The Electrical Environment of the Earth's Atmosphere: a Review

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THE ELECTRICAL ENVIRONMENT OF THE EARTH’S ATMOSPHERE: A REVIEW

D.K. SINGH1, R.P. SINGH2 and A.K. KAMRA1 1Indian Institute of Tropical Meteorology, Pune-411 008, India [email protected], [email protected] 2Department of Physics, Banaras Hindu University, Varanasi-211 005, India

Abstract. The study of the electrical environment of the Earth’s atmosphere has rapidly advanced during the past century. Great strides have been made towards the understanding of lightning and thunderstorms and in relating them to the global electric circuit. The electromagnetic fields and currents connect different parts of the Earth’s environment, and any type of perturbation in one region affects another region. Starting from the traditional views in which the electrodynamics of one region has been studied in isolation from the neighboring regions, the modern theory of the global electrical circuit has been discussed briefly. Interconnection and electrodynamic coupling of various regions of the Earth’s environment can be easily studied by using the global electric circuit model. Deficiencies in the model and the possibility of improvement in it have been suggested. Application of the global electric circuit model to the understanding of the Earth’s changes of climate has been indicated.

1. Introduction

The Earth’s environment is filled with electrons, negative ions and positive ions, comprising of very low plasma density. These charged particles interact amongst themselves and also interact with electric and magnetic fields present in the medium, leading to the control of space–plasma environments by an electrodynamic process. The outer boundary of the Earth’s environment extends up to the magnetosphere, which is formed by interaction of the solar wind with the geomagnetic field (Hines, 1963; Dungey, 1978). As a result of this interaction the geomagnetic field is compressed on the dayside, and a tail trailing up to a distance of more than 500 Earth radii is created on the nightside (Dungey, 1978). The electric current developed in the magnetopause helps in the creation of the tail. The cavity carved as a result of the deflection of the solar wind by the geomagnetic field and enclosed by the magnetopause is called magnetosphere (Figure 1), in which the geomagnetic field is contained. The magnetosphere contains the radiation belt composed of energetic charged particles trapped in the magnetic field. The number density of electron - ion pairs in the magnetosphere is highly variable, ranging in order of magnitude from a low of 106 m−3 in parts of the tail up to 1012 m−3 in the densest portions of the dayside atmosphere. As a result of the solar wind’s interaction with the geomagnetic field, mass, momentum, and energy are transferred from the solar wind to the magnetosphere, and a complex pattern of several current systems is

Figure 1. Three-dimensional schematic diagram of the Earth’s magnetosphere (thin arrows indicate the direction of the magnetic field and thick arrows show the magnetopause’s current, ring current, field-aligned current, neutral sheet current, and tail current). generated in different parts of the magnetosphere (Figure 1). The variability of solar conditions reflects in the variability of the solar wind, which results in variability of the current system. In the magnetosphere a natural phenomenon involving electric discharge, somewhat like a thunderstorm, occurs which is called a magnetospheric substorm. During substorms the cross-tail current is disrupted and diverted towards the ionosphere as a field-aligned current. The energy stored in the magnetotail is converted into plasma heat and bulk flow energy, and it is dumped towards the inner magnetosphere. Energetic precipitating particles cause enhanced auroral activity. Relations between atmospheric electricity and both solar activity (Cobb, 1967; Markson, 1971, 1978; Hays and Roble, 1979; Roble and Hays, 1979; Markson and Muir, 1980; Roble and Tzur, 1986; Roble, 1991; Tinsley and Heelis, 1993; Rycroft et al., 2000; Tinsley, 2000) and volcanic activity (Meyerott et al., 1983) have been reported. Any perturbation in the interplanetary or atmospheric environment causes a variation in electrical conductivity and hence variation in the current/electric field system of the atmosphere. The atmospheric electric conductivity depends on the ionization rate, the recombination rate, and various meteorological and solar activity conditions. In the ionosphere ionization is caused mainly by the extreme ultraviolet and X-ray radiation from the Sun. The precipitating energetic charged particles from the magnetosphere can cause significant ionization, mainly at high latitudes. The ionization in the lower atmosphere depends on solar activity in the sense that at a particular height the ion production rate is lower during the sunspot maximum period than during the sunspot minimum period (Neher, 1967). The mechanism is not fully understood, but it appears that irregularities and enhancements of the interplanetary magnetic field (IMF) tend to exclude part of the lower THE ELECTRICAL ENVIRONMENT OF THE EARTH’S ATMOSPHERE 377 energy cosmic rays from the inner solar system (Barouch and Burlaga, 1975). The effect becomes more pronounced with increasing height/increasing geomagnetic latitude (L). At L = 50◦ the reduction of the ion production rate during the periods of sunspot maximum is about 30% at 20 km and about 50% at 30 km (Gringel et al., 1986). The solar cycle dependence was confirmed by measurement with the open balloon-borne ionization chambers (Hofmann and Rosen, 1979). Analytical expressions for computing the ionization rates dependent on latitude and the period of solar cycles are given by Heaps (1978). Superimposed on the 11 year solar cycle variation are the Forbush decreases, which are somehow related to solar flares and exhibit a temperature reduction of the incoming cosmic ray flux for periods of a few hours to a few days or weeks (Duggal and Pomerantz, 1977). Measurements of electric current and fields in the lower atmosphere show variations associated with solar flares (Cobb, 1967; Muhleisen, 1971; Holzworth and Mozer, 1979; Michnowski, 1998; Tinsley, 2000), solar magnetic sector boundary crossings (Markson, 1971, Park, 1976), geomagnetic activity (Cobb, 1967; Marcz, 1976; Tanaka et al., 1977; Bering et al., 1980), auroral activity (Freier, 1961; Olson, 1971; Lobodin and Paramonov, 1972; Shaw and Hunsucker, 1977), and solar cycle variations (Muhleisen, 1977; Olson, 1977; Tinsley, 2000). A specific mechanism for the solar terrestrial coupling through atmospheric electricity has been suggested and studied (Markson, 1971, 1978; Markson and Muir, 1980; Roble, 1985; Lakhina, 1993; Rycroft et al., 2000). In this paper we present briefly the electrical structure of the Earth’s atmosphere, source of electric field, and its connection with the global electric circuit (GEC). Recent results in this emerging field are summarized.

2. Electrical Structure of the Earth’s Atmosphere

It has been known for over two centuries that the solid and liquid Earth and its atmosphere are almost permanently electrified. The surface has a net negative charge, and there is an equal and opposite positive charge distributed throughout the atmosphere above the surface. The Earth’s atmosphere has been studied by divid- ing it into various regions based on temperature profiles, conductivity, or electron density (Figure 2). Each region has been studied more or less in isolation as far as electrodynamical processes are concerned, although processes operating in one region are influenced by the presence of neighboring regions and the processes at work in them.

2.1. THE TROPOSPHERE

The lowest region of the atmosphere, where the temperature decreases with an increase in altitude, is called the troposphere, and which is the chief focus of meteorologists, for it is in this layer that essentially all phenomena to which we 378 SINGH ET AL.

