Atmospheric disturbances

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Atmospheric interference is called natural impulse-like and noise-like electromagnetic signals in the entire frequency range of radio waves , which limit the sensitivity of radio reception systems.

You step z. B. in the frequency range less than about 10 MHz as cracking noises in radio receivers. There they are primarily generated by lightning discharges as coherent and non-coherent pulses. At higher frequencies it is essentially extraterrestrial noise-like interference sources (e.g. galactic noise ). In urban areas, electrical devices of all kinds can contribute to these disturbances.

The Russian physicist Alexander Stepanowitsch Popow was the first to register electromagnetic pulses from lightning with the help of a simple radio receiver ( coherer ) in 1895 .

Atmospherics

The electric current in a lightning channel with all its branches behaves like a huge antenna that emits electromagnetic waves in a wide frequency band. Beyond a distance where luminous phenomena can be seen and thunder can be heard (up to about ten kilometers), this electromagnetic radiation is the only source of direct information about thunderstorm activity. Pulse-shaped electrical currents during a main discharge in a cloud-earth lightning channel (R-discharge) or in an internal cloud lightning (K-discharge) are the main source for the generation of coherent pulse-shaped electromagnetic signals ( spherics , also sferics , or atmospherics ).

While this pulse shape dominates in the frequency range below about 100  kHz , the signal changes into incoherent pulses as the frequency increases.

The long-wave electromagnetic wave propagation of spherics takes place in the area between the earth and the ionospheric D-layer . Whistlers generated by lightning discharges can propagate into the magnetosphere along the lines of force of the earth's magnetic field . Finally, there are luminous phenomena in the middle atmosphere ( sprites ). These are short-lived electrical phenomena that are presumably generated by lightning bolts of extremely large dimensions.

Electric current in the lightning channel

Figure 1. Vertical electric field (in V / m) as a function of time (in μs) of a typical spheric impulse at a distance of 25 kilometers from the source. Solid curve: Dipole radiation over an ideal electrically conductive ground. Dashed curve: Dipole radiation over a ground with finite conductivity (σ = 3 × 10 −3 S / m). Dotted curve: radiation from a lightning canal at a height of five kilometers.

In a typical main discharge between cloud and ground (R discharge), negative electrical charge (electrons) of the order of magnitude of Q = 1 C, which is stored in the lightning channel, is carried to the ground within a typical pulse time of τ = 100 μs. This corresponds to an electric current of the order of magnitude . The maximum spectral energy is emitted at the frequency of or at a wavelength of ( c is the speed of light).

With a typical internal cloud discharge (K discharge), a positive charge of the order of magnitude C = 10 mC in the upper branch and a corresponding negative charge in the lower branch of the lightning channel are neutralized in a typical time of τ = 25 μs. The corresponding values ​​for electric current, frequency and wavelength are J = 400 A, f = 40 kHz, and λ = 7.5 km.

The typical length of a lightning channel is thus of the order of magnitude of for R discharges and for K discharges. Often a continuous current flows between two main discharges. Its “pulse” time varies between 10 and 150 ms, its electrical current is approximately J = 100 A. This corresponds to the sizes Q = 1 to 20 C, f = 7 to 100 Hz and λ = 3 to 40 mm. Both R-discharges and K-discharges produce coherent pulses in a broadband receiver that is tuned to the frequencies 1–100 kHz. The electric field of such a pulse grows to a maximum within a few microseconds and then decreases like a damped oscillator (Figure 1). The direction of the field strength depends on whether the discharge is positive or negative.

The visible part of the channel of an earth lightning bolt has the typical length of five kilometers. Another part of comparable length is hidden in the cloud and may have a significant horizontal branch. This also applies to the internal K discharge. It can be seen that the fundamental wavelength of the electromagnetic waves of R and K discharges is many times greater than the length of the lightning channel. The physics of electromagnetic wave propagation within the lightning channel is therefore a wave-optical phenomenon, and the ray-optical solution is not applicable here.

