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The Global Atmo spheric- Electrical Circuit RAYMOND G. ROBLE arid ISRAEL TZUR National Center for Atmospheric Research Lightning was recognized as a grand manifestation of static electricity within thunderstorm clouds in the eighteenth century. It was also recognized that electri- cal phenomena are not confined to thunderclouds and that a weak electrification exists as a permanent prop- erty of the atmosphere even during fair weather. Fur- ther research established that the Earth's surface is charged negatively and the air is charged positively, with a vertical electric field of about 100 V/m existing in the atmosphere near the Earth's surface. An electro- static explanation for the phenomena was sought at first, and one theory suggested that the electric field of the atmosphere was the result of an intrinsic negative charge on the Earth, probably collected during the Earth's formation. With the discovery of cosmic-ray ionization in the early twentieth century, it was realized that air possesses an electrical conductivity due to its ion content. As a result of the finite electrical conductivity, vertical conduction currents flow from the atmosphere to the Earth, tending to neutralize the charge on the Earth. On the basis of actual conductivity values it was calculated that charge neutralization would take place in less than an hour, and the continued existence of an electric field suggested some generation mechanism to oppose the leakage currents flowing to the Earth. The search for this generation mechanism soon became the main object of research on global atmospheric electric- ity. 206 In the early twentieth century the concept of a global circuit of atmospheric electricity slowly began to evolve (Israel, 1973; Pierce, 1977~. The net positive space charge in the air between the ground and a height of about 10 km is nearly equal to the.negative charge on the surface of the Earth. The electrical conductivity of the air increases rapidly with altitude, and the product of the local vertical electric field and local conductivity at any altitude within an atmospheric column gives a con- stant air-earth current flowing downward. This con- stant air-earth current with respect to altitude implies that the current flow is mainly driven by a constant dif- ference in potential between the surface of the Earth and some higher altitude in the atmosphere. The discov- ery of the highly conducting ionosphere in the 1920s ex- plained the long-range propagation of radio waves and was important for the evolution of the concept of the global electric circuit. The ionosphere, with its large electrical conductivity, provided a means of closing the global circuit. It, however, is not a perfect conductor parallel to the Earth's surface, but it possesses a finite conductivity, and the electric currents and fields within it are driven by the combined action of the ionospheric and magnetospheric dynamo systems as well as by cur- rent generation from the lower atmosphere. Wilson (1920) first demonstrated that a thunderstorm supplies a negative charge to the Earth. In the 1920s, it was also known that over the oceans and in polar areas

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THE GLOBAL ATMOSPHERIC-ELECTRICAL CIRCUIT the diurnal maximum of the fair-weather potential gra- dient at the Earth's surface occurred at the same univer- sal time (about 1900 UT). Furthermore, radio measure- ments of atmospherics showed that global thunderstorm activity also peaked near 1900 UT, with the main thun- derstorm centers being in Africa and South America. Scientists studying meteorological statistics of thunder- storm activity found similar diurnal variations. The di- urnal UT variation of potential gradient over the oceans was similar to the diurnal UT variation of thunderstorm occurrence frequency with no phase delay. These exper- imental facts all contributed to the concept of the Earth's global electrical circuit and furthermore sug- gested that thunderstorms were the generators within the circuit. This concept of the global electrical circuit persists today, although there are still basic problems and details that need to be resolved (Dolezalek, 1971, 1972; Kasemir, 1979~. THUNDERSTORMS AS GENERATORS IN THE GLOBAL CIRCUIT The vast majority of clouds in the atmosphere form and dissipate without ever producing precipitation or lightning. A cloud, however, interacts with atmo- spheric ions and becomes electrified to a certain degree. The small, fast ions in the atmosphere almost exclusively provide the electrical conductivity. The intermediate and large ions do not greatly contribute because of their lower mobility. The fast ions within clouds become at- tached to more massive cloud particles and thereby de- crease the electrical conductivity within the cloud rela- tive to the surrounding clear air. As a result of this electrical conductivity change alone, clouds act as an electrical obstacle; space charge develops