Figure 2. Proﬁles distribution of the temperature, conductivity, and electron density of the Earth’s atmosphere. collectively refer as weather occur. Almost all clouds, and certainly all precipitation, as well as all the violent storms, occur in this region of the atmosphere. There should be little wonder why the troposphere is often called as the ‘weather sphere’. It extends from the surface of the Earth up to about 10 to 12 km. Throughout this layer there is a general decrease of temperature with altitude at a mean rate of 6.5 ◦C/km, which is called the environmental lapse rate. This lapse rate is highly variable from place to place, but never exceeds 10 ◦C/km, except near the ground. Sometimes shallow layers up to about 1 km deep in which the temperature actually increases with height are observed in the troposphere. When such a reversal occurs a temperature inversion is said to exist. The main source of ionization in the troposphere is galactic cosmic rays, apart from radioactive materials exhaling from the soil. The radioactive ionization component depends on different meteorological parameters and can exceed the cosmic rays component by an order of magnitude (Hoppel et al., 1986). It decreases rapidly with increasing height, and at 1 km it is already signiﬁcantly less than the contribution owed to cosmic rays (Pierce and Whitson, 1965). The temperature of the Earth’s surface rises as a result of absorption of solar radiation. Heat from the Earth’s surface is transferred to the air near the ground by conduction and radiation, and is distributed upwards through the atmosphere by turbulent mixing. Convection, which involves the ascent of warm air and downward movement of cold air is effective in transporting heat upward. Owing to this process THE ELECTRICAL ENVIRONMENT OF THE EARTH’S ATMOSPHERE 379 the average air temperature is usually highest near the ground and decreases with height until it reaches a level called the tropopause, at an average height of approximately 12 km in the tropics and 10 km near the poles. The minimum temperature at the tropopause level can be between −70 ◦Cto−90 ◦C in the tropics.

2.2. THE STRATOSPHERE

The stratosphere lies between the tropopause and the stratopause (∼ 50 km altitude). In the lower part of the stratosphere the temperature is nearly constant with height or increases slowly. Throughout the stratosphere galactic cosmic rays provide the principal ionization source, and the ionization rate does not vary diurnally but does vary with geomagnetic latitude and with the phase of the 11 year solar cycle. Roughly speaking, the ion production rate at 30 km height increases by a factor of 10 from the geomagnetic equator to the polar caps at a sun spot minimum (cosmic-ray maximum) and by a factor of 5 at a sun spot maximum. The solar cycle modulation is near zero at the equator, increasing to a factor of about 2 in a polar cap region. The ionization rate above 30 km is approximately proportional to the atmospheric density. These properties result because of (a) the shielding effect of the geomagnetic field, which allows cosmic ray particles to enter the atmosphere at successively higher latitudes for successively lower energies, and (b) the reduction in cosmic ray flux in the inner solar system as solar activity intensifies. In addition, solar proton events (SPE) provide a sporadic and intense source of ionization at high latitudes. Solar flares produced one of the largest SPE event recorded in terms of the total energy input into the middle atmosphere during 4–9 August, 1972. The largest Forbush decrease in cosmic rays intensity that has been observed also occurred during this event (Pomerantz and Duggal, 1973, 1974; Duggal, 1979). The conductivity, which is roughly of the order of 10−14 mho/m at the Earth’s surface, increases exponentially with altitude in the troposphere - stratosphere region; the main charge carriers are the small positive and negative ions. The warming of the stratosphere results from the absorption of ultraviolet radiation by ozone (at wavelengths between about 200 nm and 310 nm). Although the stratosphere contains much of the total atmospheric ozone (peak density ∼ 20– 22 km), the maximum temperature occurs at the stratopause, where the temperature may exceed 0 ◦C.

2.3. THE MESOSPHERE

This is the region of the second decrease of temperature with height, like the troposphere, to a minimum of about −90 ◦C around 80 km. This layer is commonly known as the mesosphere, which literally means the middle sphere and the level corresponding to the minimum temperature is referred to as the mesopause. The mesosphere extends from about 50 km to 85 km, and many of the atmospheric variations encountered in it are linked to complicated processes in the underlying layers. The major sources of ionization are the solar Lyman alpha radiation, X-ray 380 SINGH ET AL. radiation, and the intense auroral particle precipitation. The conductivity increases with height rather sharply. The main charge carriers are electrons, positive ions + + + − (e.g., N2 O2 ,NO ) and the negative ions O2 . The major daytime source of ionization in undisturbed conditions is provided by the NO molecule, whose low ionization potential of 9.25 electron volts allows it to be ionized by the intense Solar Lyman alpha radiation. The concentration of NO in the mesosphere is not well known and is almost certainly variable in response to meteorological factors (Solomon et al., 1982).

2.4. THE IONOSPHERE

The ionosphere starts from above the mesopause and extends to a height of about 500 km. The upper boundary is not well deﬁned, i.e., the so called thermosphere is included in the ionosphere. In this region ionized species do not necessarily recombine quickly, and there is a permanent population of ions and free electrons. The net concentration of ions and free electrons (generally in equal numbers) is greatest at a height of a few hundred kilometers and has a profound effect on the properties and behavior of the medium. The major sources of ionization are EUV and X-rays radiation from the sun, and energetic particle precipitation from the magnetosphere into the auroral ionosphere. The Current carriers are electrons + + + and the positive ions such as NO ,O2 ,andO . Electrical conductivity becomes anisotropic in this region with the parallel conductivity (with respect to an Earth’s ﬁeld line) exceeding the transverse conductivity by several orders of magnitude. The ionized medium also affects radio waves, and as a plasma it can support and generate a variety of waves, interactions, and instabilities which are not found in a neutral gas.

2.5. THE MAGNETOSPHERE

The magnetic field decreases with altitude as the atmosphere becomes more sparse and its degree of ionization increases. The electrodynamic properties of the medium above the ionosphere are dominated by the geomagnetic field, and this region is called the magnetosphere. The lower boundary of the magnetosphere is the ionosphere and it extends up to magnetopause. At the magnetopause energy is coupled into the magnetosphere from the solar wind, and here is determined much of the behavior of the magnetosphere and of the ionosphere at high latitudes. In the sunward direction the magnetopause is encountered at about 10 Earth radii, but in the anti-solar direction the magnetosphere is extended downward in a long tail, the magnetotail, within which occur plasma processes of great significance for the geospacial regions. THE ELECTRICAL ENVIRONMENT OF THE EARTH’S ATMOSPHERE 381

Figure 3. Various currents which flow in the vicinity of an active thunder cloud. There are five contributions to the total current (the Maxwell current) JM = JE +JC +JL +JP +∂D/∂t,belowthe thunder clouds. Above the thunder cloud, JM = JE + ∂D/∂t. Here, JE = σE is the field-dependent current, JC is the convection current, JL is the lightning current, JP is the precipitation current, and ∂D/∂t is the displacement current (Roble, 1991).

