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Atmospheric electricity
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Atmospheric electricity is the regular diurnal variations of the Earth's atmospheric electromagnetic network (or, more broadly, any planet's electrical system in its layer of gases). The Earth’s surface, the ionosphere, and the atmosphere is known as the global atmospheric electrical circuit. Atmospheric electricity is a multidisciplinary topic.
There is always free electricity in the air and in the clouds, which acts by induction upon the earth and the electromagnetic devices.
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Encyclopedia
Atmospheric electricity is the regular diurnal variations of the Earth's atmospheric electromagnetic network (or, more broadly, any planet's electrical system in its layer of gases). The Earth’s surface, the ionosphere, and the atmosphere is known as the global atmospheric electrical circuit. Atmospheric electricity is a multidisciplinary topic.
There is always free electricity in the air and in the clouds, which acts by induction upon the earth and the electromagnetic devices. The atmospheric medium, by which we are surrounded, contains not only combined electricity, like every other form of matter, but also a considerable quantity in a free and uncombined state; sometimes of one kind, sometimes of the other; but as a general rule it is always of an opposite kind to that of the earth. Different layers, or strata of the atmosphere, placed only at small distances from each other, are frequently found to be in different electric states. The phenomena of atmospheric electricity are of three kinds. There is the electrical phenomena of thunderstorms and there are the phenomena of continual electrification in the air, and the phenomena of the polar Aurora constitute a third
branch of the subject.
History
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, Benjamin Franklin observed lightning to possess almost all the properties observable in electrical machines.
In July of 1750, Franklin hypothesized that electricity could be taken from clouds via a tall metal aerial with a sharp point. Before Franklin could carry his experiment, in 1752 Thomas-François Dalibard erected a 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 of 1752, Thomas François d'Alibard 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);
- magnetising needles;
- the similarity between St. Elmo's Fire and glow discharge.
Around June of 1752, Franklin reportedly performed his famous kite experiment. The kite experiment was repeated by Romas, who drew from a metallic string sparks long, and by Cavallo, who made many important observations on atmospheric electricity. L. G. Lemonnier (1752) also reproduced Franklin 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. Saussure found that there was a variation with height, as well. In 1785, C. A. Coulomb discovered the 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 camera 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 thunderstorm's 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 and studying the left-over magnetic fields (basalt, being a ferromagnetic mineral, becomes magnetically polarised when exposed to a large external field such as those generated in a lightning strike).
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 transmit energy wirelessly over large distances (see his Wardenclyffe Tower and Magnifying Transmitter). The Polish Polar Station, Hornsund, has researched the magnitude of the earth's electric field and recording 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. It is thought that any apparatus which might be used to extract useful energy from atmospheric electricity would be prohibitively costly to build and maintain, which is probably why the field has not attracted much interest.
Description
Atmospheric electricity abounds in the environment; some traces of it are found less than four feet from the surface of the earth, but on attaining greater height it becomes more apparent. The main concept is that the air above the surface of the earth is usually, during fine weather, positively electrified, or at least that it is positive with respect to the earth's surface, the earth's surface being relatively negative. Additionally, the presence of electrical action in the atmosphere, due to the accumulation of enormous static charges of current generated presumably by friction of the air upon itself, can account for the various phenomena of lightning and thunderstorms. Other causes to produce electricity in the atmosphere are, evaporation from the earth's surface, chemical changes which take place upon the earth's surface, and the expansion, condensation, and variation of temperature of the atmosphere and of the moisture contained in it.
According to M. Peltier, the terrestrial globe is completely negative, and inter-planetary space positive; the atmosphere itself has no electricity, and is only in a passive state; so that the effects observed are due to the relative influence of these two great stores of electricity. Researchers are disposed to assume that the terrestrial globe possesses, at least on its solid part, an excess of negative electricity, and that it is the same with bodies placed at its surface; but it appears to them to follow, from the various observations made, that the atmosphere itself is positively electrified. This positive electricity evidently arises from the same source as the negative of the globe. It is probable that it is essentially in the aqueous vapors with which the atmosphere is always more or less filled that it resides, rather than in the particles of the air itself; but it does not the less exist in the atmosphere.
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 .