on the surface of the cloud, and the distribution of fair-weather con- duction currents and fields flowing in the vicinity of the cloud are altered. As convective activity intensifies, electrification increases. Strong electrification generally begins with the rapid vertical and horizontal develop- ment of a fair-weather cumulus cloud into a cumulo- nimbus type. Most of the lightning on Earth is produced by strongly convective cumulonimbus clouds with a vig- orous system of updrafts and downdrafts. The updrafts and downdrafts associated with convec- tion and the interactions between cloud and precipita- tion particles (Tzur and Levin, 1981; Rawlins, 1982) act in some manner that eventually separates charges within the thundercloud. Charge-separation processes usually fill the upper portion of the thundercloud with the main positive charge and the lower portion with the main negative charge. The measured charge structure within individual clouds is complex (Krehbiel et al., 207 1979), and a simplifying assumption that is usually made for modeling purposes considers the charge to be distributed in a spherical fashion within some finite vol- ume. A finite spherical distribution of charge has the same distant electric-field structure in space as an equiv- alent point charge. Therefore, for simplicity in studies of the electrical interaction of a thunderstorm with its immediate environment, the lightning charges, which are lowered to ground during a lightning stroke, and the main thunderstorm charges are usually represented as a series of point charges. The main negative charge in the lower part of a thun- derstorm (Krehbiel et al., 1979) occurs at a height where the atmospheric temperature is between -10C and - 20C. This temperature range is typically between 6 and 8 km for summer thunderstorms and around 2 km for winter thunderstorms. 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 oc- cur between - 25C and - 60C depending on the size of the storm. This temperature range usually lies be- tween 8 and 16 km in altitude. The ensemble of thun- derstorms occurring over the globe at any given time will have positive and negative charge centers located over a large altitude range depending on the atmo- spheric structure, the size of the thunderstorm, its type (e.g., air mass, frontal), its location (e.g., ocean, plains, or mountains), its latitude, its stage of development, and other factors. There is no generally accepted model of thunder- storm electrification that can be used to calculate the current that storms release into the global electrical cir- cuit. Measurements by Gish and Wait (1950), Stergis et al. (1957a), Vonnegut et al. (1966, 1973), and Kasemir (1979) showed that the total current flowing upward from thunderstorms areas ranges from 0.1 to 6 A with an average of about 0.7 A per thunderstorm cell. It is, thus, possible to use this value in a theoretical model without referring to the details of the charge-separation mecha- nisms. The simplest model used to investigate the elec- trical interactions of a thunderstorm with its immediate environment assumes a quasi-static dipolar charge dis- tribution embedded within the thundercloud, which is immersed in a conducting atmosphere whose electrical conductivity increases exponentially from the surface of the Earth to a highly conducting region somewhere within the ionosphere above about 60 km. In the quasi-static state these charges are maintained in equilibrium against discharge currents by assuming that a steady convection current acts between the two charge centers in the updraft and downdraft regions of the storm. In earlier studies only conduction currents were assumed to flow in the environment, and although

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208 this assumption is probably valid in the highly conduct- ing region above a storm it is not valid within and below the storm, where corona, lightning, precipitation, con- vection, and displacement currents all contribute to the charge exchange between charge centers and between the storm and the surface of the Earth. Holzer and Saxon (1952) and Kasemir (1959) presented analytic so- lutions for the potential distribution around thunder- storms whose charge distributions are represented as point dipole current sources. A calculated normalized potential distribution around a quasi-static dipolar charge structure within the atmosphere (Kasemir, 1959) is shown in Figure 15. liar . The lines represent stream- lines of current flow between the two charge centers and between the charge centers and the ionosphere and the ground. The highly conducting Earth's surface is con- sidered by including image point sources within the Earth for the solution of the electrostatic potential dis- tribution around the thunderstorm. The currents flow upward from the positive charge center toward the ion- osphere and from the ground toward the negative charge. There is also a current flow between the charges. All currents are calculated as conduction cur (a) \~ oo 01 02 /' ~ . . .l, I/,/, / I/,,/, ,//, //////.