Extremely low frequency

Extremely low frequency (ELF) is the ITU designation for electromagnetic radiation (radio waves) with frequencies from 3 to 30 Hz, and corresponding wavelengths of 100,000 to 10,000 kilometers, respectively. In atmospheric science, an alternative definition is usually given, from 3 Hz to 3 kHz. In the related magnetosphere science, the lower frequency electromagnetic oscillations (pulsations occurring below ~3 Hz) are considered to lie in the ULF range, which is thus also defined differently from the ITU radio bands. ELF radio waves are generated by lightning and natural disturbances in Earth's magnetic field, so they are a subject of research by atmospheric scientists. Because of the difficulty of building antennas that can radiate such long waves, ELF frequencies have been used in only a very few man-made communication systems. ELF waves can penetrate seawater, which makes them useful in communication with submarines, and a few nations have built military ELF transmitters to transmit signals to their submerged submarines, consisting of huge grounded wire antennas (ground dipoles) 15 - 60 km long driven by transmitters producing megawatts of power. The US, Russia, India, and China are the only nations known to have constructed these ELF communication facilities. The U.S. facilities were used between 1985 and 2004 but are now decommissioned. ELF is a subradio frequency. Some medical peer reviewed journal articles refer to ELF in the context of 'extremely low frequency (ELF) magnetic fields (MF)' with frequencies of 50 Hz and 50–80 Hz. United StatesGovernment agencies, such as NASA, describe ELF as non-ionizing radiation with frequencies between 0 and 300 Hz. The World Health Organization (WHO) have used ELF to refer to the concept of 'extremely low frequency (ELF) electric and magnetic fields (EMF)' The WHO also stated that at frequencies between 0 and 300 Hz, 'the wavelengths in air are very long (6000 km at 50 Hz and 5000 km at 60 Hz), and, in practical situations, the electric and magnetic fields act independently of one another and are measured separately.' Due to their extremely long wavelength, ELF waves can diffract around large obstacles, and are not blocked by mountain ranges or the horizon and can travel around the curve of the Earth. ELF and VLF waves propagate long distances by an Earth-ionosphere waveguide mechanism. The Earth is surrounded by a layer of charged particles (ions and electrons) in the atmosphere at an altitude of about 60 km at the bottom of the ionosphere, called the D layer which reflects ELF waves. The space between the conductive Earth's surface and the conductive D layer acts as a parallel-plate waveguide which confines ELF waves, allowing them to propagate long distances without escaping into space. In contrast to VLF waves, the height of the layer is much less than one wavelength at ELF frequencies, so the only mode that can propagate at ELF frequencies is the TEM mode in vertical polarization, with the electric field vertical and the magnetic field horizontal. ELF waves have extremely low attenuation of 1–2 dB per 1000 km, giving a single transmitter the potential to communicate worldwide.