Vol50,3,2007 435 ANNALS OF GEOPHYSICS, VOL. 50, N. 3, June 2007 Key words radio noise – background noise 1. Introduction Natural electromagnetic noise sources exist- ing since the origin of the Earth may be influ- encing the evolution of living systems. Under- standing how this background noise is generat- ed and distributed and how it interacts with liv- ing systems, can contribute to the general knowledge of life on Earth. In this review we consider the different frequency ranges, starting from the lowest frequencies generated within the ionospheric and magnetospheric cavities, up to the microwave band, including galactic noise, for both natural and man-made noise. Different sources generate natural noise inside the magnetospheric cavity at different frequen- cies. A primary source of noise is given by the interaction of particles and waves coming from outer space with the magnetosphere plasma at various altitudes; another source is inside the ionospheric cavity where atmospheric lightning discharges produce several interesting propa- gating phenomena. At higher frequency radio noise originated in the atmosphere becomes less important and cosmic noise is prevalent up to the millimetric wavelength. Human technologies implanted for power transmission and communications are the well known causes of man made noise. Man-made noise, mainly due to communication and broad- casting systems, electric energy transport sys- tems, automotive ignition, industrial thermal processes and instruments for scientific/med- ical appliances is distributed, albeit not uni- formly, in all bands. In VLF-HF band atmos- pheric noise is still larger than man-made noise in the order of a tenth of dB in rural areas and Natural and man-made terrestrial electromagnetic noise: an outlook Cesidio Bianchi and Antonio Meloni Istituto Nazionale di Geofisica e Vulcanologia, Roma, Italy Abstarct The terrestrial environment is continuously exposed to electromagnetic radiations which set up a «background» electromagnetic noise. Within the Non Ionizing Radiation band (NIR), i.e. for frequencies lower than 300 GHz, this background can have a natural or an artificial origin. Natural origins of electromagnetic radiations are gen- erally atmospheric or cosmic while artificial origins are technological applications, power transmission, commu- nications, etc. This paper briefly describes the natural and man-made electromagnetic noise in the NIR band. Natural noise comes from a large variety of sources involving different physical phenomena and covering a wide range of frequencies and showing various propagation characteristics with an extremely broad range of power levels. Due to technological growth man-made electromagnetic noise is nowadays superimposed on natural noise almost everywhere on Earth. In the last decades man-made noise has increased dramatically over and above the natural noise in residential and business areas. This increase has led some scientists to consider pos- sible negative effects of electromagnetic waves on human life and living systems in general. Accurate measure- ments of natural and man-made electromagnetic noise are necessary to understand the relative power levels in the different bands and their influence on life. Mailing address: Dr. Cesidio Bianchi, Istituto Naziona- le di Geofisica e Vulcanologia, Via di Vigna Murata 605, 00143 Roma, Italy; e-mail: bianchi@ingv.it 436 Cesidio Bianchi and Antonio Meloni simply overcomes man-made noise in business areas. In general man-made noise is concentrat- ed in particular frequencies in association with technological applications. Both contributions, natural and man-made, affect technological applications such as com- munication, remote sensing, etc. The purpose of this work is to describe the electromagnetic terrestrial environment and the main sources of emission both natural and man-made. Figure 1 shows some propagation mode names and acronyms for natural contributions in the terres- trial environment inside the magnetosphere and ionospheric cavity. 2. Natural radio noise Non Ionizing Radiation (NIR) electromag- netic waves range from the mHz to 300 GHz spanning over 14 orders of magnitude (table I). The corresponding wavelengths range from the dimension of the solar system, for waves around the mHz frequencies, to millimeters for frequencies around 300 GHz. 2.1. Magnetic noise in ULF/ELF band and other propagating phenomena The Earth’s magnetic field is not only static but also includes very slow time variations: sec- ular, annual, 27 days, diurnal and substorm magnetic bay type variations at very low fre- quency (<1 mHz); all this can be considered a sort of «natural magnetic» noise. Variation am- plitudes can be quantified in tenths of nT for the diurnal variation and hundreds, or