3. Sources of electric ﬁelds in different regions and their coupling

A thunderstorm is the main source of electric fields in the lower atmosphere com- prised of the troposphere, stratosphere, and mesosphere. The thunderstorm activity produces vertical electric fields on a global scale. The main negative charge in the lower part of a thunderstorm occurs at a height at which the atmospheric temperature is between −10 ◦Cand−20 ◦C (this temperature range is typically between 6 to 8 km for summer and 2 km for winter thunderstorms) (Krehbiel et al., 1979). The positive charge at the top of the storm does not have so clear a relationship with temperature as the negative charge, but can typically occur between −25 ◦C and −60 ◦C depending on the size of the storm (this temperature range usually lies between 8 to 16 km in altitude). There is no generally accepted model of thunderstorm electrification that can be used to calculate the current which storms release into the GEC. Freier (1979) presented a thunderstorm model with more details than the other models. He con- 382 SINGH ET AL. sidered the atmosphere between the Earth and the ionosphere to be sub-divided into three separate regions (Figure 3). In the first region - below the negative layer of charge in a thunderstorm he considered conduction, displacement, and precipitation current densities, allowing all to vary with altitude. In the second region (between the bottom and top of the thunderstorm cloud) he considered charging, conduction, displacement and precipitation current densities varying in space and time. In the third region (above the thunder storm) only conduction and displacement currents are considered. All lightning currents are considered as discontinuous charge transfers. In the fair weather regions, far away from the thunderstorm, only conduction current flows. The time variation in thunderstorm electric fields, both aloft and at ground level, can be interpreted in terms of the total Maxwell current density that varies slowly. The total Maxwell current, ∂D J = J + J + J + , M E C L ∂t where JE is the field dependent current which includes the point discharge current, JC is the convection current produced by the mechanical transport of charge such as by the air motion or by precipitation, JL is the lighting current representing discontinuous transfer of charge in both space and time, and hence occurs impulsively, and ∂D/∂t is the displacement current. The corona and lightning usually transfer negative charge to the ground, and precipitation and turbulence transfer positive charge. Krider and Musser (1982) showed that the average Maxwell current density is usually not affected by lightning discharges and varies slowly throughout the evolution of a storm. Since the Maxwell current is steady at a time when the electric field both at the ground and aloft undergoes large changes in amplitude, and sometimes even in polarity, they inferred that the cloud electrification processes are substantially independent of the electric field. Also, since the Maxwell current varies slowly throughout the storm, it probably represents an electrical quantity that is coupled directly to the meteorological structure of the storm. Thus the thunderstorm appears to be a current source that produces a quasi-static Maxwell current density. Figure 3 shows the electric charges carried into the cloud from below by the updraught, constituting an electric current density JM (Roble, 1991). Current (∼ 0.5 A) flowing upward (Gish and Wait, 1950; Blakeslee et al., 1989) from the top of cloud charges the ionosphere, which is a good conductor, and therefore the ionosphere behaves as an equipotential surface having a potential of ∼ 300 kV with respect to the Earth (Wilson, 1916). This happens over 10% of the Earth’s surface for all of the ∼ 200 thunderclouds which are active at any time, and which are mainly concentrated over the tropical land masses during the local afternoons. On the remaining 90% of the Earth’s surface remote from the thunderstorms, a return current of 1000 A (∼ 1pA/m2) flows back to the Earth in the so called fair weather field between the ionosphere and the Earth (Figure 4). The electrical resistance of the fair weather atmosphere is ∼ 300 , and the field strength near the Earth’s surface is ∼ 130 V/m (Markson, 1978). Ogawa (1985) and Roble (1991) THE ELECTRICAL ENVIRONMENT OF THE EARTH’S ATMOSPHERE 383

Figure 4. Diagram of the global electric circuit. Ionizing radiation is mainly owed to galactic cosmic rays in the middle atmosphere (Markson, 1978). gave alternative numerical values, an average upward current of 1.7 A from the top of the thundercloud over an area whose radius is ∼ 55 km. Thus for an ionospheric potential of ∼ 300 kV with respect to the Earth only ∼ 3% of the Earth’s surface is required for thunderclouds, with 97% for the fair weather field. The fair weather electric conduction current varies according to the ionospheric potential difference and the columnar resistance between ionosphere and ground. Horizontal currents flow freely along the highly conducting surface of the Earth and in the ionosphere. A current flows upward from a thunderstorm cloud’s top towards the ionosphere and also from the ground into the thunderstorm generators, closing the circuit. There are temporal variations on time scales varying from microseconds (lightning discharge) to milliseconds (sprites), minutes to hours (thunderstorm regeneration), hour to a day (diurnal variations), months (seasonal variations), and to a decade (solar cycle effect). The region between the top of the thunderstorm and the ionosphere was earlier considered to be the seat of a vertical current maintaining the electrification of the fair weather atmosphere and radio waves through the ionosphere. In the recent years thunderstorms have been projected to give rise to newly observed optical phenomena, namely, red sprites, blue jets, elves (emissions of light and VLF perturbations from a source of an electromagnetic), blue starters, etc. (Franz et al. 1990; Lyons, 1994; Sentman et al., 1995; Boeck et al. 1998; Reising et al., 1999; Barrington-Leigh and Inan, 1999; Rodger, 1999; Singh et al., 2002; Sato et al., + 2003). Sprites are red in colour as result of the excitation of the N2 line. The life time of the sprites is much longer, up to 50 ms, than that of elves and they occur 384 SINGH ET AL. in the 40–90 km altitude range, with a maximum altitude of 88 ± 5kmanda maximum brightness at 66 km (Wescott et al. 2001). Sprites triggered by negative lightning discharges have also been reported (Barrington-Leigh et al., 1999). It has been suggested that large discharges of a positive cloud to ground simultaneously excite both the Earth - ionosphere cavity resonances (EICR) and sprites (Singh et al., 2002; Sato et al., 2003), and hence EICR may be used as a diagnostic tool for sprites. Price et al. (2002) observed that all the sprites detected optically in the United States produced detectable ELF/VLF transients in their observations some 11,000 km away in Israel. All of these transients were of positive polarity, showing that they were associated with positive lightning. Bering et al. (2002) observed that sprite produced a vertical electric field perturbation of ∼ 0.275 V/m in stratosphere. We find that upward escape of the lightning signal is an old phenomenon popularly known as ‘blue’ or ‘green’ pillars and rocket discharges like columns of optical emissions. Everett (1903), Boys, (1926), Malan (1937), Wright (1950) and Wood (1951) discuss the possibility of lightning discharge propagation upwards from a cloud top and undergoing multiple reflections between the cloud and the ionosphere. In a thundercloud discharge various possibilities exist for the flow of the return stroke current which depend upon its orientation type (positive cloud to ground or negative ground to cloud), and intensity. It may be terminated in the originating cloud or intercepted by the originating cloud top while propagating between the ground and ionosphere. Under certain circumstances return strokes may undergo multiple reflections between the cloud and the ionosphere. The out- wards escaping lightning impulse may interact with the ionospheric region and give rise to optical emissions and γ rays. This basic predictive idea of Wilson (1925) has been validated by Huang et al. (1999). Sentman and Wescott (1993) recorded a large number of upward directed electrical discharges during a single NASA airborne DC flight over thunderstorms in Iowa. The optical emissions mainly occur in the altitude range of 50–90 km with lateral dimensions of 20–50 km. The duration is estimated to be less than 16 ms and the brightness was estimated to be 25–50 kR, which is almost same as the bright aurora. To explain the mechanism of optical emissions Pasko et al. (1995, 1997) carried out a two dimensional quasi-electrostatic simulation and showed the existence of a strong electric field between the thundercloud top and the ionosphere. Following a cloud to ground discharge Cummer et al. (1998) and Cummer (2003) have confirmed the existence of large electrical currents. The unrelaxed strong electric field increases the electron temperature as well as electron density, leading to emission of the first and second positive bands of N2, with emission intensity peaking at altitudes between 70 and 80 km. Rowland et al. (1996) discussed whether electromagnetic pulses driven by lightning can cause the breakdown of the neutral atmosphere in the lower D region and hence generation of optical emissions. Cho and Rycroft (1998), using electrostatic and electromagnetic codes, studied the electric field structure and optical emissions from cloud top to the ionosphere and concluded that a single red sprite can be successfully explained THE ELECTRICAL ENVIRONMENT OF THE EARTH’S ATMOSPHERE 385 by the electromagnetic simulation. However, sprites are often observed as clusters. To explain that they suggested that the positive charge is distributed in spots, and once the positive CG discharge occurs the current flows by connecting these spots of positive charges. This non-uniform distribution of the source current may lead to clusters of red sprites. Rycroft and Cho (1998) have studied the acceleration of electrons, heating and ionization of the atmosphere as a result of redistribution of electric charge and the electromagnetic pulse during lightning discharge. The situation becomes strongly non-linear and runaway electrons/electrical breakdown of the atmosphere can also occur. Nickolaenko and Hayakwa (1998) showed that the bending of the current wave substantially modifies the electric field that may be detected at higher altitudes. Nagano et al. (2003) have evaluated the modification in electron density and collision frequency of the ionosphere by the intense electric field of the EM pulse radiated by the lightning current strokes, and hence discussed his generation of elves. The above discussions clearly support the observation that the electrical conductivity of the atmosphere above thunderstorms might be different from that of the surrounding atmosphere. It has been reported that for seven out of the nine largest thunderstorm events conductivities were enhanced by up to a factor of 2 from ambient values (Pinto et al., 1988; Hu et al., 1989; Holzworth and Hu, 1995). Hu et al. (1989) have speculated that these alterations in conductivity could be owed to the gravity wave produced by the thunderstorm or x-rays from lightning induced electron precipitation. Rowland (1998) has studied lightning driven electric fields at high altitudes and has shown that the electric field was sufficient to cause thermal breakdown and runaway breakdown at the height corresponding to the observation of sprites/elves. The electrical condition of the atmosphere depends upon its state of ionization. The main source of the ionization up to the altitude of 60 km is Galactic cosmic rays, although additional ionization near the ground can be produced by radioactive emissions from the ground and owing to release of radioactive gases from the soil, and above 60 km solar ultraviolet radiation becomes important. During the geomagnetic storm period energetic auroral electron precipitation, auroral X-ray bremsstrahlung radiation, and proton bombardment during solar proton events, all can become significant sources of additional ionization for the high latitude middle atmosphere. The full cosmic ray spectrum is only capable of reaching the Earth’s surface at geomagnetic latitudes higher than ∼ 60◦. At lower geomagnetic latitudes the lower energy particles are successively excluded by the Earth’s geomagnetic field, and only particles with energies greater than about the 15 GeV range can reach the equator. The cosmic ray spectrum thus hardens with decrease in geomagnetic latitude. As a result the height of the maximum ion production rate decreases from about 20 km at mid latitudes to about 15 km near the equator. The profile of the ion production rate for cosmic rays for various geomagnetic latitudes during the solar cycle minimum, using the data from Neher (1967), is shown in Figure 5. The ionization rate increases with geomagnetic latitude, and both the height of the 386 SINGH ET AL.