//////,,,, //,,l,,,,,/////,li,,,i,,/,i,/,, (b) ,03 []Wo (:Wj bwu we [ ] ,,,, ,l,,,,7,,,,,,,,,,,,,,,,,,,,,,/////////,////////~/////// FIGURE 15.1 (a) Schematic giving the current streamlines of dipo- lar point current sources embedded within an atmosphere with expo- nentially increasing conductivity over a perfectly conducting Earth, (b) schematic of a lumped parameter representation of the global at- mospheric electrical circuit where the thunderstorm is represented as a current generator with internal resistance Wi, WO represents the resis- tance between cloud top and the ionosphere, Wa represents the fair- weather load resistance, and W., represents the resistance between the thunderstorm and the ground (Kasemir, 1959). RAYMOND G. ROBLE and ISRAEL TZUR rents in this simple model and have considerably more complexity in actual storms. Holzer and Saxon (1952) showed that the current out- put from such a thunderstorm model is sensitive to the charge-separation distance, with a greater current out- put from larger storms having intense vertical velocities and charge-separation capability. They also showed that storms with frequent cloud-to-ground lightning strokes move negative charge to the Earth's surface and leave the cloud with a net positive charge that has a greater current output than a dipole. Thus, according to their calculations, even a weak storm with charge sepa- ration could supply a current to the ionosphere and the global circuit. Kasemir (1959) showed that the exponential conduc- tivity increase with altitude in the atmosphere is impor- tant for the current flow from thunderstorms toward the ionosphere. He pointed out that if the atmosphere had a constant conductivity at all altitudes the primary thunderstorm source current would flow downward to the image source, with no current flow toward the iono- sphere and into the global circuit. Anderson and Freier (1969) considered both the limiting fast and slow time variations within thunderstorms. Their dipole quasi- static solutions for slow time variation show that the po- tential distribution about thunderstorms is affected by both the conductivity and the charge-separation dis- tance within storms. Their calculations also suggest that the thunderstorm, in a quasi-static sense, may be electri- cally closed and only the pumping action of lightning discharges can provide the current necessary to main- tain the ionospheric potential. Park and Dejnakarintra (1973, 1977a) considered a dipolar thunderstorm model and analytically solved for the current output and the mapping of thundercloud electric fields into the ionosphere. This model consid- ered the anisotropy of the electrical conductivity above about 60 km and assumed that the Earth's geomagnetic- field lines were vertical. The largest ionospheric fields were calculated to occur at night over giant thunder- storms with values on the order of 10 ~ 4 V/m at 100 km. During the day the calculated do electric fields are 1 to 2 orders of magnitude smaller because of the increased ionospheric electrical conductivity caused by solar ex- treme ultraviolet (EUV) radiation ionization. Dejna- karintra and Park (1974) examined the penetration of lightning-induced ac fields into the ionosphere and found that the lightning electric-field signal recovery time decreases rapidly with increasing altitude until at 100 km the electric-field wave form appears as a sharp pulse. The ac fields are also larger at night than during the day, when ionospheric conductivities are larger. Wait (1960) pointed out that there is a constant radial

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THE GLOBAL ATMOSPHERIC-ELECTRICAL CIRCUIT potential associated with the lower-order (n = 0) mode in a concentric spherical cavity excited by a radial cur- rent-moment element. He estimated that the potential across the earth-ionosphere gap caused by a single 3-km- length lightning discharge of 1000 A would be on the order of 1 V, and he suggested that the omnipresent con- stant voltage during fair weather might result from the accumulated action of a number of lightning strokes. Hill (1971) further examined the resonant hypothesis and showed that the atmospheric field is generated through electrostatic induction by equivalent charge di- poles from thunderclouds. Using parameters of light- ning frequency and the magnitude of the current mo- ment within storms available at that time, he found an ionospheric potential only about one third of the ob- served intensity. Schumann resonances in the earth-ionosphere cavity are also excited by worldwide lightning activity, and this subject has been reviewed by Polk (1982~. Willett (1979) pointed out that the total current sup- plied to the global electric circuit from a thunderstorm differs depending on whether the storm is considered to be a current or a voltage generator. For a current thun- derstorm generator, where the current supplied to the global