exceptional- ly, thousands of nT in the case of strong mag- netic storms (see Kivelson and Russell, 1995; Merrill et al., 1998). At higher frequencies in this band various other phenomena, for exam- ple geomagnetic pulsations, take place. Geo- magnetic pulsations, i.e., Ultra-Low-Frequency (ULF) waves cover roughly the frequency range from 1 mHz to 1 Hz, i.e., from the lowest frequency the magnetospheric cavity can sus- tain, up to the various ion gyrofrequencies (see for example: Hughes, 1994; Richmond and Lu, 2000). Pulsation frequency is considered to be «ultra» low when it is lower than expected plas- ma frequencies, like plasma frequency and the ion gyrofrequency. Geomagnetic pulsations were first observed in the ground-based meas- urements of the 1859 Great Aurora events (Stewart, 1861) and were subsequently studied in depth. It is well known now that lower fre- quency pulsations are generally related to the Kelvin Helmholtz instability that takes place at the magnetopause, and is generated by the solar wind interacting with the magnetosphere, or by upstream waves in the foreshock region. Table II reports geomagnetic pulsations’ average in- tensity, frequency and other information (mod- ified from Lanzerotti et al., 1990). Other electromagnetic phenomena in the ULF/ELF band are originated by particles im- pinging on the magnetosphere causing electro- magnetic emissions that propagate inside the magnetosphere cavity. Chorus emissions and auroral hiss are two other relevant phenomena. Chorus emissions are among the most intense plasma waves in the outer magnetosphere that propagate as far as the Earth surface and are ob- served at intermediate invariant latitudes. Since these emissions occur in matching audible sound bands, a simple tuned radio receiver can Fig. 1. A schematic representation of terrestrial nat- ural and man-made radio noise sources within the magnetosphere-ionosphere system. 437 Natural and man-made terrestrial electromagnetic noise: an outlook Table I. NIR frequency bands and the main natural radio noise sources. Frequency Wavelengh Main natural radio Environment range (m) noise sources ULF 1-3000 mHz 3×1011-3×108 Resonances Magnetospheric (Ultra Low Frequency) in the magnetospheric cavity cavity, interaction with particles of solar origin and radiative pressure with the magnetosphere ELF 3-3000 Hz 108-105 Resonances Ionospheric (Extremely Low in the ionospheric cavity cavity Frequency) VLF (Very Low 3-30 kHz 105-104 Propagation Ionospheric Frequency) in the ionospheric cavity cavity of the atmospheric disharge radiate energy LF 30-300 kHz 104-103 Atmospheric noise Ionospheric (Low Frequency) cavity MF 300-3000 kHz 103-102 Atmospheric noise Ionospheric (Medium Frequency) cavity HF 3-30 MHz 102-10 Atmospheric Ionospheric (High Frequency) and cosmic noise cavity VHF 30-300 MHz 10-1 Atmospheric Earth surfaces (Very High and cosmic noise (mainly due Frequency) to the cosmic noise that penetrates the ionospheric layers) UHF 300-3000 MHz 1-10-1 Cosmic noise As above (Ultra High Frequency) SHF 3-30 GHz 10-1-10-2 Cosmic noise As above (Super High Frequency) EHF 30-300 GHz 10-2-10-3 Cosmic noise As above (Extremely High Frequency) convert natural radio signals into sound. The Chorus spectral features (from 500 Hz to 1.2 kHz) consist in the succession of predominant- ly rising tones which resemble a chorus of chirping birds from which these emissions take their name. Auroral hiss emissions are broad, intense electromagnetic emissions which occur over a wide frequency range from a few hun- dred Hz to several tens of kHz occurring main- ly in the auroral zone. This spreading at high frequencies is caused by the anisotropic charac- ter of whistler mode propagation (see later). The resulting tones are strongly modulated hiss-like tones. 438 Cesidio Bianchi and Antonio Meloni 2.2. Atmospheric noise in ELF/VLF band and related electromagnetic phenomena Lightning flashes are the main source of en- ergy for the electromagnetic background inside the ionospheric cavity. Starting from the lower band ELF (few Hz) up to several VHF (hundreds MHz) the noise originates mainly from the ener- gy radiated by lightning strokes (see for exam- ple: Cummer and Inan, 2000; Mika et al., 2005). Several million lightning strokes occur daily from an estimated 2000 storms worldwide, and the Earth is hit about 100 times a second by lightning. The discharge is very violent and can easily reach 10000 A. The amount of energy re- leased by each discharge can vary from units to tenths of GJ. Hence, for the duration of the dis- charge (less than 1 s), the power involved in this phenomenon is of the order of 1-10 GW. The an- nual total released energy is in the order of 1019 J. If only 10% of this energy is radiated as elec- tromagnetic energy (fig. 2) it