are known it is possible to establish its interaction with the electromagnetic fields. A biological system is practically transparent to the static magnetic field, life on Earth, where a natural quasi static magnetic field in the range of 20-70 μT order of magnitude exists, has developed in such environment. Time varying magnetic fields interact with living systems according to their respective wavelengths and sizes. Static and slow varying electric fields inside the biological system reduce their strength of several order of magnitude. Cell membranes play a key role in protecting biological systems against large electric fields starting from static to ELF band. Higher frequencies electromagnetic waves interact with the polar water molecules which constitute generally a large fraction of the living systems. Cell membrane confers to biological systems peculiar electromagnetic properties especially at the lower frequencies. Increasing the frequency the real part of the dielectric permittivity decreases and the internal electric field is comparable with the external field, at high frequencies thermal effects become prevalent. The characterization of the natural and man-made radio noise allows us to establish also how biological systems had interacted with the electromagnetic fields. At the present time the characterization of man-made noise is important to determine how life can be affected by this new contribution that is superimposed to the natural noise in the planetary environment (figure 1).

FIGURE 1 A schematic pictorial of terrestrial natural and man-made radio noise sources A schematic pictorial of terrestrial natural and man-made radio noise sources