At any given moment about 2,000 thunderstorms roll over Earth, producing
some 50 flashes of lightning every second. Each lightning burst creates
electromagnetic waves that begin to circle around Earth captured
between Earth's surface and a boundary about 60 miles up. Some of the
waves - if they have just the right wavelength - combine, increasing in
strength, to create a repeating atmospheric heartbeat known as Schumann
resonance. This resonance provides a useful tool to analyze Earth's
weather, its electric environment, and to even help determine what types
of atoms and molecules exist in Earth's atmosphere.
The waves
created by lightning do not look like the up and down waves of the
ocean, but they still oscillate with regions of greater energy and
lesser energy. These waves remain trapped inside an atmospheric ceiling
created by the lower edge of the "ionosphere" - a part of the atmosphere
filled with charged particles, which begins about 60 miles up into the
sky. In this case, the sweet spot for resonance requires the wave to be
as long (or twice, three times as long, etc) as the circumference of
Earth. This is an extremely low frequency wave that can be as low as 8
Hertz (Hz) - some one hundred thousand times lower than the lowest
frequency radio waves used to send signals to your AM/FM radio. As this
wave flows around Earth, it hits itself again at the perfect spot such
that the crests and troughs are aligned. Voila, waves acting in
resonance with each other to pump up the original signal.
While
they'd been predicted in 1952, Schumann resonances were first measured
reliably in the early 1960s. Since then, scientists have discovered that
variations in the resonances correspond to changes in the seasons,
solar activity, activity in Earth's magnetic environment, in water
aerosols in the atmosphere, and other Earth-bound phenomena.
See: Schumann resonance animation
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's surface and the
ionosphere.
Description
This global electromagnetic resonance phenomenon is named after physicist
Winfried Otto Schumann
who predicted it mathematically in 1952. Schumann resonances occur
because the space between the surface of the Earth and the conductive
ionosphere acts as a closed
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 electric currents in
lightning. Schumann resonances are the principal background in the
electromagnetic spectrum
[1] beginning at 3 Hz and extend to 60 Hz,
[2] and appear as distinct peaks at extremely low frequencies (ELF) around 7.86 (fundamental),
[3] 14.3, 20.8, 27.3 and 33.8 Hz.
[4][5]
In the normal mode descriptions of Schumann resonances, the
fundamental mode 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 7.86 Hz, but this frequency can vary slightly from a
variety of factors, such as solar-induced perturbations to the
ionosphere, which comprises the upper wall of the closed cavity
[citation needed]. The higher resonance modes are spaced at approximately 6.5 Hz intervals
[citation needed],
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.
[citation needed]
Observations of Schumann resonances have been 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 water vapor
in the upper troposphere. It has been speculated that extraterrestrial
lightning (on other planets) may also be detected and studied by means
of their Schumann resonance signatures. Schumann resonances have been
used to study the lower ionosphere on Earth and it has been suggested as
one way to explore the lower ionosphere 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.
History
The first documented observations of global electromagnetic resonance were made by
Nikola Tesla
at his Colorado Springs laboratory in 1899. This observation led to
certain peculiar conclusions about the electrical properties of the
Earth, and which made the basis for his idea for
wireless energy transmission.
[6]
Tesla researched ways to transmit power and energy wirelessly over long distances (via
transverse waves and
longitudinal waves). He transmitted extremely low frequencies through the ground as well as between the Earth's surface and the
Kennelly-Heaviside layer.
He received patents on wireless transceivers that developed standing
waves by this method. Making mathematical calculations based on his
experiments, Tesla discovered that the resonant frequency of the Earth
was approximately 8 hertz (Hz).
[citation needed]
In the 1950s, researchers confirmed that the resonant frequency of the
Earth's ionospheric cavity was in this range (later named the Schumann
resonance).
The first suggestion that an ionosphere existed, capable of trapping
electromagnetic waves, was made by
Heaviside and Kennelly in 1902.
[7][8] It took another twenty years before
Edward Appleton and Barnett in 1925,
[9] were able to prove experimentally the existence of the ionosphere.
Although some of the most important mathematical tools for dealing with spherical
waveguides were developed by
G. N. Watson in 1918,
[10] it was
Winfried Otto Schumann who first studied the theoretical aspects of the global resonances of the earth–ionosphere
waveguide system, known today as the Schumann resonances. In 1952–1954 Schumann, together with
H. L. König, attempted to measure the resonant frequencies.
[11][12][13][14] However, it was not until measurements made by Balser and Wagner in 1960–1963
[15][16][17][18][19]
that adequate analysis techniques were available to extract the
resonance information from the background noise. Since then there has
been an increasing interest in Schumann resonances in a wide variety of
fields.
