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Atmospheric electricity is the pattern of electrical charges in the Earth's atmosphere (or less commonly, that of another planet). The normal movement of electric charges among the Earth's surface, the various layers of the atmosphere, and especially the ionosphere, taken together, are known as the global atmospheric electrical circuit. Much of the reasoning required to explain these currents lies within the field of electrostatics, but also requires understanding of other disciplines within Earth science.
Eliminating, for the moment, consideration of the extremely dense charge populations that exist in the upper reaches of the atmosphere, a region called the ionosphere, filled with hot, dense, plasma gas whose ions give the ionosphere its name, we note that there is always some amount of unbound positive and negative, but net positive, electric charge in the atmosphere closest to the surface of the negatively charged Earth on a 'fine day'. When days are not so 'fine', the net unbound charge that exists in the clouds of thunderstorms can be exceedingly negative.
The 'fine day' net positive charge sets up an electric field between the negative Earth and the net positive charge in the air, and this electric field stores electrical energy. The positive charge acts by induction on the earth and electromagnetic devices.
Experiments have shown that the intensity of this electric field is greater in the middle of the day than at morning or night and is also greater in winter than in summer. In 'fine weather', the potential, aka 'voltage', increases with altitude at about 30 volts per foot (100 V/m), when climbing against the gradient of the electric field. This electric field gradient continues up into the atmosphere to a point where the voltage reaches its maximum, in the neighborhood of 300,000 volts. This occurs at approximately 30–50 km above the Earth's surface. From that point in the atmosphere up to its outer limit, nearly 1,000 km, the electric field gradient produced in the lower atmosphere either ceases or has reversed.
Global daily cycles, with a minimum and a peak at roughly 16:00 hours later, were researched by the Carnegie Institution of Washington in the 20th century. This Carnegie curve variation has been described as "the fundamental electrical heartbeat of the planet".
The phenomena characterizing atmospheric electricity are of at least three kinds. There are thunderstorms, which create lightning bolts that 'instantaneously' discharge huge amounts of atmospheric charge to ground in a rapid release of energy stored in the electric field that built up to a particularly extreme degree in the storm clouds. There is a related phenomenon of continual electrification (re-charging) of the air in the lower atmosphere. A third phenomenon is that of the polar auroras.
Most authorities agree that whatever may be the origin of the net unbound positive charge in the atmosphere, the generation of enormous currents (flow of electrons, negative charges), that flow between clouds and ground during a lightning discharge, begins with condensation of water vapor within the clouds; each minute water droplet moving through the air collects upon its surface a certain amount of negative charge by collecting 'free' electrons. As these tiny drops coalesce into larger drops, and still larger drops, there is a corresponding decrease in the total exposed surface upon which the collected electronic charges can be carried, raising the negative voltage as droplets combine. This is because an object's potential rises as the electrical capacitance of the object holding the charge is decreased. The combined negative electric potential of all the coalescing water drops rises until it overcomes the breakdown voltage of the, usually non-conductive, air, and jumps to earth as a lightning bolt. The similarity of lightning to the discharge of accumulated electrons developed on an electrical machine was demonstrated by Franklin in his memorable kite experiments.
- 1 History
- 2 Description
- 3 Historical research
- 4 Lightning research
- 5 See also
- 6 References and external articles
- 7 Further reading
- 8 External links
The detonating sparks drawn from electrical machines and from Leyden jars suggested to the early experimenters, Hauksbee, Newton, Wall, Nollet, and Gray, that lightning and thunder were due to electric discharges. In 1708, Dr. William Wall was one of the first to observe that spark discharges resembled miniature lightning, after observing the sparks from a charged piece of amber.
In the middle of the 18th century, Benjamin Franklin's experiments showed that electrical phenomena of the atmosphere were not fundamentally different from those produced in the laboratory. By 1749, Franklin observed lightning to possess almost all the properties observable in electrical machines.
In July 1750, Franklin hypothesized that electricity could be taken from clouds via a tall metal aerial with a sharp point. Before Franklin could carry out his experiment, in 1752 Thomas-François Dalibard erected a 40-foot (12 m) iron rod at Marly-la-Ville, near Paris, drawing sparks from a passing cloud. With ground-insulated aerials, an experimenter could bring a grounded lead with an insulated wax handle close to the aerial, and observe a spark discharge from the aerial to the grounding wire. In May 1752, Dalibard affirmed that Franklin's theory was correct.