circuit is independent of the load, his results showed that current output is proportional to the as- sumed source current strength and to a proportionality factor that depends on the ratio of the height of the iono- sphere to the electrical conductivity scale height. For a voltage-source thunderstorm generator, where the charging current increases with time until limited by some voltage-sensitive dissipative mechanism such as lightning or corona, the current output may be some- what smaller depending on assumptions made concern- ing the storm's internal resistance. Not enough is known about the thunderstorm generator to distinguish be- tween the current and voltage source models. Freier (1979) presented a thunderstorm model with more details than point-charge sources and conduction currents. He considered the atmosphere between the Earth and the ionosphere divided into three separate re- gions as shown in Figure 15.2. In region 1, below the negative layer of charge in a thunderstorm, he consid- ered conduction, displacement, and precipitation cur- rent densities, allowing all to vary with altitude. In re- gion 2, between the bottom and top of the thunderstorm cloud, he considered charging, conduction, displace- ment, and precipitation current densities that also vary in space and time. In region 3, above the storm, only conduction and displacement are considered. All light- ning currents are considered to be discontinuous charge transfers, and in the fair-weather regions far to the side of the thunderstorm only conduction currents flow. 209 ~ T ~ Conduction, JE ~ ~ ~ _ Con \ ~ / 12 km \ + + + ,\ + + + ~/ i<' (-40 to-60C) ~ Charge Separation Current 7 - - 1- 10 to -20C) /. . ... Lightning. JL\3! pO.t I I I I I I I ~ Precipitation, UP Discharge, ~ ~ I ~ Wf /// ////f~f,Wf~fif/~,7 J M = JE + JO + JL + JP + At Below T. S. JM = JE + ) ~Above T. S. FIGURE 15.2 Schematic illustrating the various currents that flow within and in the vicinity of thunderstorms: JE is the conduction cur- rent, Jr is a convection current, Jo is the lightning current, JP is the precipitation current, ~D/8t is the displacement current, and JM is the total Maxwell current. This model is an improvement over previous thunder- storm models, which considered lumped circuit param- eters of columnar resistance as shown in Figure 15.1(b) for the dipolar conduction current generator. Freier (1979) used it to describe a charge-transfer mechanism that may occur during large severe storms when precipi- tation is heavy and when the storms are, in certain in- stances, accompanied by tornadoes. The precipitation current during such storms may be so large that it re- moves much of the negative charge in the lower portion of the storm and allows the generator to operate be- tween the positive charge and the ground. During such storms much more electrical energy is produced, with a possibly greater current output that must be considered in any global electric model. Krider and Musser (1982) pointed out that the time variations in thunderstorm electric fields, both aloft and at the ground, can be interpreted as a total Maxwell cur- rent density that varies slowly in intervals between lightning discharges. The total Maxwell current, ~M, consists of field-dependent currents, both linear and nonlinear (corona), ~E; convection currents, ~c; light- ning currents, If; and the displacement current density, aDIdt.

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210 J\I = JE + JC + JL + Edict. (15.1) The field-dependent currents and convection currents are distributed over large areas and, in the absence of lightning, tend to vary rather slowly with time. The lightning current occurs impulsively and represents a discontinuous transfer of charge in both space and time. Corona and lightning usually transfer negative charge to the ground, and precipitation and turbulence trans- fer positive charge. Krider and Musser (1982) showed that the average Maxwell current density is usually not affected by light- ning discharges and varies slowly throughout the evolu- tion of the storm. Since the Maxwell current is steady at times when the electric field both at the ground and aloft undergoes large changes in amplitude, and some- times even polarity, they inferred that the cloud electri- fication 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 mete- orological structure of the storm. Thus, the thunder- storm appears to be a current source that produces a quasi-static Maxwell current density. Grenet (1947, 1959) and Vonnegut (1953, 1965) pro- posed a convective thunderstorm generator in which the electrification process is related to the updrafts and downdrafts acting within the thunderstorm. In this gen- eration process positive space charge is convected to the top of the thunderstorm cloud by updrafts within it, and the negative charge attracted to the top of the cloud is swept downward by convective downdrafts at the edges of the cloud. For this generator, the magnitude of the cloud electrification