Basic theory
Lightning discharges are considered to be the primary natural source
of Schumann resonance excitation; lightning channels behave like huge
antennas that radiate
electromagnetic energy at frequencies below about 100 kHz.
[20] These signals are very weak at large distances from the lightning source, but the Earth–ionosphere
waveguide behaves like a
resonator at ELF frequencies and amplifies the spectral signals from lightning at the resonance frequencies.
[20]
In an ideal cavity, the
resonant frequency of the
-th mode
is determined by the
Earth radius and the
speed of light .
[11]
The real Earth–ionosphere
waveguide is not a perfect electromagnetic resonant cavity. Losses due to finite ionosphere
electrical conductivity
lower the propagation speed of electromagnetic signals in the cavity,
resulting in a resonance frequency that is lower 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
produce other effects in the Schumann resonance power spectra.
Measurements
Today Schumann resonances are recorded at many separate research
stations around the world. The sensors used to measure Schumann
resonances typically consist of two horizontal
magnetic inductive coils for measuring the north-south and east-west components of the
magnetic field, and a vertical electric dipole antenna for measuring the vertical component of the
electric field.
A typical passband of the instruments is 3–100 Hz. The Schumann
resonance electric field amplitude (~300 microvolts per meter) is much
smaller than the
static fair-weather electric field (~150 V/m) in the
atmosphere. Similarly, the amplitude of the Schumann resonance magnetic field (~1 picotesla) is many
orders of magnitude smaller than the
Earth magnetic field (~30–50 microteslas).
[21]
Specialized receivers and antennas are needed to detect and record
Schumann resonances. The electric component is commonly measured with a
ball antenna, suggested by Ogawa et al., in 1966,
[22] connected to a high-impedance
amplifier. The magnetic
induction coils typically consist of tens- to hundreds-of-thousands of turns of wire wound around a core of very high
magnetic permeability.
Dependence on global lightning activity
From the very beginning of Schumann resonance studies, it was known
that they could be used to monitor global lightning activity. At any
given time there are about 2000
thunderstorms around the
globe.
[23] Producing ~50 lightning events per
second,
[24] 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,
based on the known properties of lightning
climatology. An alternative approach is placing the receiver at the North or South Pole, which remain approximately
equidistant from the main thunderstorm centers during the day.
[25] One method not requiring preliminary assumptions on the lightning distribution
[26]
is based on the decomposition of the average background Schumann
resonance spectra, utilizing ratios between the average electric and
magnetic spectra and between their linear combination. This technique
assumes the cavity is spherically symmetric and therefore does not
include known cavity asymmetries that are believed to affect the
resonance and propagation properties of electromagnetic waves in the
system.
Diurnal variations
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
is independent of the direction of the source relative to the observer,
and is therefore a measure of global lightning. The diurnal behavior of
the vertical electric field shows three distinct 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.
"Chimney" ranking
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. Schumann
resonance observations made from Europe show a greater contribution from
Asia than from South America. This contradicts optical satellite and
climatological lightning data that show the South American thunderstorm
center stronger than the Asian center.,
[24]
although observations made from North America indicate the dominant
contribution comes from South America. The reason for such disparity
remains unclear, but may have something to do with the 60 Hz cycling of
electricity used in North America (60 Hz being a mode of Schumann
Resonance). Williams and Sátori
[27]
suggest that in order to obtain "correct" Asia-America chimney ranking,
it is necessary to remove the influence of the day/night variations in
the ionospheric conductivity (day-night asymmetry influence) from the
Schumann resonance records. On the other hand, such "corrected" records
presented in the work by Sátori et al.
[28]
show that even after the removal of the day-night asymmetry influence
from Schumann resonance records, the Asian contribution remains greater
than American. Similar results were obtained by Pechony et al.
[29]
who calculated Schumann resonance fields from satellite lightning data.
It was assumed that the distribution of lightning in the satellite maps
was a good proxy for Schumann excitations sources, even though
satellite observations predominantly measure in-cloud lightning rather
than the cloud-to-ground lightning that are the primary exciters of the
resonances. Both simulations – those neglecting the day-night asymmetry,
and those taking this asymmetry into account, showed same Asia-America
chimney ranking. As for today, the reason for the "invert" ranking of
Asia and America chimneys in Schumann resonance records remains unclear
and the subject requires further, targeted research.
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.
[15] It was concluded that no particular systematic variations of the ionosphere (which serves as the upper
waveguide boundary) are needed to explain these variations.