Franklin listed the following similarities between electricity and lightning:
- producing light of a similar color;
- rapid motion;
- being conducted by metals, water and ice;
- melting metals and igniting inflammable substances;
- "sulfurous" smell (which is now known to be due to ozone);
- magnetizing needles;
- the similarity between St. Elmo's Fire and glow discharge.
Around June 1752, Franklin reportedly performed his famous kite experiment. The kite experiment was repeated by Romas, who drew from a metallic string sparks 9 feet (2.7 m) long, and by Cavallo, who made many important observations on atmospheric electricity. L. G. Lemonnier (1752) also reproduced Franklin's experiment with an aerial, but substituted the ground wire with some dust particles (testing attraction). He went on to document the fair weather condition, the clear-day electrification of the atmosphere, and the diurnal variation of the atmosphere's electricity. G. Beccaria (1775) confirmed Lemonnier's diurnal variation data and determined that the atmosphere's charge polarity was positive in fair weather. H. B. Saussure (1779) recorded data relating to a conductor's induced charge in the atmosphere. Saussure's instrument (which contained two small spheres suspended in parallel with two thin wires) was a precursor to the electrometer. Saussure found that the fair weather condition had an annual variation, and found that there was a variation with height, as well. In 1785, Coulomb discovered the electrical conductivity of air. His discovery was contrary to the prevailing thought at the time, that the atmospheric gases were insulators (which they are to some extent, or at least not very good conductors when not ionized). His research was, unfortunately, completely ignored. P. Erman (1804) theorized that the Earth was negatively charged. J. C. A. Peltier (1842) tested and confirmed Erman's idea. Lord Kelvin (1860s) proposed that atmospheric positive charges explained the fair weather condition and, later, recognized the existence of atmospheric electric fields.
Over the course of the next century, using the ideas of Alessandro Volta and Francis Ronald, several researchers contributed to the growing body of knowledge about atmospheric electrical phenomena. With the invention of the portable electrometer and Lord Kelvin's 19th century water-dropping condenser, a greater level of precision was introduced into observational results. Towards the end of the 19th century came the discovery by W. Linss (1887) that even the most perfectly insulated conductors lose their charge, as Coulomb before him had found, and that this loss depended on atmospheric conditions. H. H. Hoffert (1888) identified individual lightning downward strokes using early cameras and would report this in "Intermittent Lightning-Flashes". J. Elster and H. F. Geitel, who also worked on thermionic emission, proposed a theory to explain thunderstorms' electrical structure (1885) and, later, discovered atmospheric radioactivity (1899). By then it had become clear that freely charged positive and negative ions were always present in the atmosphere, and that radiant emanations could be collected. F. Pockels (1897) estimated lightning current intensity by analyzing lightning flashes in basalt (c. 1900) and studying the left-over magnetic fields.
Using a Peltier electrometer, Luigi Palmieri researched atmospheric electricity. Nikola Tesla and Hermann Plauson investigated the production of energy and power via atmospheric electricity. Tesla also proposed to use the atmospheric electrical circuit to transceive wireless energy over large distances. The Polish Polar Station, Hornsund, has researched the magnitude of the Earth's electric field and recorded its vertical component. Discoveries about the electrification of the atmosphere via sensitive electrical instruments and ideas on how the Earth’s negative charge is maintained were developed mainly in the 20th century. Whilst a certain amount of observational work has been done in the branches of atmospheric electricity, the science has not developed to a considerable extent. Up to the contemporary day, apparatus which extract industrial energy from atmospheric electricity have not been built and maintained.
Atmospheric electricity abounds in the environment above the surface of Earth. While some traces of it are found less than a metre above the land and water surfaces, on attaining greater height, it becomes more apparent. In general, during fine weather, the air above the surface of Earth is positively charged, while the Earth's surface charge is negative.
Additionally, the presence of electrical action in Earth's atmosphere, due to the accumulation of static charges, generated by friction of the air upon itself, released to ground in an instant, in a massive down-rush of current (moving electronic charges) the moment the accumulated charge exceeds the breakdown voltage of air, accounts for the phenomenon of lightning.
Other sources of atmospheric charge include evaporation of water from Earth's surface, chemical reactions which take place upon Earth's surface that release charged particles into the atmosphere, and expansion or condensation of moisture contained in the atmosphere due to variation of temperature.