is related to the strength of thun- derstorm convective activity. The convection mecha- nism also involves conduction currents flowing from the upper atmosphere to the top of the cloud. The magni- tude of the electrification process can, therefore, in- crease with an increase in the electrical conductivity of the atmosphere over the cloud. The above discussion reveals the complexity and diffi- culty involved in modeling the thunderstorm as a gener- ator in the global circuit. Significant progress has been made in recent years, but there is still a long way to go before a generally accepted thunderstorm model will be available for use in a global model of atmospheric elec- tricity. Yet such a model is of prime importance, and research in this area should be pursued vigorously. SOME PROPERTIES OF THE GLOBAL CIRCUIT According to the classical picture of atmospheric elec- tricity (Dolezalek, 1972; Israel, 1973), the totality of RAYMOND G. ROBLE and ISRAEL TZUR thunderstorms acting together at any time charges the ionosphere to a potential of several hundred thousand volts with respect to the Earth's surface. This potential difference drives a vertical electric conduction current downward from the ionosphere to the ground in all fair- weather regions on the globe. The fair-weather electric conduction current varies according to the ionospheric potential difference and the columnar resistance be- tween the ionosphere and the ground. Horizontal cur- rents flow freely along the highly conducting Earth's surface and in the ionosphere. A current flows upward from a thunderstorm cloud top toward the ionosphere and also from the ground into the thunderstorm genera- tor, closing the circuit. A lumped parameter schematic of the global circuit is shown in Figure 15.1(b). This schematic does not represent the real circuit but rather illustrates its basic concepts. The global fair-weather load resistance is given as Wa and is about 250 Q. The thunderstorm generator source is shown along with its equivalent internal resistance, Wi, which is not well known. The total resistance between the thunderstorm and ionosphere is represented as WO and is about 105-106 Q. and the total resistance between the thunderstorm and ground is represented by W7, and is also not well known. Markson (1978), however, suggests that W7t is small because of corona discharge beneath the storm, having a value of about 104-lOs Q. Some of the overall properties of the global circuit are summarized in Table 15.1. There are many additional elements that compli- cate this simplified classical picture, and these are dis- cussed in the following subsections. Global Thunderstorms The thunderstorm generator hypothesis proposed by Wilson (1920) was based on his observations that be- neath the thundercloud negative charge is transferred to the Earth and above the thundercloud positive charge is transferred to the conductive upper atmosphere. A sub- sequent discovery was the close correlation between the diurnal universal time variation of the thunderstorm generator current (represented by the frequency of thunderstorm occurrence) and the load current (repre- sented by the fair-weather ground electric field or air- earth current density), integrated over the surface of the Earth. In about the 1920s the electric field over the oceans was found to vary diurnally in accordance with univer- sal time (Parkinson and Torrenson, 1931), as shown in the upper frame of Figure 15.3. The diurnal change of electrical conductivity over the oceans is relatively small, and therefore Ohm's law requires that the air- earth current density also follow the diurnal variation of electric field. The maximum value of both the average

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THE GLOBAL ATMOSPHERIC-ELECTRICAL CIRCUIT TABLE 15.1 Some Properties of the Global Circuit Number of Thunderstorms Acting at One Time Currents above Thunderstorms (A) (a) Range (b) Average Global Current (A) Ionospheric Potential (kV) (a) Range (b) Mean Columnar Resistance at Sea Level (Q/m0) (a) Lowlatitude (b) High latitude (e) Tibet and Antarctic plateau Total Resistance (Q) (including resistance decrease by mountains) Current Density (A/m2) (a) Inhabited and industrialized areas (b) Vegetated ground and deserts (c) South Pole Station Potential Gradient (V/mJ (a) Equator (b) 60 latitude (c) South Pole (d) Industrial areas Average Charge Transfer over the Entire World (Ckm~0yr~~) Total Charge on the Earth (C) Electrical Relaxation Times (a) 70 km (b) 18km (c) 0.01 km (d) Earth's surface Electrical Conductivity (mho/m) Sea level Tropopause Stratopause Ionosphere (a) Pedersen conductivity (b) Parallel conductivity 1500-2000 0.1to6 0.5tol 750-2000 150-600 280 1.3 X 101' 3 X 101' 2 X 10 230 200 1 x 10-12 2.4 X 10-12 2.5 X 10-12 120 155 71 300-400 +90C 500,000 10 ~ 4 see 4 see 5-40 min 10 ~ 5 see 10 - 14 10-13 10- lo 10-4-10-5 10 electric field and the current over the oceans occurs near 1900 UT, and minimum values near 0400 UT. Whipple and