would be compara- ble to the energy produced in 1970 by the elec- tric power stations in the world. The main relevant phenomena in the ELF lower band are the Schumann resonances (Bliokh et al., 1980; Sentman, 1987; Sentman, and Fraser, 1991). This phenomenon consists of a wide spectrum electromagnetic signal, com- posed by dumped waves of frequencies below 60 Hz. Schumann resonances occur because the Earth and the ionosphere form a natural wave guide that shows a fundamental resonance fre- quency at 7.8 Hz and upper harmonic compo- nents at about 15.6, 23.4 and 31.2 Hz (fig. 3). The Earth-atmosphere system can be seen, from an electromagnetic point of view, as a series of shell layers having different electrical conductiv- ity with night-day asymmetry (Greifinger et al., Table II. Magnetic noise sources in the pulsation band. Pulsation Continuous pulsations Irregular pulsations classes Pc1 Pc1 Pc3 Pc4 Pc5 Pi1 Pi2 Incoherent noise Period (s) 0.2-5 5-10 10-45 45-150 150-600 1-40 40-150 1-1000 Frequency 200-5000 100-200 22-100 7-22 2-7 25-1000 2-25 1-1000 (mHz) Intensity 1 3 10 <300 300 10 100 - (nT) Source Electromag- Electromag- Wave- Drift of Magneto- Modulation Substorms Ionospheric netic ion netic ion particle protons pause of particles current cyclotron cyclotron interaction from the instability (intensity instability instability in the bow nightside increase in the in the shock with the equatorial equatorial region magnetic magneto- magneto- activity) sphere sphere Fig. 2. Lightning frequency domain electromagnet- ic spectrum. 439 Natural and man-made terrestrial electromagnetic noise: an outlook 2007). The Earth and the ionospheric layers ap- pear as perfect conductors having air (of negligi- ble conductivity) in between, forming an Earth- ionosphere cavity, in which electromagnetic ra- diation is trapped. Lightning strikes within the troposphere radiate energy into this system and the waves travel around the Earth. In the case of constructive interference, Earth-ionosphere cavi- ty resonances are excited in the above mentioned frequency range (6-60 Hz). ELF-VLF radio waves can propagate over long distances with small attenuation rates, typ- ical 2-3 dB per Mm and phase stability. The propagation of ELF-VLF radio waves can con- veniently be considered in a spherical earth sur- rounded by a concentric reflecting layer of elec- tronic density into which VLF waves are launched and propagate. The propagation is controlled by the lowest region of the iono- sphere. During the daytime this includes the lower D-region and, at night, the lowest part of the E-region. ELF VLF propagate for very long distances nearly without attenuation. As is well known, the phase and amplitude of VLF propa- gation can be described by an Earth-ionosphere waveguide (Davies, 1990). This description was exploited in past years for radio localiza- tion purposes (Swanson, 1983). The relation between the phase and height changes, over long distances, is given by (2.1) where h is the mean height of reflection in the h d h a h h 2 2 16 2 2 ϕ λ π λ ∆ ∆ = − +b l D-region, d the path length, ϕ the phase, λ the wavelength and a is the Earth’s radius (Budden, 1985; Wait, 1970). Phase shift can be expressed in degrees or in seconds by ∆t = ∆ϕλ/2πc. The amplitude of the electric field E over long paths in a wave guide mode is approxi- mately (2.2) where α is the attenuation rate and p the radiat- ed power (in kW). The signal amplitude is gen- erally dependent on interferences of various propagation modes, that change with the varia- tions of the wave guide parameters. Observa- tion of the phase and amplitude of VLF waves were also used to study variability, morphology and other phenomena occurring in the low ionospheric region. Measurements and time- spatial statistical model is gven by Fieve et al. (2007), and Tomco and Hepner (2001). Other relevant propagating phenomena in this frequency band are the so-called sferics, tweeks and whistlers. Radio atmospherics (or sferics for short, and sometime statics) are im- pulsive signals generated by lightning strokes that travel in the Earth-ionosphere wave guide. These impulsive signals (a few ms duration) propagate for thousands of kilometers. As in a real wave guide the Earth-ionosphere guide can sustain the propagation of these signals with very low attenuation values. Since only the up- per part of the wave channel varies its position with time, the sferic propagation is determined by ionosphere conditions. Almost all AM re- ceivers detect sferics as disturbances (sounds like pops and crashes). Sparks of lightning strokes are generally powerful sources of elec- tromagnetic (radio) emission throughout the ra- dio frequency spectrum from the very low radio frequencies up to the microwave frequency ranges and the visible light spectrum, even if the radio power is concentrated in VLF range from 0.1 to 10 kHz (fig. 4). Tweeks are sferics dispersed in frequency. Their sound is similar to a bird’s song usually in a frequency range of 1-7 kHz (fig. 5). When sfer- ics propagate for long distances in a dispersive medium like the ionosphere their harmonic com- ( / ) 0.5 sin E h e a d a p300 ad λ = − − c m Fig. 3. Schumann resonance peaks. 