The Schumann resonances (SR) are a set of spectrum peaks in the extremely low frequency (ELF) portion of the Earth's electromagnetic field spectrum. Schumann resonances are global electromagnetic resonances, excited by lightning discharges in the cavity formed by the Earth surface and the ionosphere.
This global electromagnetic resonance phenomenon is named after physicist Winfried Otto Schumann who predicted it mathematically in 1952. Schumann resonance occurs because the space between the surface of the Earth and the conductive ionosphere acts as a waveguide.
The limited dimensions of the Earth cause this waveguide to act as a resonant cavity for electromagnetic waves in the ELF band. The cavity is naturally excited by energy from lightning strikes. Schumann resonance modes are observed in the power spectra of the natural electromagnetic background noise, as separate peaks at extremely low frequencies (ELF) around 7.8, 14.3, 20.8, 27.3 and 33.8 Hz.
The fundamental mode of the Schumann resonance is a standing wave in the Earth-ionosphere cavity with a wavelength equal to the circumference of the Earth. This lowest-frequency (and highest-intensity) mode of the Schumann resonance occurs at a frequency of approximately 7.8 Hz.
Further resonance modes appear at approximately 6.5 Hz intervals, a characteristic attributed to the atmosphere's spherical geometry. The peaks exhibit a spectral width of approximately 20% on account of the damping of the respective modes in the dissipative cavity. The eighth overtone lies at approximately 59.9 Hz.
Schumann resonances are used to track global lightning activity. Owing to the connection between lightning activity and the Earth's climate it has been suggested that they may also be used to monitor global temperature variations and variations of upper water vapor.
It has been speculated that extraterrestrial lightning (on other planets) may also be detected and studied with Schumann resonances. Schumann resonances have been used for research and monitoring of the lower ionosphere on Earth and was suggested for exploration of lower ionosphere parameters on celestial bodies. Effects on Schumann resonances have been reported following geomagnetic and ionospheric disturbances.
More recently, discrete Schumann resonance excitations have been linked to transient luminous events: sprites, elves, jets, and other upper-atmospheric lightning. A new field of interest using Schumann resonances is related to short-term earthquake prediction. Schumann resonances have evolved beyond the domain of geophysics where it initially began, and has since gained interest in medicine, from artists and musicians, as well as from fields such as bioenergetics, acupuncture, and psychobiology.
The idea that natural Schumann resonances, which are many orders of magnitude weaker than both the artificial fields in such studies and typical environmental fields, could yield similar effects is conjectural and highly controversial (to mainstream neurology researchers).
The first suggestion that an ionosphere existed, capable of trapping electromagnetic waves, was made by Heaviside and Kennelly in 1902. It took another twenty years before Edward Appleton and Barnett in 1925, were able to prove experimentally the existence of the ionosphere.
Lightning discharges are considered as the primary natural source of Schumann resonances. Lightning channels behave like a huge antenna which radiates electromagnetic energy as impulsive signals at frequencies below about 100 kHz.
In an ideal cavity, the resonant frequency of the n-th mode fn is determined by the Earth radius, r, and the speed of light, c:
fn = c / (2 pi r)
The real Earth-ionosphere waveguide is not a perfect electromagnetic cavity. Losses due to finite ionosphere electrical conductivity lower the propagation speed of electromagnetic signals in the cavity, resulting in a lowered resonance frequency than would be expected in an ideal case, and the observed peaks are wide. In addition there are a number of horizontal asymmetries, day-night difference in the height of the ionosphere, latitudinal changes in the Earth magnetic field, sudden ionospheric disturbances, polar cap absorption, etc. that complicate the Schumann resonance power spectra.
Today Schumann resonances are recorded at many separate research stations around the world. The electromagnetic sensors used to measure Schumann resonances typically consist of two horizontal magnetic induction coils for receiving the magnetic field in the north-south and the east-west direction and one vertical electrica dipole antenna for observing the vertical electric field.
Global lightning activity
From the very beginning of Schumann resonance studies, they were used to monitor global lightning activity by tracking changes in Schumann resonance field intensities. At any given time there are about 2000 thunderstorms around the globe. Producing ~50 lightning events per second, these thunderstorms create the background Schumann resonance signal.
Determining the spatial lightning distribution from Schumann resonance records is a complex problem: in order to estimate the lightning intensity from Schumann resonance records it is necessary to account for both the distance to lightning sources as well as the wave propagation between the source and the observer. The common approach is to make a preliminary assumption on the spatial lightning distribution, basing on the known properties of lightning climatology.
The best documented and the most debated features of the Schumann resonance phenomenon are the diurnal variations of the background Schumann resonance power spectrum.
A characteristic Schumann resonance diurnal record reflects the properties of both global lightning activity and the state of the earth-ionosphere cavity between the source region and the observer. The vertical electric field, which is equally sensitive in all directions and therefore measures the global lightning, shows three dominant maxima, associated with the three "hot spots" of planetary lightning activity: 9 UT (Universal Time) peak, linked to the increased thunderstorm activity from south-east Asia; 14 UT peak associated with the peak in African lightning activity; and the 20 UT peak resulting for the increase in lightning activity in South America. The time and amplitude of the peaks vary throughout the year, reflecting the seasonal changes in lightning activity.