The measurements of atmospheric electricity can be seen as measurements of difference of potential between a point of the Earth's surface, and a point somewhere in the air above it. The atmosphere in different regions is often found to be at different local potentials, which differ from that of the earth sometimes even by as much as 3000 volts within 100 feet (30 m). The electrostatic field and the difference of potential of the earth field according to investigations, is in summer about 60 to 100 volts and in winter 300 to 500 volts per meter of difference in height, a simple calculation gives the result that when such a collector is arranged for example on the ground, and a second one is mounted vertically over it at a distance of 2000 meters and both are connected by a conducting cable, there is a difference in potential in summer of about 2,000,000 volts and in winter even of 6,000,000 volts and more.
In the upper regions of the atmosphere the air is highly rarefied, and conducts like the rarefied gases in fluorescent tubes. The lower air is, when dry, a non-conductor. The upper stratum is positively charged, while the Earth's surface is itself negatively charged; the stratum of denser air between acting like the dielectric of a capacitor in keeping the opposite charges separate. The theory of atmospheric electricity equally explains many phenomena: free electricity causes thunderstorms, and electricity of a lower voltage is manifested during a display of the aurora borealis.
Atmospheric electricity often has an effect on electrical communications. During storms, communication may be disrupted, and electrical discharges may be seen between sharp points on signalling equipment that is inadequately earthed. In old telegraph systems, the electromagnets could be physically damaged and the wiring was known to melt. More rarely, persistent stray currents can cause more subtle effects in communication systems.
There have been various speculative conjectures regarding the origin of these semi-diurnal meteorological periods, but they have been usually of a secondary character. A primary cause is clearly to be ascribed to the many complex processes which are due to the thermodynamics of radiation. It is thought that with sufficient experience the formulas that have been deduced here, and illustrated, can be made to yield other valuable data regarding the atomic and subatomic activities which are concerned in the variations of the fundamental terms and their very numerous derivatives.
Diurnal variations found by the daily indications (during fine weather) showed two maxima occurring in summer at roughly twelve hours apart and two minima which in summer were at the hours of which were roughly nine hours apart. The maxima correspond fairly with hours of changing temperature, the minima with those of constant temperature. Atmospheric electricity, considered in a general manner, attains its maximum in January, then decreases progressively until the month of June, which presents a minimum of intensity; it increases during the following months to the end of the year. The difference between the maximum and minimum is much more sensibly felt during serene weather than during cloudy weather. During the different months, the electricity of the air is more powerful when the sky is serene than when it is cloudy, except toward the months of June and July, when the electricity attains a maximum, the value of which is nearly the same, whatever be the state of the sky.
The electric intensity observed during fogs has, at a mean, almost exactly the same value as that observed during snows. This value is very high, and corresponds to the mean maxima observed for the former and the latter months of the year. A very remarkable fact, which appears from recent observation, is that moisture acts in a manner altogether different in the cold months and in the hot ones; it increases the electricity in the winter months, it diminishes it in the summer months. The fundamental fact is, that humidity acts in two manners, the effects of which tend to oppose each other. On the one hand, it facilitates the escape of the electricity accumulated in the upper regions of the atmosphere to the stratum in which the observation is made; on the other hand, it facilitates the escape into the ground of the electricity which this stratum possesses: thus, on the one hand it increases the intensity of the electric manifestations of the instrument, on the other hand it diminishes them.
Outer space and near space
In outer space, the magnetopause flows along the boundary between the region around an astronomical object (called the "magnetosphere") and surrounding plasma, in which electric phenomena are dominated or organized by this magnetic field. Earth is surrounded by a magnetosphere, as are the magnetized planets Jupiter, Saturn, Uranus and Neptune. Mercury is magnetized, but too weakly to trap plasma. Mars has patchy surface magnetization. The magnetosphere is the location where the outward magnetic pressure of the Earth's magnetic field is counterbalanced by the solar wind, a plasma. Most of the solar particles are deflected to either side of the magnetopause. However, some particles become trapped within the Earth's magnetic field and form radiation belts. The Van Allen radiation belt is a torus of energetic charged particles (i.e. a plasma) around Earth, trapped by Earth's magnetic field.
At elevations above the clouds, atmospheric electricity forms a continuous and distinct element (called the electrosphere) in which the Earth is surrounded. The electrosphere layer (from tens of kilometers above the surface of the earth to the ionosphere) has a high electrical conductivity and is essentially at a constant electric potential. The ionosphere is the inner edge of the magnetosphere and is the part of the atmosphere that is ionized by solar radiation. (Photoionisation is a physical process in which a photon is incident on an atom, ion or molecule, resulting in the ejection of one or more electrons.)