440 Cesidio Bianchi and Antonio Meloni ponents separate along the path (Helliwell, 1965). These components penetrate the ionosphere at various depths, in such a way that higher frequen- cies penetrate more than lower ones and as a con- sequence travel for longer distances. These differ- ent paths imply different arrival times at an ob- server. In a spectrogram they appear as a de- scending tone with duration of the order of 25 to 150 ms. Tweeks are normally heard in the evening after sunset. Whistlers are remarkable bursts generated by lightning discharge. When part of the discharge energy escapes the ionospheric barrier and prop- agates through the magnetosphere, whistlers can be heard in radio receivers as a relative long whistle decreasing in frequency, from about 6 kHz to a few hundred Hz (fig. 6). In the magne- tosphere whistlers interact with free electrons and are forced to propagate along the Earth’s magnetic field lines. The harmonic components of the signals identified as whistlers correspond to electromagnetic waves that have traveled sev- eral Earth radii arriving at different times to the observer. Lower frequencies are delayed 3-6 s with respect to the higher ones. The dispersion of a whistler depends on the length of the path over which the signal travels as well as the character- istics of the propagation medium such as its elec- tron density (Kimura, 1989). 2.3. Natural LF/MF/HF noise In this frequency range as well natural elec- tromagnetic noise has its main source in the at- mospheric electric discharges; noise amplitude is generally decreasing in intensity with frequency and is affected by ionospheric conditions. Since LF/MF/HF frequencies were very soon used in radio communications and broadcasting, in this frequency range natural radio noise has been the object of scientific studies in the radio engineers community. Natural LF/MF/HF noise radio measurements started very early as radio devices developed. The electromagnetic waves radiated by impulsive lightning discharges cannot escape the ionosphere border. Waves penetrate through the lowest ionospheric layers where they are var- iously absorbed, depending on frequency. Radio waves are reflected by the upper layers of the ion- osphere up to a particular frequency, named ‘crit- ical frequency’, that is dependent on local ionos- pheric condition. Moreover the incidence angle between the wave and the ionospheric layers plays an important role. In fact depending on the path geometry several propagating modes can be Fig. 4. Typical sferics frequency spectrum. Fig. 5. Tweeks that traveled for different distances in the Earth-ionosphere waveguide. Shown for about 6000 km (1), 10 000 km (2) and 40 000 km (3) paths. Fig. 6. Whistler spectrum. 441 Natural and man-made terrestrial electromagnetic noise: an outlook established. The results are summarized in fig. 7 where the atmospheric noise versus frequency is plotted. The same figure shows cosmic noise at frequencies greater than ionosphere plasma fre- quency. The cosmic noise lowest frequency de- pends on the ionospheric conditions i.e. on elec- tron plasma frequency about (15-30 MHz). In the 1960s of last century a campaign of at- mospheric noise measurement started by Consul- tative Committee International Radio (CCIR) gave the main contribution to the knowledge of background radio noise in the LF/MF/HF fre- quency range. Sixteen radiometers were installed worldwide and the results were summarized on report CCIR N.322 in 1964 (CCIR/ITU, 1964) and N.322-3 in 1988 (CCIR/ITU, 1988). The planetary atmospheric noise distribution is re- ported in fig. 7 where cosmic noise is also report- ed. In the figure the strongest levels are associat- ed with the equatorial region and the weakest in Antarctica. Many of these propagating phenomena, in- cluding the atmospheric noise diurnal variabili- ty, can be easily explained with simple consid- erations. In the LF-MF-HF range depending on frequency, waves penetrate relatively deeply in- to ionospheric plasma. Because of the non mo- notonic trend of the electron density with height, without considering the terrestrial mag- netic field and collisions, each layer exhibits its own critical frequency up to the point where the normal incident angular frequency ω equals the angular plasma frequency ωp: (2.3) where, N is the electron density local maxi- mum, e