[30]
Subsequent theoretical studies supported the early estimations of the
small influence of the ionosphere day-night asymmetry (difference
between day-side and night-side ionosphere conductivity) on the observed
variations in Schumann resonance field intensities.
[31]
The interest in the influence of the day-night asymmetry in the
ionosphere conductivity on Schumann resonances gained new strength in
the 1990s, after publication of a work by Sentman and Fraser.
[32]
Sentman and Fraser developed a technique to separate the global and the
local contributions to the observed field power variations using
records obtained
simultaneously
at two stations that were widely separated in longitude. They
interpreted the diurnal variations observed at each station in terms of a
combination of a diurnally varying global excitation modulated by the
local ionosphere height. Their work, which combined both observations
and energy conservation arguments, convinced many scientists of the
importance of the ionospheric day-night asymmetry and inspired numerous
experimental studies. However, recently it was shown that results
obtained by Sentman and Fraser can be approximately simulated with a
uniform model (without taking into account ionosphere day-night
variation) and therefore cannot be uniquely interpreted solely in terms
of ionosphere height variation.
[33]
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. There are records showing almost clock-like accuracy of the
diurnal amplitude changes.
[28] On the other hand there are numerous days when Schumann Resonance amplitudes do not increase at
sunrise or do not decrease at
sunset. There are studies showing that the general behavior of Schumann resonance
amplitude records can be recreated from diurnal and seasonal
thunderstorm migration, without invoking ionospheric variations.
[29][31]
Two recent independent theoretical studies have shown that the
variations in Schumann resonance power related to the day-night
transition are much smaller than those associated with the peaks of the
global lightning activity, and therefore the global lightning activity
plays a more important role in the variation of the Schumann resonance
power.
[29][34]
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. Part of this controversy stems from the fact
that the Schumann resonance parameters extractable from observations
provide only a limited amount of information about the coupled lightning
source-ionospheric system geometry. The problem of inverting
observations to simultaneously infer both the lightning source function
and ionospheric structure is therefore extremely underdetermined,
leading to the possibility of nonunique interpretations.
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
because the mean rate of excitation by lightning, ~50 lightning events
per second globally, mixes up the individual contributions together.
However, occasionally there occur extremely large lightning flashes
which produce distinctive signatures that stand out from the background
signals. Called "Q-bursts", they are produced by intense lightning
strikes that transfer large amounts of charge from clouds to the ground,
and often carry high peak current.
[22] Q-bursts can exceed the
amplitude of the background signal level by a factor of 10 or more, and appear with intervals of ~10 s,
[26]
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.
[35]
showed 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. Recent observations
[35][36]
reveal that occurrences of sprites and Q bursts are highly correlated
and Schumann resonances data can possibly be used to estimate the global
occurrence rate of sprites.
[37]
Global temperature
Williams [1992]
[38]
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.
[38] The
nonlinearity of the lightning-to-temperature relation provides a natural
amplifier
of the temperature changes and makes Schumann resonance a sensitive
"thermometer". Moreover, the ice particles that are believed to
participate in the electrification processes which result in a lightning
discharge
[39]
have an important role in the radiative feedback effects that influence
the atmosphere temperature. Schumann resonances may therefore help us
to understand these
feedback effects. A strong link between global lightning and global temperature has not been experimentally confirmed as of 2008.
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,
[40] but whether this impact is a positive, or a negative
feedback is still uncertain.
[41]
The main challenge in addressing this question is the difficulty in
monitoring UTWV globally over long timescales. Continental
deep-convective
thunderstorms produce most of the lightning discharges on Earth. In addition, they transport large amount of
water vapor into the upper
troposphere, dominating the variations of global UTWV. Price [2000]
[42]
suggested that changes in the UTWV can be derived from records of
Schumann Resonances. According to the effective work made by the Upper
Tropospheric Water Vapor (( UTWV )), we should highlight that the
percentage of UTWV in normal condition of the Air mass can be meauserd
as a minimal quantity, so that its influence can be considered very very
low; in fact the higher percentage of it can be only found in the lower
Tropspheric layers. But in the case of a high quantity of UTWV in the
highest level of Troposphere, due to a warmer air mass of atlantic
origins, for istance, the Water vapor, due to the low air temperature
((about minus 60 Degrees )) it turns into ice cristal, becoming clouds
as Cirrus or Cirrus Stratus: no Water vapour exists as gas with so low
temperature. So, we can say that the affirmation that Water vapor
interacts with cloud, can be considered wrong as the clouds both those
of low level of ((Atmosphere)) and those of higher levels of it are made
of condensed or cristallised Water Vapor.