Figure 5. Variation of vertical profiles of the cosmic ray ion production rate at various geomagnetic latitudes (Neher, 1967). peak and the slope of the ionization rate above the peak also increase. Near the ground there is about 20% variation between the equatorial region and the higher latitudes. In the ionosphere the electric field is produced by a dynamo action. The ionospheric wind dynamo is driven both by tides generated within the thermosphere and by tides propagating upward from the lower atmosphere. Atmospheric wind and tides pull the weakly ionized ionosphereic plasma across the geomagnetic field, thus producing an electromotive force and field. This is called the ionospheric wind or Sq (for solar quiet) dynamo. Considering the ionospheric plasma as behaving like a conducting fluid, in the magneto-hydro-dynamic (MHD) approximation, the electric field in the frame of reference of the plasma is given by (Roederer, 1979) EPlasma = 0 = E +v × B, (1) where E is the electric field in a fixed frame of reference the Earth, v is the velocity of the plasma (Solar wind) with respect to the Earth and B is the geomagnetic field vector. From Equation (1) E =−v × B and E × B v = , (2) B2 THE ELECTRICAL ENVIRONMENT OF THE EARTH’S ATMOSPHERE 387

Equation (2) shows the inter-relationship between the electric field and plasma velocity. The MHD approximation is useful in inter-relating plasma motions with the electric and magnetic field, but it breaks down in a number of important circumstances, especially where electric current densities are large. It also breaks down in the lower ionosphere, below 150 km, where collisions between ions and the much more numerous neutrals (air molecules) are sufficiently frequent to prevent the ions from maintaining the velocity given by the Equation (2). In this region electric current readily flows across geomagnetic field lines. The winds in neutral air winds in the lower ionosphere lead to the generation of electric currents and fields, and for this reason the height range of roughly 90–150 km is called the dynamo region of the ionosphere. Volland (1972, 1977) has shown that the large scale horizontal potential difference generated in the ionosphere by the wind dynamo maps downward into the lower atmosphere and perturbs the low latitude potential and electric field near the ground by about 1–5% of the tropospheric fair weather potential and electric field stabilized by worldwide thunderstorm activity. During the geomagnetic storms period thermospheric winds acting in response to heating by high latitude auroral activity produce an ionospheric disturbance dynamo, and large scale horizontal potential differences of up to 25 kV are generated between high and low latitudes by the ionospheric disturbance dynamo (Blanc and Richmond, 1980). These disturbances are superimposed on the quiet ionospheric wind dynamo potentials. Muhleisen et al. (1971) showed that the ionospheric potential difference inferred from measurements made at widely separated mid and low latitude stations is generally small, although during one strong event they found a 60 kV ionospheric potential difference between two stations. The solar wind/magnetosphere dynamo is driven by the interaction of solar wind with the Earth’s geomagnetic field. It generates a horizontal dawn to dusk potential drop of 30–150 kV across magnetic conjugate points in polar caps, and it is associated with a current system which carries current of the order of 106 A. The magnetospheric convection pattern is aligned to the Sun relative to the geomagnetic poles (north geomagnetic pole, 78.3 ◦N latitude and 291◦E longitude; south geomagnetic pole, latitude 74.5 ◦S and longitude 127 ◦E). The pattern remains fixed relative to the Sun and moves in a complex fashion over the Earth’s surface as the Earth rotates about its geographical polar axis. Solar wind/magnetosphere dynamo is the major generator of electric fields in the magnetosphere, and the electric field is given by E = Vsw × B, (3) where Vsw is the solar wind velocity. The source of this electric field can be ap- preciated by considering the magnetosphere as a MHD generator in which a jet of plasma is forced through a static magnetic field and an electric potential is developed by dynamo action. If the magnetosphere were static the solar wind would blow across the field lines and an electric field would be generated. The magnitude 388 SINGH ET AL.