The Earth, and all living things on it, are constantly bombarded by radiation from outer space. This radiation primarily consists of positively charged ions from protons to iron and larger nuclei derived sources outside our solar system. This radiation interacts with atoms in the atmosphere to create an air shower of secondary radiation, including X-rays, muons, protons, alpha particles, pions, electrons, and neutrons. The immediate dose from cosmic radiation is largely from muons, neutrons, and electrons, and this dose varies in different parts of the world based largely on the geomagnetic field and altitude. This radiation is much more intense in the upper troposphere, around 10 km altitude, and is thus of particular concern for airline crews and frequent passengers, who spend many hours per year in this environment. During their flights airline crews typically get an extra dose on the order of 2.2 mSv (220 mrem) per year.
The Earth is constantly immersed in the solar wind, a rarefied flow of hot plasma (gas of free electrons and positive ions) emitted by the Sun in all directions, a result of the million-degree heat of the Sun's outermost layer, the solar corona. The solar wind usually reaches Earth with a velocity around 400 km/s, density around 5 ions/cc and magnetic field intensity around 2–5 nT (nanoteslas; Earth's surface field is typically 30,000–50,000 nT). These are typical values. During magnetic storms, in particular, flows can be several times faster; the interplanetary magnetic field (IMF) may also be much stronger.
The IMF originates on the Sun, related to the field of sunspots, and its field lines (lines of force) are dragged out by the solar wind. That alone would tend to line them up in the Sun-Earth direction, but the rotation of the Sun skews them (at Earth) by about 45 degrees, so that field lines passing Earth may actually start near the western edge ("limb") of the visible sun.
When the solar wind is perturbed, it easily transfers energy and material into the magnetosphere. The electrons and ions in the magnetosphere that are thus energized move along the magnetic field lines to the polar regions of the atmosphere.
Potential difference between the ionosphere and the Earth is maintained by thunderstorms' pumping action of lightning discharges. In the Earth-ionosphere cavity, the electric field and conduction current in the lower atmosphere are primarily controlled by ions. Ions have the characteristic parameters such as mobility, lifetime, and generation rate that vary with altitude.
The Schumann resonance is a set of spectrum peaks in the ELF portion of the Earth's electromagnetic field spectrum. Schumann resonance is due to the space between the surface of the Earth and the conductive ionosphere acting as a waveguide. The limited dimensions of the earth cause this waveguide to act as a resonant cavity for electromagnetic waves. The cavity is naturally excited by energy from lightning strikes.
The electrical conductivity of the atmosphere increases exponentially with altitude. The amplitudes of the electric and magnetic components depend on season, latitude, and height above the sea level. The greater the altitude the more atmospheric electricity abounds. The exosphere is the uppermost layer of the atmosphere and is estimated to be 500 km to 1000 km above the Earth's surface, and its upper boundary at about 10,000 km. The thermosphere (upper atmosphere) is the layer of the Earth's atmosphere directly above the mesosphere and directly below the exosphere. Within this layer, ultraviolet radiation causes ionization. Theories that have been proposed to explain the phenomenon of the polar aurora, but it has been demonstrated by experiments that it is due to positive charge passing from the higher regions of the atmosphere to the earth.
The mesosphere (middle atmosphere) is the layer of the Earth's atmosphere that is directly above the stratosphere and directly below the thermosphere. The mesosphere is located about 50-80/85 km above Earth's surface. The stratosphere (middle atmosphere) is a layer of Earth's atmosphere that is stratified in temperature and is situated between about 10 km and 50 km altitude above the surface at moderate latitudes, while at the poles it starts at about 8 km altitude. The stratosphere sits directly above the troposphere and directly below the mesosphere. The troposphere (lower atmosphere) is the densest layer of the atmosphere.
The planetary boundary layer (PBL), also known as the atmospheric boundary layer (ABL), is the lowest part of the atmosphere and its behavior is directly influenced by its contact with the planetary surface.
There is a potential gradient at ground level ("Atmosphere ground layer") and this vertical field corresponds to the negative charge in and near the Earth's surface. The negative potential gradient falls rapidly as altitude increases from the ground. Most of this potential gradient is in the first few kilometers. The positive potential gradient rises rapidly as altitude increases from the ground. Volta, in the 18th century, discovered that the gradient of electric potential increased as the distance from the earth increases, and, more recently, Engel has provided data to calculate the increase (image to the right).
Thunderstorms and lightning
If the quantity of water that is condensed in and subsequently precipitated from a cloud is known, then the total energy of a thunderstorm can be calculated. In an average thunderstorm, the energy released amounts to about 10,000,000 kilowatt-hours (3.6×1013 joule), which is equivalent to a 20-kiloton nuclear warhead. A large, severe thunderstorm might be 10 to 100 times more energetic.