The channel of an R-discharge can be viewed as a thin insulated vertical wire of length L and a few centimeters in diameter, in which negative electrical charge is stored. To a good approximation, it behaves like an electrical oscillating circuit with a coil, capacitor and resistor. The charge is stored in the capacitor. As soon as the wire comes into contact with the electrically conductive earth, the charge is discharged to earth. Due to the boundary conditions at the upper edge (vanishing electrical current) and at the lower edge (vanishing electrical voltage), only standing resonance waves can exist. The fundamental resonance mode , which most effectively transports electrical charge to earth, has a wavelength that is four times the length of the channel ( λ / 4 antenna). In the case of the K discharge, the channel behaves like a λ / 2 antenna. Of course, this picture no longer applies to the higher modes, since their wavelengths are already comparable or smaller than the real curved channel. They contribute to the incoherent noise in the higher frequency ranges ( ).

Transfer function of the ionospheric waveguide

The lightning channel emits electromagnetic waves ( spherics ). These can be approximated by the electromagnetic far field of a Hertzian dipole . In a spectral analysis , the electromagnetic field of the signal in Figure 1 has a spectral maximum at 4 kHz. Beyond this maximum, the spectral amplitude decreases almost like 1 / f (Figure 2).

Figure 2. Spectral amplitude of the vertical electric field strength of the spherics in Figure 1 as a function of frequency. The ionospheric waveguide modifies this waveform as it propagates. The different curves apply to different distances from the source (1 mm = 1000 km). The bump is dominant up to a distance of 200 km.

The upper curve in Fig. 2 shows that R-discharges radiate their energy preferably in the ELF / VLF range ( ELF = extremely low frequencies (ultra- long waves) ( ); VLF = very low frequencies (longest waves) ( )). These waves are reflected on the ground and on the ionospheric D-layer (approx. 70 km altitude during the day and 90 km altitude at night). The reflection and attenuation of the waves on the ground depend on frequency, distance and orography , in the ionosphere also on time of day, season, latitude and geomagnetic field.

VLF propagation in the ionospheric waveguide can be described by the ray-optical theory and by the wave-optical theory. For distances less than about 500 km (depending on frequency), the ray-optical theory is suitable. Ground waves and waves simply reflected by the ionosphere interfere with each other. At distances greater than about 500 km, the waves reflected several times by the ionosphere become more important. Here the application of the wave-optical solution becomes necessary. The first wave mode is dampened the least and therefore plays the most important role for distances greater than about 1000 km.

The ionospheric waveguide is dispersive . Its propagation properties are described by a transfer function that is mainly dependent on the distance ρ and the frequency f . In the VLF range and at distances greater than 1000 km, only mode no. 1 is important. The lowest attenuation occurs at around 15 kHz. The ionospheric waveguide therefore behaves like a bandpass (Fig. 2), so that the 15 kHz signal dominates at distances greater than about 5000 km.

The optical ray solution loses its validity for ELF waves ( ). The zeroth mode dominates here and is responsible for the second window in Fig. 2 at greater distances. Resonance waves of this zeroth mode can be generated in the ionospheric waveguide by the continuous electrical current between two R discharges. Their wavelengths are an integral multiple of the circumference of the earth, and their resonance frequency can be approximated by (with ; a is the earth's radius). These resonance modes with their fundamental frequency are known as Schumann resonances .

Determination of thunderstorm activity

There are around 100 main lightning discharges (R discharges) per second worldwide, mainly over the continents in low and medium latitudes. K discharges are far more common than R discharges. However, their energies are several times weaker than those of R-discharges and do not play a role outside a distance range of about 100 km.

The observation of spherics is a suitable means of recording thunderstorm activity. Measurements of the Schumann resonances worldwide from only a few stations determine the global thunderstorm activity quite well. The dispersion properties of the transfer function of the ionospheric waveguide can be exploited by measuring the group velocity of a spheric signal at two adjacent frequencies. The group delay difference between neighboring frequencies in the lower VLF range is directly proportional to the distance between the source and receiver. Together with a bearing of the signal, the location of the source can be determined. Since the propagation attenuation of the VLF waves is smaller with west-east propagation than the other way round and smaller at night than during daylight, thunderstorm activity can be observed from sources up to a distance of approx. 10,000 km at night and with west-east propagation . Usually the range is on the order of 5000 km.

For the regional area (<1000 km) it is common to take bearings on a Spheric at the same time, or to measure the arrival time of the signal from different stations. The prerequisite for such measurements is concentration on a single pulse. Without separation of the individual pulses, interference occurs with a beat frequency that is equal to the inverse pulse repetition time.