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 Geissler's tubes. The lower air is, when dry, a non-conductor. The upper stratum is believed to be charged with positive electricity, while the earth's surface is itself negatively charged; the stratum of denser air between acting like the glass of a Leyden jar in keeping the opposite charges separate. The theory of atmospheric electricity explains equally many phenomena; free electricity, which is manifested during thunder-storms, being the cause of the former; and electricity of a lower tension, manifested during a display of the aurora borealis, causing the latter.
The electric atmosphere is the most frequent cause which deters or prevents electrical transmissions. During storms, it is seen that the some apparatus works irregularly, interrupting the passage of strong currents instantaneously, and often produces upon the apparatus in the offices, between metallic points, bright sparks; in telegraphic systems the armatures of the electro-magnets are drawn up with great force, and the wires and other metallic substances about the instruments fused. It is also observed, but more rarely, currents, which continue for a longer or shorter time, that prevent working of communication systems.
Variations
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.)
Polar Aurora
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.
Earth-Ionosphere cavity
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.
Atmospheric layers
The 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 currents of positive electricity 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. It is also known as the "exchange layer".
There is a potential gradient at ground level and this corresponds to the negative charge in and near the Earth's surface. This negative potential gradient falls rapidly as altitude increases from the ground. Most of this potential gradient is in the first few kilometers. Conversely, the positive potential gradient rises rapidly as altitude increases from the ground.
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 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 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; this works out to approximately one gigavolt (one billion volts) for a 300 m (1000 ft) lightning bolt. With an electric current of 100 kA, this gives a power of 100 terawatts. 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 a 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. 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 lights to glow.
Research and investigation
Low altitude
For ascertaining the electric state of the atmosphere near the surface of the earth, Volta's electrometer is sufficient. An electrometer is an instrument which serves to indicate and measure electricity. The one just mentioned consists of a glass jar, surmounted by a pointed, metallic rod; and to the lower end of the rod, which enters the jar, two fine straws are loosely attached. The pointed rod, collecting the electricity from the air, the two straws become similarly electrified and recede from each other; the amount of divergence measuring the intensity of the fluid.
High altitude
Experiments are made in the higher regions of the atmosphere by the aid of kites and balloons. The string of the kite must be wound with fine wire, in order to convey the electricity from the sky; and it must also be insulated, by attaching the lower end either to a silken cord or glass pillar. Small, stationary balloons are sometimes employed, the strings of which are arranged and fastened in the same manner. Occasionally meteorologists ascend in balloons for the purpose of making observations.
Lightning
A lightning rocket consists of a rocket launcher that is in communication with a detection device that measures the presence of electrostatic and ionic change in close proximity to the rocket launcher that also fires the rocket launcher. This system is designed to control the time and the location of a lightning strike.
See also
General: Geophysics, Atmospheric sciences, Atmospheric physics, Atmospheric dynamics, Journal of Geophysical Research, Earth system model, Atmospheric chemistry, Air quality
Electromagnetsm: Earth's magnetic field, Sprites and lightning, Whistler (radio), Telluric currents, relaxation time, electrode effect, potential gradient
Other: Charles Chree Medal, Electrodynamic tethers, Solar radiation
People: Egon Schweidler, Charles Chree, Nikola Tesla, Hermann Plauson
Citations and notes
General references
Pre-1930s
Post-1930s
Journals
- Articles
- General
Other readings
Websites
Further reading
- James R. Wait, Some basic electromagnetic aspects of ULF field variations in the atmosphere. Journal Pure and Applied Geophysics, Volume 114, Number 1 / January, 1976 Pages 15-28 Birkhäuser Basel ISSN 0033-4553 (Print) 1420-9136 (Online) DOI 10.1007/BF00875488
- Charles Chree, . Proceedings of the Royal Society of London, Vol. 60, 1896 - 1897 (1896 - 1897), pp. 96-132
- G. C. Simpson, C. S. Wrigh, . Proceedings of the Royal Society of London. Series A, Containing Papers of a Mathematical and Physical Character, Vol. 85, No. 577 (May 10, 1911), pp. 175-199
External links
- , phys.uh.edu
- 'Fair weather' measurements important to understanding thunderstorms. science.nasa.gov
- Clifford E Carnicom, Oct 21 2002
- , uah.edu
- Tjt, . ava.fmi.fi
- ICAE - International Commission on Atmospheric Electricity
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