Figure 6. Schematic diagram of Earth’s magnetic field and plasma flow in the solar wind/ magnetosphere. Solid lines show the magnetic field, open arrows show the plasma velocity’s direction. a, b, c,andd denote magnetic regions of different topology (Lyons and Williams, 1984). of this field is just that required to produce the circulation. The total potential drop is given by

VT = VswLBn, (4) where L is the width of the magnetosphere and Bn is estimated from the magnetic flux leaving the polar caps which ultimately connects to the IMF. For a polar cap of radius Rp and magnetic flux density Bp, a magnetotail of length ST and radius RT (= L/2), (Hargreaves, 1992). 2 2 πRP BP Bn = , (5) 2πRT ST 6 −5 −2 Taking RP = 1.7 × 10 m, BP = 5.5 × 10 Wbm , RT = 20 RE,andST = −10 −2 200 RE, one obtains Bn = 4.5 × 10 Wbm ∼ 0.45 nT. A solar wind speed of Vsw = 500 km/sec gives VT = 58 kV. Although this calculation assumes values which may not be well known, it does give a result of the right magnitude, and, so reinforces the validity of the electric potential approach. Furthermore, it is often more convenient to treat the dynamics of the magnetosphere in terms of the electric field. The electric potential developed during the dynamo process depends on the orientation of the Earth’s magnetic field which extends indefinitely into interplanetary space (Hill and Wolf, 1977; Stern, 1977; Lyons and Williams, 1984). For ready reference we present a schematic diagram of the magnetic field configuration (Figure 6) where the IMF is directed southward. If the IMF had an east - west component, as it usually does, we would require a three-dimensional representation THE ELECTRICAL ENVIRONMENT OF THE EARTH’S ATMOSPHERE 389 of the magnetic field configuration which is very complex. In fact, Figure 6 represents a conceptual tool rather than a true representation of the magnetospheric field lines. In the figure there are four classes of magnetic field lines shown: (a) closed magnetic field lines connected to the Earth in both northern and southern hemi- sphere; (b) interplanetary field lines unconnected to the Earth; (c) open field lines connecting the northern polar cap to interplanetary space; and (d) open field lines connecting the southern polar cap to interplanetary space. The interplanetary electric field, obtained from (2), is directed out of the paper in Figure 6. To the extent that the MHD approximation is valid and electric fields map along the magnetic field, the polar ionosphere is also subjected to an electric field out of the paper, causing ionospheric plasma to convect anti-sunward. The ionospheric electric field is greater than that of the interplanetary electric field because the bending of magnetic field lines at the ionosphere causes electric potential gradients to intensify. On the other hand, the drift velocity of the plasma in the upper ionosphere is much lower than the solar wind velocity because of the inverse dependence on magnetic field strength. The polar cap electric field is 20 mV/m, giving an ionospheric convection velocity of roughly 300 m/sec. The physical processes that determines the amount of magnetic flux that interconnect the geomagnetic field and the IMF are not well understood (Cowely, 1982). It involves violation of the MHD approximation, which occurs to some extent through the magnetosphere, but is particularly important in at least two regions: at the sunward the magnetopause and somewhere in the magnetospheric tail. In the sunward direction the magnetopause plasma flows together through unconnected interplanetary and magnetospheric magnetic fields and flows out northward and southward on interconnected magnetic field lines. In the tail the plasma flows together on interconnected field lines and flows out through unconnected magnetospheric and IMF. In the closed portion of the magnetosphere the plasma flows generally towards the sun, passing around the Earth on the morning and evening side. However, some of the outermost portion of the cloud field region convects away from the sun because of momentum transfer from the nearby solar wind (Hones, 1983). The magnetospheric plasma convection causes energization of plasma and particle precipitation into the ionosphere and hence affects the electrodynamic properties of the ionosphere. The convection of plasma from the magnetosphere to the ionosphere is compressionally heated because the volume occupied by plasma on neighboring magnetic field lines is reduced as magnetic field strength increases. Other processes also help in the energization of the plasma. Some of the energized particles precipitate into the ionosphere as a result of wave - particle interaction and create ionization enhancements, especially in the auroral oval (Tsurutani and Lakhina, 1997; Singh and Singh, 2002; Singh et al., 2003). The energized plasma also has an important influence on the flow of the electric currents and on the distribution of electric fields (Spiro and Wolf, 1984). Energetic particles drift in the Earth’s magnetic field, electrons towards the east and positive ions towards the west, so that a westward ring current flows within the hot plasma. This current 390 SINGH ET AL.

flowing in the geomagnetic field essentially exerts an electromagnetic force on the plasma directed away from the Earth; thus tending to oppose the Earthward convection. Charge separation associated with the ring current tends to create an eastward electric field component, opposite to the nightside westward convection electric field, largely canceling the convection electric field in the inner magnetosphere. The overall pattern of magnetospheric convection tends to map along the magnetic field line into the ionosphere, even though this mapping is imperfect because of the net electric field which tends to develop within the non-uniform energetic plasma. In the upper ionosphere, where (2) is valid, the general convection pattern looks as shown in Figure (7). The flow lines in Figure (7) correspond to lines of constant electrostatic potential in a steady state. There is a potential high on the dawn side of the polar cap and low on the dusk side, with a potential difference of the order of 50 kV. The electric field strength in the auroral oval tends to be somewhat larger than the polar cap electric field. We have tried to show above that the electrical properties of different regions are linked and that one is not justified in studying the electrodynamics of the Earth’s atmosphere region by region, i.e., separately for troposphere, stratosphere, mesosphere, ionosphere, magnetosphere, and interplanetary medium. The electric field and current map from one region to the other, and control the electrodynamic properties of the entire atmosphere. Therefore a global approach is required in order to understand the electrical environment of the Earth’s atmosphere (Lakhina, 1993).

4. Global Electric Circuit

The concept of a GEC began to evolve in the early twentieth century with the re- cognition of the following facts: (a) the net positive space charge in the atmosphere between the Earth’s surface and a height of ∼ 10 km is nearly equal to the negative charge on the Earth’s surface; (b) the electrical conductivity of the air increases with altitude and the air – earth current within an atmospheric column remains constant with altitude, which implies that this is being driven by a constant voltage drop between the surface of the Earth and upper atmosphere. The discovery of the ionosphere during the same period provided the means of closing the global circuit through this conducting layer and played an important role in framing the concept of the GEC. According to the classical picture of atmospheric electricity the totality of thunderstorms acting together at any time charges the ionosphere to a potential of several hundred thousand volts with respect to the Earth’s surface (Dolezalek, 1972). This potential difference drives a vertical current downward from the ionosphere to the ground in all fair weather regions of the globe. The fair weather electric conduction current varies according to the ionospheric potential difference and the columnar resistance between the ionosphere and the ground. The fair weather electric ﬁeld varies typically between 100–300 V/m at THE ELECTRICAL ENVIRONMENT OF THE EARTH’S ATMOSPHERE 391