Atmospheric noise

The signal-to-noise ratio determines the sensitivity of telecommunication systems (e.g. radio receivers). An analog signal must be significantly larger than the noise amplitude in order to be recognized. Atmospheric noise is one of the most important causes for the limitation of the reception quality of radio signals.

The incessant discharge processes in connection with the development of lightning phenomena (pre-discharges, intermediate discharges, etc.) generate a sequence of incoherent pulses in the entire frequency range, the mean amplitude of which decreases almost proportionally to the reciprocal frequency. In the ELF range, technically caused noise due to the 50 Hz mains voltage and harmonics, high voltage conductors, the 16.7 Hertz traction current network , natural signals of magnetospheric origin, etc. predominate. In the VLF range, the coherent R and K discharges dominate as isolated impulses become visible from the background noise. The noise amplitude becomes increasingly incoherent above about 100 kHz . In urban regions, technical noise from electrical devices (electric motors, ignition systems in car engines, etc.) is also superimposed. Above the shortwave range (3–30 MHz), extraterrestrial noise (galactic noise, solar noise) dominates. This also includes the 2.7 Kelvin cosmic background radiation in the mm and cm wave range.

The atmospheric noise depends on the frequency, the time of year, the time of year and the geographical location. It is measured worldwide and is recorded in CCIR reports (CCIR = "Comité Consultatif International des Radiocommunications").

Electromagnetic environmental compatibility

The electromagnetic environmental compatibility deals with the influence of electromagnetic fields, in particular of technical electrical devices of all kinds, on the environment. Guidelines for limit values ​​for both high-frequency and low-frequency systems are established there. In contrast to the omnipresent atmospheric disturbances, technical devices are usually point sources whose emitted energy density decreases with the third power of the distance from the source. Evidence for an undesirable biological effect of atmospheric disturbances on humans has not yet been provided.