and me are the charge and the mass of the electron, and εo is the permittivity of free space. This gives a good approximate descrip- tion of the confinement limits of waves inside the ionospheric cavity. The refractive phase in- dex µ simple is . (2.4) This relation establishes the condition of pene- tration, for ω = ωp the refractive index µ is zero. To take into account the attenuation suffered by the radio waves, collisions between free elec- trons and neutral molecules, indicated with let- ter ν, must be considered. In that case the phase refractive index µ becomes a complex quantity . (2.5) The imaginary term is responsible for the ab- sorption suffered by the radio wave during its propagation in the ionospheric plasma. In the lower ionospheric region, the attenuation rate α is relevant (up to 20 dB for each ionospheric hop). In fact α is proportional to the quantity N ν/ω 2 which reaches its maximum in the ionospheric D layer. This seems in contradic- tion with the fact that waves of lower frequen- cies described in the relation (2.2) are less at- tenuated. This can be easily explained by rela- tion (2.3), because of waves in ELF-VLF band do not penetrate deeply into the ionospheric plasma and therefore they are not absorbed. Terrestrial magnetic field and oblique incidence of the wave in the plasma are beyond the scope of this work. In the terrestrial surface the condi- tions of reflection and absorption are described by the reflection coefficient and the attenuation rate α and they are both dependent on the elec- tric conductivity of the surface. j 1 1 1 p p 2 2 2 2 2 2 2 2 2 µ ω ω ν ω ω ω ν ω ω ν = − + − +b b b l l l 1 p 2 2 µ ω ω = − m Ne p o e 2 ω ω ε= = Fig. 7. Electric field strength versus frequency for atmospheric and cosmic noise. 442 Cesidio Bianchi and Antonio Meloni 2.4. Natural UHF/SHF/EHF noise In this frequency range the natural predom- inant noise is the background cosmic (or galac- tic) noise. In many ways cosmic noise is simi- lar to terrestrial natural noise, like distant light- ning strikes, or man-made radio noise; for this reason at the beginning of radio measurements it was difficult to identify the different sources. The discovery of the origin of this background noise is usually also marked as the birth of ra- dio astronomy. In 1931 Karl Jansky built an an- tenna operating at 20.5 MHz and after months of careful observations he concluded that the source of the recorded noise was outside the so- lar system. Since then those frequency bands have been carefully investigated by radio as- tronomers (Kraus, 1988). In 1940 Grote Reber made the first radio map of our Milky Way galaxy at a frequency of 160 MHz. After that the radio emission from the Sun, Jupiter and other celestial bodies have been also identified by radio astronomers. Figure 8 reports the spec- trum of solar and other main sources. Some of these sources are very strong or show a particu- lar behavior (fig. 9). The synchrotron radiation has a characteristic wide spectrum like other impulsive radio sources. Moreover galactic or stellar systems generate a strong and common noise source: the thermal noise due to random motion of electrically charged particles in space. Thermal noise arises from electrons and ions in motion in a dissipative media. Others sources are the bremsstrahlung radiation main- ly due to electron proton collision (solar chro- mosphere, Orion nebula etc.). The Earth is not the only planet that shows a large variety of radio signals. Almost all plan- ets have an electromagnetic background, for in- stance Venus is quite similar to the Earth, and the four gas giants, Jupiter, Saturn, Uranus and Neptune are natural emitters of HF radio sig- nals. Due to its huge atmospheric disharges Jupiter is one of the major radio emitters. The sun is the most powerful electromagnetic emit- ter in our solar system, able to generate very broadband radio emissions. In the solar radio emission, several phenomena like flares, mag- netic waves, and storms of electrically charged nuclear particles and ions, directly cause and/or influence the propagation of electromagnetic waves throughout the known solar spectrum. 3. Man-made electromagnetic noise Man-made noise is originated by human tech- nologies. It is strongly dependent on the distance from the sources (power lines, radio, TV commu- nication installations and other) which can be very variable, on frequency and on emitted pow- er. Power and frequency in their time and spatial distribution are the quantities that best describe all man-made radio sources. Other important characteristics are the type of emitted waves, con-Fig. 8. Solar radio emissions. Fig. 9. Some sources of radio emissions and cos- mic background. 