Figure 7. Schematic diagram of the magnetic north pole region showing the auroral oval and ionospheric convection. The convection contours also represent electric potential contours, with a potential difference of the order of 8 kV between them (Burch, J.A.: 1977, ’The Magnetosphere, Upper Atmosphere and Magnetosphere, NRC Geophysics Study Committee, National Academy of Science, Washington, DC, pp. 42–56). the ground surface and shows diurnal, seasonal, and other time variations caused by many factors. The fair weather conductivity of the atmosphere near the Earth’s surface is of the order of 10−14 mho/m and shows considerable variations with particulate pollution (Cobb and Wells, 1970; Kamra and Deshpande, 1995; Kamra et al. 2001), relative humidity (Kamra et al., 1997), and radioactivity of the air and ground surface (Israelsson and Knudsen, 1986; Israelsson et al., 1987). The electric conductivity increases nearly exponentially with altitude up to 60 km with the scale length of 7 km (Figure 2). This conductivity is maintained primarily by galactic cosmic rays ionization. These Galactic cosmic rays flux reduces in the mid latitudes, when the solar activity increases, thus reducing the atmospheric conductivity in this region, while in the same period solar protons may be ‘funnelled’ by the Earth’s magnetic field to polar regions resulting in an increased atmospheric conductivity there. A dawn to dusk potential difference is also applied across the polar regions as a result of the interaction of solar wind and the Earth’s magnetic field (Tinsley and Heelis, 1993). Cho and Rycroft (1998) have presented a simple 392 SINGH ET AL. model profile for the atmospheric conductivity ranging from 10−13 mho/m near the surface to 10−7 mho/m at 80 km altitude in the lower ionosphere. Hale (1994) has presented a more complex profile, which shows variations in both space and time. He has shown that the conductivity is three orders of magnitude higher at the height of 35 km compared to that at the Earth’s surface, whereas the air density at 35 km is 1% of the Earth’s surface. Below 60 km the main charge carriers are small positive and negative ions which are produced primarily by galactic cosmic rays, and above 60 km free electrons become more important as charge carriers, and their high mobility abruptly increases the conductivity throughout the mesosphere. However, near the Earth’s surface the conductivity is large enough to dissipate any field in just 5–40 min (depending on the amount of pollution); therefore the local electric field must be maintained by some almost continuous current source. Above 80 km the conductivity becomes anisotropic because of the influence of the geomagnetic field and shows diurnal variation owed to solar photo-ionization processes. The arena for the subject is included in the system shown in Figure 8 which shows the Earth at the center, surrounded by the atmosphere, ionosphere, the Van Allen belts, and the magnetosphere deformed by the solar wind coming from the Sun. During the geomagnetically active periods the energetic charged particles precipitating from the Earth’s inner and outer magnetospheric radiation belts (Fig- ure 8) interact with the middle and lower atmosphere by depositing their energy in the atmosphere, by creating ionization directly or via bremmstrahlung radiation, by altering its chemistry (Jackman et al., 1995), or by affecting the nucleation by electrofreezing of water droplets to form clouds, thereby influencing the dynamics of storm, and the atmosphere (Tinsley and Heelis, 1993; Tinsley, 1996, 2000). Thus the electrical behaviour of the Earth’s environment is controlled by the dynamics of the solar atmosphere. The Earth’s surface and the ionosphere provide two conducting plates where current flows horizontally. Figure 4 shows that the GEC is driven by the upward current from a thunderstorm’s top towards the ionosphere and also from the ground into the thunderstorm generators, thus closing the circuit (Markson, 1978). The global fair weather load resistance is of the order of 100 . About 2000 thunderstorms occurring around the globe at any one time are the source of current (the Wilson current) in the circuit (Figure 3). They occur mostly in the tropics in the local afternoon and evening. Aircraft and balloon measurements above thunderstorms show that a total current of 0.1–6 A, with an average of 0.4 A, flows from the top of a thunderstorm cell to the ionosphere (Gish and Wait, 1950; Stergis et al., 1957; Vonnegut et al., 1973; Kasemir, 1979). Below a thunderstorm the transfer of charge by point discharge, lightning, precipitation, convection and displacement currents contributes to the net current flowing between the thunderstorm’s base and the ground. Blakeslee et al. (1989) carried out measurements of air conductivity and vertical electric field with a high altitude NASA U2 airplane flying over thunderstorms in the Tennessee valley region of the United States and reported that the Wilson current varied from 0.09–3.7 A with an average of 1.7 A, and the area- THE ELECTRICAL ENVIRONMENT OF THE EARTH’S ATMOSPHERE 393

Figure 8. Schematic diagram of Earth’s magnetosphere, showing the Earth at the center, surrounded by the atmosphere, the ionosphere, the Van Allen radiation, and the magnetosphere deformed by the flowing solar wind (Davies, K., Ionospheric Radio, Peter Peregrinus, London, 1990). averaged Maxwell current varied from 0.09–5.9 A with an average of 2.2 A. They have also shown that the relative efficiency of a thunderstorm to supply current to the GEC is inversely related to the storm flash rate. Thus the current generated within the cloud is divided between production of lightning and maintenance of the Wilson current. Intra-cloud discharges do not support the Wilson current. The ratio of cloud to ground and intra-cloud discharge increases from about 0.1 in the equatorial region to about 0.4 near latitude 50◦ (Pierce, 1970; Prentice and Macher- ras, 1977). Thunderstorm activity is maximum near the equator and decreases with latitude. Thus the supply of Wilson current varies with latitude and shows a peak at low latitudes. Using the global model of atmospheric electricity Roble and Hays (1979) com- puted the electric field and air earth current density along the Earth’s orographic surface which is shown in Figure 9. They present variation of the calculated electric field and the air earth current density over the Earth’s surface caused by the downward mapping of the magnetospheric convection potential pattern. Under the maximum positive ionospheric potential, the calculated surface electric field is +15 V/m (positive ionospheric potential regions; the air – earth current flows 394 SINGH ET AL.

Figure 9. Contours illustrating the downward mapping of the ionospheric potential pattern: (a) im- posed ionospheric potential (kV) at ionospheric height (> 100 km); (b) calculated electric field (V/M) − along the Earth’s orographic surface; and (c) calculated ground current (A/m2, multiplied by 10 13) along the Earth’s orograpic surface. All figures are plotted at 19:00 UT in geomagnetic coordinates (Roble, and Hays, 1979). THE ELECTRICAL ENVIRONMENT OF THE EARTH’S ATMOSPHERE 395 into the ground) and under the minimum negative potential the calculated surface electric field is −20 V/m (a negative ionospheric potential; the current flows from the ground towards the ionosphere). The maximum ground current density occurs over the mountainous regions of Antarctica, Greenland and the Northern Rocky Mountains. The calculated air earth current shows considerable variations as a result of the Earth’s orography that are associated with changes in the columnar resistance. In the vicinity of the high Antarctic mountain plateau the air earth current has positive perturbations of up to 0.2 × 10−12 A/m2 on the dawn side and to −2 × 10−12 A/m2 on the dusk side. This potential pattern moves over the Earth’s surface during the day, rotating about the geomagnetic pole. Roble and Hays (1979) showed that in sun-aligned geomagnetic coordinates at a ground station, balloon, or aircraft at a given geographic location should detect variations that are organized in magnetic local time. At an early magnetic local time the ionospheric potential perturbations of the Earth’s potential gradient are positive, and at a later magnetic local time negative potential overtakes times and the perturbations are negative. Owing to orographic variations the globally integrated ground current varies between net upward and downward values, which in turn causes the difference in the fair weather ionospheric potential to vary between positive and negative values in a diurnal cycle (Roble and Hays, 1979). Analyzing the data recorded at the South Pole and Thule, Greenland, Kasemir (1972) showed that the diurnal Universal time (UT) variations of potential gradient are about 30% less than the global low latitude UT variations, which are attributed to variations in thunderstorm frequency. The polar curve has a similar shape to the curve derived from the Carnegie cruise, but at a much reduced amplitude. From these results Kasemir (1979) concluded that another agent, besides worldwide thunderstorm activity, may modulate the global circuit at high latitudes. Kamra et al. (1994) have challenged the concept of the classical GEC on the basis of their electric field observations in the Indian Oceans. Recent observations of Deshpande and Kamra (2001) also show that diurnal variation of the electric field at Antarctica significantly differs from the Carnegie diurnal curve of the electric field. Analyzing the balloon measurements of magnetospheric convection fields data Mozer and Serlin (1969), Mozer (1971), Holzworth and Mozer (1979), Holzworth (1981), and D’Angelo et al. (1982) have studied the correlation of the vertical electric field with the magnetic activity parameters. During quiet geomagnetic conditions the classical Carnegie curve could be reproduced at the location of the balloon’s height, and during more active geomagnetic conditions the dawn – dusk potential difference of the magnetospheric convection pattern was found to clearly influence the vertical fair weather field as it intensified and presumably moved from its quiet time position to over the balloon’s height. Measurements of the vertical electric field at Syowa station (Antarctica) showed that it increases in response to a magnetospheric substorm (Tanaka et al., 1977). All these measurements suggested an electrical coupling between the magnetospheric dynamo and the GEC at 396 SINGH ET AL.