Schumann resonances on other planets
The existence of Schumann-like resonances is conditioned primarily by two factors: (1) a closed, planetary-sized spherical
[dubious – discuss]
cavity, consisting of conducting lower and upper boundaries separated
by an insulating medium. For the earth the conducting lower boundary is
its surface, and the upper boundary is the ionosphere. Other planets may
have similar electrical conductivity geometry, so it is speculated that
they should possess similar resonant behavior. (2) source of electrical
excitation of
electromagnetic waves in the ELF range. 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 in situ capability exists today to validate the results,
but in the case of Mars there have been terrestrial observations of
radio emission spectra that have been associated with Schumann
resonances.
[43]
The reported radio emissions are not of the primary electromagnetic
Schumann modes, but rather of secondary modulations of the nonthermal
microwave emissions from the planet at approximately the expected
Schumann frequencies, and have not been independently confirmed to be
associated with lightning activity on Mars. There is the possibility
that future lander missions could carry in situ instrumentation to
perform the necessary measurements. Theoretical studies are 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. Theoretical calculations of the Schumann resonances
at Venus were reported by Nickolaenko and Rabinowicz [1982]
[44] and Pechony and Price [2004].
[45] Both studies yielded very close results, indicating that Schumann resonances should be easily detectable on that
planet given a lightning source of excitation and a suitably located sensor.
On
Mars detection of lightning activity has been reported by Ruf et al. [2009].
[43]
The evidence is indirect and in the form of modulations of the
nonthermal microwave spectrum at approximately the expected Schumann
resonance frequencies. It has not been independently confirmed that
these are associated with electrical discharges on Mars. In the event
confirmation is made by direct, in situ observations, it would verify
the suggestion of the possibility of charge separation and lightning
strokes in the Martian dust storms made by Eden and Vonnegut [1973]
[46] and Renno et al. [2003].
[47] Martian global resonances were modeled by Sukhorukov [1991],
[48] Pechony and Price [2004]
[45] and Molina-Cuberos et al. [2006].
[49]
The results of the three studies are somewhat different, but it seems
that at least the first two Schumann resonance modes should be
detectable. Evidence of the first three Schumann resonance modes is
present in the spectra of radio emission from the lightning detected in
Martian dust storms.
[43]
It was long ago suggested that lightning discharges may occur on Titan,
[50] 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.
Schumann resonances on Titan have received more attention than on any
other
celestial body, in works by Besser et al. [2002],
[51] Morente et al. [2003],
[52] Molina-Cuberos et al. [2004],
[53] Nickolaenko et al. [2003]
[54] and Pechony and Price [2004].
[45] It appears that only the first Schumann resonance mode might be detectable on
Titan.
Jupiter
is the only planet where lightning activity has been optically
detected. Existence of lightning activity on that planet was predicted
by Bar-Nun [1975]
[55] and it is now supported by data from
Galileo,
Voyagers 1 and 2,
Pioneers 10 and 11 and Cassini.
Saturn is also expected to have intensive lightning activity, but the three visiting spacecrafts –
Pioneer 11 in 1979,
Voyager 1 in 1980 and
Voyager 2 in 1981, failed to provide any convincing evidence from optical observations. The strong storm monitored on
Saturn by the
Cassini spacecraft
produced no visible lightning flashes, although electromagnetic sensors
aboard the spacecraft detected signatures that are characteristic of
lightning. Little is known about the electrical parameters of
Jupiter and
Saturn interior. Even the question of what should serve as the lower
waveguide boundary is a non-trivial one in case of the gaseous planets. There seem to be no works dedicated to Schumann resonances on
Saturn. To date there has been only one attempt to model Schumann resonances on
Jupiter.
[56]
Here, the electrical conductivity profile within the gaseous atmosphere
of Jupiter was calculated using methods similar to those used to model
stellar interiors, and it was pointed out that the same methods could be
easily extended to the other gas giants Saturn, Uranus and Neptune.
Given the intense lightning activity at Jupiter, the Schumann resonances
should be easily detectable with a sensor suitably positioned within
the planetary-ionospheric cavity.
Speculation about Schumann resonance effects in non-geophysics domains
Interest in Schumann resonances extends beyond the domain of
geophysics where it initially began, to the fields of
bioenergetics[57] and
acupuncture.
[57] Critics
[who?] claim that the studies that support these applications are inconclusive and that further studies are needed.
A small study in Japan found that blood pressure was lowered by the
Schumann resonance, with the effects on human health needing to be
investigated further.
[58]
See also
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