443 Natural and man-made terrestrial electromagnetic noise: an outlook tinuous or impulsive, their modulation and wave polarization. Terrestrial man-made sources are mainly located in business, industrial and residen- tial areas. In rural (countryside) and ‘quiet rural’ areas the sources become scarce (see for example report CCIR/ITU, 1990). In spite of its relevance, man made noise power, depending on frequency band, is on average 20-30 dB lower, in ‘quiet ru- ral’ areas than the business and residential area (fig. 10). Of course, starting from the 1960’s, satellite communication systems also contribute to man-made noise measurable on Earth. As previously done for natural electromag- netic noise, the following man-made noise con- tributions are described in the diverse frequen- cy bands with the indication of their main sources. 3.1. ELF band In this band the strongest source of man- made noise comes from the electric power lines which operate ideally at a single frequency, generally 50 Hz (60 Hz in U.S.A.). The electric and magnetic fields generated at this frequency are practically decoupled and because of power lines’ multi-polar configuration, decrease dra- matically with the distance from the lines. The power lines are now spread over the planet in all continents except Antarctica where natural ELF waves close to 50-60 Hz can be recorded be- cause there is a smaller amount of disturbance. Also, some uninhabited regions in other conti- nents are relatively free from this kind of distur- bance but in general, in the ELF band, the 50- 60 Hz electric and magnetic fields are the strongest man-made source of noise both as emitted power and as extension of the source. 3.2. VLF-HF band As in the case of other frequencies, man- made noise emissions are due to electronic de- vices for indoor use, industrial and radio com- munication apparatuses, including broadcasting radios, since all this equipment employs electri- cal oscillators that become unwanted sources of artificial radio emission. The main localized sources are radio AM broadcasting (fig. 11) and some industrial equip- ment. Among the latter relevant sources are heaters that exploit magnetic induction and di- electric losses in the material. Other weaker but numerous sources, such as automotive ignition, heavily contribute to the background noise. We have also immeasurable indoor sources such as electrical household appliances, PC monitors TV, etc. Other scientific and medical devices are irrelevant in the background radio noise configu- ration in the considered range of frequency. Fig. 10. Median values of man-made radio noise power (solid lines) expressed in terms of Fam (dB above thermal noise at T0=288 K). Atmospheric noise (dashed lines) and cosmic background noise (dotted line) are re- ported for comparison with man-made noise. Fig. 11. Power emitted by the most common com- munication systems. 444 Cesidio Bianchi and Antonio Meloni 3.3. VHF-UHF band Electromagnetic radiations in these bands are due to broadcasting FM radio TV and mo- bile phone service stations (fig. 11) and more important automotive ignition. Radar and satel- lite appliances do not make a very important contribution to background noise level. Main indoor sources are cordless phones, microwave, ovens, etc. This equipment does not make a sig- nificant impact outdoors. 3.4. SHF-EHF band Because of the complexity of the system of radio communications and limitations in indus- trial applications operating at these frequencies, in this range only particular equipment is em- ployed. These bands include satellite communi- cation systems and survey, radar systems, scien- tific and medical appliances, for which there are intrinsic technological difficulties in manufactur- ing. For this reason man-made noise is very low in SHF and EHF bands. Moreover, their physical emission processes determine propagation modes for SHF and EHF waves which are strict- ly directional. Other technological applications in this range are related to passive observation and measurements as in radio astronomy. 4. Conclusions This paper described the terrestrial natural electromagnetic noise and its main characteris- tics. Natural electromagnetic noise constitutes a background radio noise in which living systems are immersed and have consequently evolved since the origin of life on Earth. Superimposed on the natural background radio noise is man- made radio noise. The main man-made radio noise sources and its characteristics were also described and compared with the natural noise ones. Artificial noise levels have been constant- ly increasing in the last hundred years or so with the expansion of electromagnetic applica- tions. A digression of the main characteristics, features and sources was made in the frequency range of non ionizing radiation. 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