Figure 10. A simplified equivalent circuit for the global electric circuit showing the thunderstorm as the main generator (Ogawa, 1985). high latitudes and indicated a need for more measurements in order to gain better understanding of the nature of the interaction. Only a few mathematical models of global atmospheric electricity have ap- peared over the years (Kasemir, 1963, 1977; Hill, 1971; Hays and Roble,1979; Volland, 1982; Ogawa, 1985). The widely referred model of Ogawa (1985), considering the simple equivalent circuit for the atmosphere and an equipotential surface for the ionosphere, is shown in Figure 10. The thundercloud is treated as a constant current generator with a positive charge at the top and negative charge at its bottom. Here r is the global resistance between the Earth and the ionosphere and R1, R2 and R3 are the resistances between the ionosphere and top of the thundercloud, between two charge centers within the thundercloud and between the bottom negative charge and the Earth’s surface, respectively. Since R1, R2,andR3 are much greater than r, the upward current from the thundercloud to the ionosphere is given by R I I = 2 0 (6) R1 + R2 + R3 It should the noted that the current I is also the fair weather current from the ionosphere to the Earth’s surface and it is related to the thunderstorm generator current I0. The measurement of I at a high altitude observatory remote from active thunderstorms can give some information about I0,ifR1 and R2 are considered as constant. Both may, however, be reduced at times of enhanced fluxes of energetic charged particles associated with enhanced geomagnetic activity. The contribution to R1 above an active, sprite-producing thundercloud may be greatly reduced owing to ionization produced in the rarefied mesosphere by the large transient electric field during large, positive cloud to ground lightning discharges (Cho and Rycroft, THE ELECTRICAL ENVIRONMENT OF THE EARTH’S ATMOSPHERE 397

Figure 11. The equivalent global electric circuit showing typical numerical values (Rycroft et al., 2000).

1998). Recent observations of optical emission between the top of the thunderstorm and the ionosphere suggest the existence of intense current which may be an extension of the return stroke current (Cummer et al., 1998; Cummer, 2003). This current should be included in the mathematical modeling of GEC. Recently Pasko et al. (2002) have reported a video recording of a blue jet propagating upwards from a small thunder cloud cell to an altitude of about 70 km. As relatively small thunder cloud cells are very common in the tropics, it is probable that optical phenomena from the top of the clouds may constitute an important component of the GEC. Rycroft et al. (2000) presented a new model of GEC treating the ionosphere and the magnetosphere as passive elements (Figure 11). Three different regions of the fair weather circuit are given. One of these is for the high altitude part of the Earth where the profiles of J and E through the fair weather atmosphere will differ from those of low and mid latitudes. The capacitance C of the concentric shell of atmosphere between the Earth and the ionosphere, over one scale height of atmosphere rather than over the full height of the ionosphere, is 4πε R2 C = 0 e ≈ 0.7F, (7) H where Re is the radius of the Earth, ε0 the permittivity of free space, and H is the scale height which is equal to 7 km. The time constant for the atmospheric GEC 398 SINGH ET AL. is τ = Cr ≈ 2 min (Rycroft et al., 2000). The energy associated with the global electric circuit is enormous, ≈ 2×1010 J(Rycroftet al., 2000). This value has been obtained by considering a charge of 200 C associated with each storm and 1,000 storms operating at a time. The electric current density through the fair weather atmosphere ≈ 2 × 10−12 A/m−2. Taking the conductivity of air at ground level to be ≈ 2 × 10−14 mho/m, the fair weather electric field is ≈ 102 V/m at ground level, ≈ 1 V/m at altitude 20 km and ≈ 10−2 V/m at altitude 50 km (Rycroft et al., 2000). Thus even though the fair weather current remains the same, the vertical electric field goes on decreasing with altitude. The current remaining the same if the atmosphere conductivity changes owing to some reason, then accordingly the fair weather electric field also changes. For example, following a Forbush decrease, if the atmospheric conductivity is everywhere reduced by 10% then the fair weather electric field will be increased by ∼ 10% (Ogawa, 1985). The ionospheric potential would reduce to 99% of the initial value only for a few milliseconds after sprites, and would have little effect on the fair weather electric field (Rycroft et al., 2000). Is there any affect of sprites on the global electric circuit? This is a completely unanswered question to this date. A sprite occurs over large convective thunderstorms and affects the conductivity of the upper atmosphere (Rycroft and Cho, 1998; Rycroft et al., 2000; Singh et al., 2002). The frequency of occurrence of sprits is far less than that of lightning (only 1 sprite out of ∼ 200 lightning discharges). Based on such observations Rycroft et al. (2000) have suggested that sprites do not play any major role in the GEC. However, intensive research into this topic is required in future. In all GEC models electrostatic phenomena have been considered, whereas during lightning discharges electromagnetic waves having frequencies from a few Hz to 100 MHz are generated and propagated through the atmosphere. To account for the effect of these waves electrodynamic or electromagnetic effects should be considered by relating electromagnetic fields to charge and current densities in a time varying situation. At higher frequencies (ω σ/ε0) the medium can be considered as a leaky dielectric whereas at lower frequencies (ω σ/ε0)itcanbe considered as a conductor. Even in the absence of radiation displacement current should be considered. In fact, the Maxwell current shown in Figure 3 is by its nature very variable, and not a great deal is known about it. Further studies of this topic are required. The GEC model has several advantages over the traditional methods, based directly or indirectly on the solar heating mechanism put forward for explaining solar – terrestrial – weather relationships. The main drawback of the solar heating mechanism is that the solar constant variations are very small (< 0.1%). Secondly, they require efficient coupling from the thermosphere to the lower atmosphere, which in reality is rather weak. Thirdly, the solar heating mechanisms are too slow; they require at least several days before atmospheric dynamics would be affected significantly. The GEC model by passes all these difficulties, at the same time it offers a novel approach to understanding the electrical environment of our planet. THE ELECTRICAL ENVIRONMENT OF THE EARTH’S ATMOSPHERE 399