How lightning initially forms is still a matter of debate: Scientists have studied root causes ranging from atmospheric perturbations (wind, humidity, and atmospheric pressure) to the impact of solar wind and accumulation of charged solar particles. Ice inside a cloud is thought to be a key element in lightning development, and may cause a forcible separation of positive and negative charges within the cloud, thus assisting in the formation of lightning.
An average bolt of lightning carries a negative electric current of 40 kiloamperes (kA) (although some bolts can be up to 120 kA), and transfers a charge of five coulombs and energy of 500 MJ, or enough energy to power a 100-watt lightbulb for just under two months. The voltage depends on the length of the bolt, with the dielectric breakdown of air being three million volts per meter, and lightning bolts often being several hundred meters long. However, lightning leader development is not a simple matter of dielectric breakdown, and the ambient electric fields required for lightning leader propagation can be a few orders of magnitude less than dielectric breakdown strength. Further, the potential gradient inside a well-developed return-stroke channel is on the order of hundreds of volts per meter or less due to intense channel ionization, resulting in a true power output on the order of megawatts per meter for a vigorous return-stroke current of 100 kA .
Electrification in the air
Electrostatics involves the buildup of charge on the surface of objects due to contact with other surfaces. Although charge exchange happens whenever any two surfaces contact and separate, the effects of charge exchange are usually only noticed when at least one of the surfaces has a high resistance to electrical flow. This is because the charges that transfer to or from the highly resistive surface are more or less trapped there for a long enough time for their effects to be observed. These charges then remain on the object until they either bleed off to ground or are quickly neutralized by a discharge: e.g., the familiar phenomenon of a static 'shock' is caused by the neutralization of charge built up in the body from contact with nonconductive surfaces.
St. Elmo's Fire is an electrical phenomenon in which luminous plasma is created by a coronal discharge originating from a grounded object. Ball lightning is often erroneously identified as St. Elmo's Fire. They are separate and distinct phenomena. Although referred to as "fire", St. Elmo's Fire is, in fact, plasma. Saint Elmo's fire is another phase of atmospheric electricity to be considered in this connection. It is otherwise known as the fire of Saint Elias, of Saint Clara, of Saint Nicholas and of Helena, as well as composite, composant or corposant (that is, corpus sanctum [ed., holy body]). The phenomenon is observed, usually during a thunderstorm, at the tops of trees, spires, etc., or on the heads of animals, as a brush or star of light.
The electric field around the object in question causes ionization of the air molecules, producing a faint glow easily visible in low-light conditions. Approximately 1,000 – 30,000 volts per centimetre is required to induce St. Elmo's Fire; however, this number is greatly dependent on the geometry of the object in question. Sharp points tend to require lower voltage levels to produce the same result because electric fields are more concentrated in areas of high curvature, thus discharges are more intense at the end of pointed objects. St. Elmo's Fire and normal sparks both can appear when high electrical voltage affects a gas. St. Elmo's fire is seen during thunderstorms when the ground below the storm is electrically charged, and there is high voltage in the air between the cloud and the ground. The voltage tears apart the air molecules and the gas begins to glow. The nitrogen and oxygen in the Earth's atmosphere causes St. Elmo's Fire to fluoresce with blue or violet light; this is similar to the mechanism that causes neon signs to glow.
The earliest equipment for detecting electrical charge in the air was a pointed metal rod projecting into the air several feet and connected at its lower end to a gold leaf electroscope. When this rod was projected into the air a few feet, the leaves diverged. Another variation was known as Volta's electrometer.
For high-altitude measurements, kites were once used, and weather balloons or aerostats are still used, to lift experimental equipment into the air. Early experimenters even went aloft themselves in hot-air balloons.
A lightning rocket is a device used to control the time and the location of a lightning strike. It consists of a rocket launcher that is fired by a sensor that detects nearby electrostatic and ionic charge.
- Geophysics, Atmospheric sciences, Atmospheric physics, Atmospheric dynamics, Journal of Geophysical Research, Earth system model, Atmospheric chemistry, Ionosphere, Air quality, Sky voltage
- Earth's magnetic field, Sprites and lightning, Whistler (radio), Telluric currents, relaxation time, electrode effect, potential gradient
- Charles Chree Medal, Electrodynamic tethers, Solar radiation
- Egon Schweidler, Charles Chree, Nikola Tesla, Hermann Plauson, Joseph Dwyer
References and external articles
Citations and notes
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