In general, the African peak is the strongest, reflecting the major contribution of the African "chimney" to the global lightning activity. The ranking of the two other peaks - Asian and American - is the subject of a vigorous dispute among Schumann resonance scientists.
Influence of the day-night asymmetry
In the early literature the observed diurnal variations of Schumann resonance power were explained by the variations in the source-receiver (lightning-observer) geometry. It was concluded that no particular systematic variations of the ionosphere (which serves as the upper waveguide boundary) are needed to explain these variations.
The interest in the influence of the day-night asymmetry in the ionosphere conductivity on Schumann resonances gained a new strength in the 1990s, after publication of a work by Sentman and Fraser.
Schumann resonance amplitude records show significant diurnal and seasonal variations which in general coincide in time with the times of the day-night transition (the terminator). This time-matching seems to support the suggestion of a significant influence of the day-night ionosphere asymmetry on Schumann resonance amplitudes.
It is generally acknowledged that source-observer effects are the dominant source of the observed diurnal variations, but there remains considerable controversy about the degree to which day-night signatures are present in the data.
The "inverse problem"One of the interesting problems in Schumann resonances studies is determining the lightning source characteristics (the "inverse problem"). Temporally resolving each individual flash is impossible,[clarify] but there are intense ELF transient events, also named "Q bursts."
Q-bursts are triggered by intense lightning strikes, associated with a large charge transfer and often high peak current. Q-bursts can exceed the amplitude of the background signal level by a factor of 10 or more, and appear with intervals of ~10sec, which allows to consider them as isolated events and determine the source lightning location.
The source location is determined with either multi-station or single-station techniques, and requires assuming a model for the earth-ionosphere cavity. The multi-station techniques are more accurate, but require more complicated and expensive facilities.
Transient luminous events research
It is now believed that many of the Schumann resonances transients (Q bursts) are related to the transient luminous events (TLEs). In 1995 Boccippio et al. suggested that sprites, the most common TLE, are produced by positive cloud-to-ground lightning occurring in the stratiform region of a thunderstorm system, and are accompanied by Q-burst in the Schumann resonances band.
Climate change research
Global climate change is the subject of intense debate and concern. One of the important aspects in understanding global climate change is the development of tools and techniques that would allow continuous and long-term monitoring of processes affecting the global climate. It has been suggested that Schumann resonances are one of only a few tools that can provide such global information reliably and cheaply.
Williams  suggested that global temperature may be monitored with the Schumann resonances. The link between Schumann resonance and temperature is lightning flash rate, which increases nonlinearly with temperature. The nonlinearity of the lightning-to-temperature relation provides a natural amplifier of the temperature changes and makes Schumann resonance a sensitive "thermometer".
Upper tropospheric water vapor
Tropospheric water vapor is a key element of the Earth's climate, which has direct effects as a greenhouse gas, as well as indirect effect through interaction with clouds, aerosols and tropospheric chemistry. Upper tropospheric water vapor (UTWV) has a much greater impact on the greenhouse effect than water vapor in the lower atmosphere, but whether this impact is a positive or a negative feedback is still uncertain.
Existence of Schumann resonances is conditioned primarily by two factors:
Within the Solar System there are five candidates for Schumann resonance detection besides the Earth: Venus, Mars, Jupiter, Saturn and its moon Titan.
Modeling Schumann resonances on the planets and moons of the Solar System is complicated by the lack of knowledge of the waveguide parameters. No capability to validate the results exists today, but in the case of Mars there exists the possibility that future lander missions could carry instrumentation to perform the necessary measurements. Theoretical studies are therefore primarily directed to parameterizing the problem for future planetary explorers.
The strongest evidence for lightning on Venus comes from the impulsive electromagnetic waves detected by Venera 11 and 12 landers. Schumann resonances on Venus were studied by Nickolaenko and Rabinowicz  and Pechony and Price . Both studies yielded very close results, indicating that Schumann resonances should be easily detectable on this planet given a lightning source of excitation and a suitably located sensor.
On Mars, lightning activity has not been detected, but charge separation and lightning strokes are considered possible in the Martian dust storms. Martian global resonances were modeled by Sukhorukov , Pechony and Price  and Molina-Cuberos et al.  . The results of the three studies are somewhat different, but it seems that at least the first two Schumann resonance modes should be detectable.
It was long ago suggested that lightning dischargers may occur on Titan, but recent data from Cassini-Huygens seems to indicate that there is no lightning activity on this largest satellite of Saturn. Due to the recent interest in Titan, associated with the Cassini-Huygens mission, its ionosphere is perhaps the most thoroughly modeled today.
Jupiter is the only planet where lightning activity has been optically detected. Existence of lightning activity on this planet was predicted by Bar-Nun  and it is now supported by data from Galileo, Voyagers 1 and 2, Pioneers 10 and 11 and Cassini.