For example, a change of ionospheric potential caused by solar flares would rapidly affect electric field intensities all over the world. The state of ionization of the lower atmosphere is controlled by cosmic rays of both solar and galactic origins, which again depend upon solar activity. A slight change in the ionization over the cloud top can affect the electric field throughout the lower atmosphere. Theoretical modeling shows that a slight change in the initial background electric field during cloud electrification can lead to an entirely different final voltage being developed (Sartor, 1980). This is because the Earth’s atmosphere is a highly nonlinear system. The main source of electrical phenomena upon the Earth’s environment is thunderstorm activity, which is affected by solar activity. Brooks (1934) analyzed worldwide data collected from 22 stations and suggested a positive correlation between the frequency of thunderstorms and relative sun spot numbers (R). The correlation was low at mid latitudes and increased both towards the equator and the pole. Stringfellow (1974) analyzed data collected in Britain between 1930 and 1973 and found the correlation coefficient between lightning frequency and sun spot numbers to be ∼ 0.8. Recently Schlegel et al. (2001) analyzed data from the Ger- man lightning detection system (BLIDS) and showed a significant correlation of lightning frequency with AP and R, and a significant anti-correlation with cosmic ray flux. However, a similar analysis with data from the Austrian System (ALDIS) yielded inconclusive results, although the two observing regions are quite close to each other. The difference in lightning activity can be understood in terms of the weather system. The operational region of the ALDIS system lies in the south and east of the Alps mountains and is dominated by the continental Mediterranen weather system, whereas the area of the BLIDS system is mostly within the influence of the north Atlantic weather system (Schlegel et al., 2001). The solar activity influences the lower D region (Volland, 1995) and Schumann resonance (Schlegel and Fullekrug, 1999), which in turn affect the electrical environment of the Earth. A modern detection system should be used to explore new aspects of the Sun – thunderstorm/lightning relationship and its variation with solar/geophysical indices. Recently GEC has been being used as a tool for studying the Earth’s climate and changes in it (Price and Rind, 1990, 1994) because of its direct connection with lightning activity. The subject has been recently reviewed by Williams (2003). A close relationship has been shown between: (a) tropical surface temperature and monthly variability of the Schumann Resonance (Williams, 1992); (b) ELF observations in Antarctic/Greenland and global surface temperature (Fuellekrug and Fraser-Smith, 1997); (c) diurnal surface temperature changes and the diurnal variability of the GEC (Price, 1993); and (d) ionospheric potential and global/tropical surface temperature (Markson and Price, 1999). Reeve and Toumi (1999), using satellite data, showed agreement between global temperature and global lightning activity. Aerosols in the atmospheric boundary layer and stratosphere have a strong influence on the electrical phenomenon in the atmosphere. Adlerman and Williams (1996) found large effects from several factors such as seasonal changes, variations 400 SINGH ET AL. in mixed layer heights, variations in the production rates and anthropogenic aerosols, and variations in surface wind speed on the seasonal variations of the GEC. These aerosol particles can even influence the charge generating mechanisms in storms and thus affect the charging currents in the GEC (Williams et al., 2002). Williams and Heckman (1993) concluded that the conduction currents other than lightning is the dominant charging agent for the Earth’s surface. Recently Price (2000) extended this study and showed a close link between the variability of upper troposphere water vapour (UTWV) and the variability of global lightning activity. UTWV is closely linked to other phenomena such as tropical cirrus cloud, stratospheric water vapour, and tropospheric chemistry (Price, 2000; Price and Asfur, 2003). These examples suggest that by monitoring the GEC it is possible to study the variability of surface temperature, tropical deep convection, rainfall, upper troposphere water vapour, and other important parameters which affect the global climate system.

5. Conclusions

We have summarized the electrical behavior of different regions of the Earth’s environment. Sources of electric ﬁelds and the electrodynamic processes involved in each region have been discussed brieﬂy. It has been suggested that the GEC model, if properly solved, is able to provide short term and long term variations in the electrical processes of various regions and their inter-coupling. Possible causes of changes in the GEC are discussed and its role in monitoring the Earth’s climate is indicated. Recently observed optical phenomena (sprite, blue jets, blue starter, etc.) between the ionosphere and the cloud top produces transient plasma which affects the electrodynamics of the atmosphere, and it should be explored in detail. In fact, further research is needed to better understand the natural environment and its variability, so that future evolution may be predicated.

Acknowledgements

One of the authors (DKS) thanks V. Gopalakrishnan and S.D. Pawar for helpful discussion during the preparation of the manuscript. We are grateful to the referee for his constructive and valuable suggestions.

Glossary

ELF: Extremely low frequency electromagnetic radiation (3–300 Hz). Fair weather: Normal sunny weather without any precipitation, without any ap- preciable amount of low cloud, and with calm/low winds. THE ELECTRICAL ENVIRONMENT OF THE EARTH’S ATMOSPHERE 401

L-parameter: The equation of a geomagnetic field line is r = L cos2 φ,wherer is the geocentric distance from the center of the Earth, φ is the magnetic latitude. The parameter is the distance in Earth radii (RE) from the center of Earth to the point where a geomagnetic field line crosses the equator (φ). L is useful in identifying field lines and magnetic drift shells. Interplanetary magnetic field: The magnetic field resulting from the outward transport of the solar magnetic field by the expanding solar plasma is known as the solar wind. Lightning: A discharge of atmospheric electricity accompanied by a vivid flash of light. During thunderstorms static electricity builds up within the clouds. A positive charge builds up in the upper part of the cloud, while a large negative charge builds up in the lower portion. When the difference between the positive and negative charges becomes large the electrical charge jumps from one area to another, creating a lightning bolt. Most lightning bolts are intra-cloud, but they can also strike the ground. Magnetotail: In the anti-sunward direction the magnetosphere is extended into a long tail, the basic form of the magnetotail in the plane containing the magnetic poles is shown in Figure 1. The flux density is about 20 in the tail lobes. Plasma: A fourth state of matter (in addition to solid, liquid, and gas) which ex- ists in space. In this state atoms are positively charged and share space with free negatively charged electrons (but overall electrically neutral). Plasma can conduct electricity and interact strongly with electric and magnetic fields. The solar wind is actually hot plasma blowing from the Sun. Schumann resonance: Schumann resonances are the eigenfrequencies of the Earth – ionosphere cavity oscillations excited by global lightning activity, and lie in the frequency range 6–50 Hz. Solar cycle: Eleven year cycle of sunspots and solar flares that affects other solar indexes such as the solar output of ultraviolet radiation and the solar wind. The Earth’s magnetic field, temperature, and ozone levels are affected by this cycle. Solar flare: A solar flare is a sudden brightening of the small area of the photosphere that may last between a few minutes and several hours. Solar wind: A plasma of protons and electrons, with an admixture of He++ and other lesser solar ions, streaming continuously together with the solar magnetic field from the solar atmosphere out to the solar system at a speed of ∼ 200– 1000 km/s. Substorm: Basic disturbance in the terrestrial magnetosphere, involving success- ive reconfiguration of the geomagnetic field and explosive dissipation of energy in the high latitude ionosphere (auroral displays), the inner plasma sheet (acceleration of energetic ions) and the distant magnetotail (formation of magnetic neutral lines and plasmoids). The energy is provided by the solar wind and is temporarily stored in the form of increased magnetic flux in the magnetotail. Sunspot: A region on the surface (the photosphere) of the Sun that is temporarily cool and dark compared to surrounding areas. 402 SINGH ET AL.

Thermosphere: High temperature (T>1000 ◦K) region of the upper atmosphere above 100 km. Thunderstorm: Local storm resulting from warm humid air rising in an unstable environment. Air may start moving upward because of unequal surface heating, the lifting of warm air along a frontal zone, or diverging upper level winds (these diverging winds draw air up beneath them). Severe thunderstorms can produce large hail, very strong winds, ﬂash ﬂoods, and tornados. Wave - particle Interaction: Interactions between waves and particles in a plasma that result in the exchange of energy and momentum and can result in whistler- induced electron precipitation.

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