See also

Individual evidence

  1. ↑ Hans Volland (Ed.): CRC Handbook of Atmospherics. 2 volumes. CRC Press, Boca Raton FL 1982.
  2. ↑ Hans Volland (Ed.): Handbook of Atmospheric Electrodynamics. 2 volumes. CRC Press, Boca Raton, FL 1995.
  3. ^ A b E. A. Lewis: High frequency radio noise. In: Hans Volland (Ed.): CRC Handbook of Atmospherics. Volume 1. CRC Press, Boca Raton, FL 1982, ISBN 0-8493-3226-5 , pp. 251-288.
  4. ^ A b D. E. Proctor: Radio noise above 300 kHz due to Natural Causes. In: Hans Volland (Ed.): Handbook of Atmospheric Electrodynamics. Volume 1. CRC Press, Boca Raton, FL 1995, ISBN 0-8493-8647-0 , pp. 311-358.
  5. ^ M. Hayakawa: Whistlers. In: Hans Volland (Ed.): Handbook of Atmospheric Electrodynamics. Volume 2. CRC Press, Boca Raton, FL 1995, ISBN 0-8493-2520-X , pp. 155-194.
  6. ^ CG Park: Whistlers. In: Hans Volland (Ed.): CRC Handbook of Atmospherics. Volume 2. CRC Press, Boca Raton, FL 1982, ISBN 0-8493-3227-3 , pp. 21-77.
  7. ↑ Martin Füllekrug, Eugene A. Mareev, Michael J. Rycroft: Sprites, Elves and Intense Lightning Discharges (= NATO Science Series. Series 2: Mathematics, Physics and Chemistry. Volume 224). Springer, Dordrecht 2006, ISBN 1-4020-4627-8 .
  8. ↑ GI Serhan, MA Uman, DG Childers, YT Lin: The RF spectra of first and subsequent lightning return strokes in the 1- to 200-km range. In: Radio Science. Volume 15, No. 6, November / December 1980, ISSN  0048-6604 , pp. 1089-1094, doi : 10.1029 / RS015i006p01089 .
  9. ^ A b H. Volland: Longwave sferics propagation within the atmospheric waveguide. In: Hans Volland (Ed.): Handbook of Atmospheric Electrodynamics. Volume 2. CRC Press, Boca Raton, FL 1995, ISBN 0-8493-2520-X , pp. 65-93.
  10. ^ Martin A. Uman: The Lightning Discharge (= International Geophysics Series. Volume 39). Academic Press, Orlando, Fla. 1987, ISBN 0-12-708350-2 .
  11. ↑ YT Lin, MA Uman, JA Tiller, RD Brantley, WH Beasley, EP Krider, CD Weidman: Characterization of lightning return stroke electric and magnetic fields from simultaneous two-station measurements. In: Journal of Geophysical Research. Series C: Oceans. Volume 84, No. C10, October 1979, ISSN  0196-2256 , pp. 6307-6314, doi : 10.1029 / JC084iC10p06307 .
  12. ^ Charles D. Weidman, E. Philip Krider: The radiation field wave forms produced by intracloud lightning discharge processes. In: Journal of Geophysical Research. Series C: Oceans. Volume 84, No. C6, June 1979, pp. 3159-3164, doi : 10.1029 / JC084iC06p03159 .
  13. ↑ Hans Volland: Atmospheric Electrodynamics (= Physics and Chemistry in Space. Volume 11). Springer, Berlin et al. 1984, ISBN 3-540-13510-3 .
  14. ↑ James R. Wait: Wave Propagation Theory. = Lectures on Wave Propagation Theory. Pergamon Press, New York NY et al. 1982, ISBN 0-08-026345-3 .
  15. ^ W. Harth: Theory of low frequency wave propagation. In: Hans Volland (Ed.): CRC Handbook of Atmospherics. Volume 2. CRC Press, Boca Raton, FL 1982, ISBN 0-8493-3227-3 , pp. 133-202, here p. 182.
  16. ^ C. Polk: Schumann resonances. In: Hans Volland (Ed.): CRC Handbook of Atmospherics. Volume 1. CRC Press, Boca Raton, FL 1982, ISBN 0-8493-3226-5 , pp. 111-178.
  17. ↑ a b D. D. Sentman: resonances Schumann. In: Hans Volland (Ed.): Handbook of Atmospheric Electrodynamics. Volume 1. CRC Press, Boca Raton, FL 1995, ISBN 0-8493-8647-0 , pp. 267-295.
  18. ^ B. Vonnegut: The physics of thunderclouds. In: Hans Volland (Ed.): CRC Handbook of Atmospherics. Volume 1. CRC Press, Boca Raton FL 1982, ISBN 0-8493-3226-5 , pp. 1-22.
  19. ^ ER Williams: Meteorological aspects of thunderstorms. In: Hans Volland (Ed.): Handbook of Atmospheric Electrodynamics. Volume 1. CRC Press, Boca Raton, FL 1995, ISBN 0-8493-8647-0 , pp. 27-60.
  20. ↑ Christoph Grandt: Thunderstorm monitoring in South Africa and Europe by means of Very Low Frequency sferics. In: Journal of Geophysical Research. Series D: Atmospheres. Volume 97, No. D16, November 1992, ISSN  0148-0227 , pp. 18215-18226, doi : 10.1029 / 92JD01623 .
  21. ^ RE Orville: Lightning detection from ground and space. In: Hans Volland (Ed.): Handbook of Atmospheric Electrodynamics. Volume 1. CRC Press, Boca Raton, FL 1995, ISBN 0-8493-8647-0 , pp. 137-149.
  22. ^ AC Fraser-Smith: Low-frequency radio noise. In: Hans Volland (Ed.): Handbook of Atmospheric Electrodynamics. Volume 1. CRC Press, Boca Raton, FL 1995, ISBN 0-8493-8647-0 , pp. 297-310.
  23. ^ AD Spaulding: Atmospheric noise and its effects on telecommunication system performance. In: Hans Volland (Ed.): Handbook of Atmospheric Electrodynamics. Volume 1. CRC Press, Boca Raton, FL 1995, ISBN 0-8493-8647-0 , p. 359 ff.
  24. ^ R. Reiter: Biological effects of electromagnetic man-made noise, atmospherics, and small ions. In: Hans Volland (Ed.): Handbook of Atmospheric Electrodynamics. Volume 2. CRC Press, Boca Raton, FL 1995, ISBN 0-8493-2520-X , pp. 117 ff.