Scrase (1936) obtained the average thunderstorm probability as a function of local time at Kew, England, from corona current records. Assuming that the same thunderstorm probability curve also exists on other continents as a function of local time, they com- bined this curve with the world thunderstorm day statis- tics of Brooks (1925) and obtained the diurnal variation of worldwide thunderstorm activity as a function of uni- versal time, shown in the bottom frame of Figure 15.3. The three major component curves with their maxima at 0800, 1400, and 2000 UT represent the contributions of the major thunderstorm regions of Asia and Austra- lia, Africa and Europe, and America, respectively. The summation of the component curves represents the diur- nal variation of worldwide thunderstorm activity. The similarity of the diurnal variation of electric field over 211 the ocean and the diurnal variation of worldwide thun- derstorm activity supports the hypothesis that thunder- storms are the electrical generator in the global circuit. The maxima and minima of both curves occur at about the same universal times. The amplitudes of modulation for the two curves, however, are different. The ampli- tude of the electric-field curve is about 20 percent, and the amplitude of the thunderstorm curve is about 45 percent. Whipple and Scrase (1936) suggested that this difference might be resolved if a steady supply current from worldwide ocean thunderstorms is added to the worldwide continental distribution of storms. This sug- gestion, however, is not supported by current data. The difference in amplitudes probably arises from the enor- mous variability in thunderstorm electrification. Although the similarity between the diurnal UT worldwide thunderstorm frequency curve and the diur- nal UT electric-field curve suggests that thunderstorms

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212 120 110 100 on 12OI 100 8O 60j~ 4OI 20 o Carnegie _' _ r ~7N 12 16 20 HOUR The World t' _ ~_ _ _ ` ~America New Zealand ~| ~ ~ 0 48 12 16 20 _ HOUR FIGURE 15.3 (a) Annual curve of the diurnal variation of the atmo- spheric electric field on the oceans (volts per meter) as measured by the Carnegie and Maud expeditions (Parkinson and Torrenson, 1931) and (b) annual curve of the diurnal variations of global thunderstorm ac- tivity according to Whipple and Scrase (1936~. are the generators in the global electrical circuit, there is still considerable uncertainty concerning the details, as discussed by Dolezalek (1972) and Kasemir (1979~. Moreover, the data on which the thunderstorm activity is based are only of a qualitative nature: "a thunder- storm day is a day when thunder has been heard." The commonly quoted UT diurnal patterns, shown in Fig- ure 15.3, are averages over a long time that were made to reduce the influence of various disturbing factors. When shorter time averages are used and even single diurnal variations the correlations show great depar- tures from the average curves. The variability of thun- derstorm frequency can be large, with significant de- partures from the average; for example, whole continents may be cloudless for a long time. The mea- sured electric field on the ground is also highly variable as a result of local influences, and it generally takes a week's worth of averaging or more to bring out the diur- nal UT pattern. Most measurements are made on conti- nents, where the electric field displays variations with local time, and these measurements do not fit into a daily worldwide pattern but must be averaged to deter- mine worldwide characteristics. Paramanov (1950) sug- gested that local influences might cancel out if the elec- tric fields measured on many continents were synchronized to universal time and averaged. RAYMOND G. ROBLE and ISRAEL TZUR Dolezalek (1972), in examining the data available at the time, concluded that a globally controlled current does flow vertically through the atmosphere but that its connection to thunderstorm activity is tenuous and, in fact, is often contradicted by the proper interpretation I of available measurements. 24 More recently, Orville and Spencer (1979) examined lightning flashes recorded in photographs by two satel lites in the Defense Meteorological Satellite Program (DMSP) and found that most of the lightning is confined to land areas and that the ratio of global lightning fre quency during northern summer to that of southern summer is about 1.4 for both the dusk and midnight sat ellite data. They pointed out that this summer-winter difference in global lightning frequency is opposite to the electric-field measurements. The hypothesized rela tion of the global atmospheric electric current to thun 24 derstorms is still an unsettled question and clearly needs to be resolved to make further progress in understanding the Earth's global atmospheric electric circuit. Current above Thunderstorms A few measurements have been made that give the magnitude of the current flowing upward over the whole area of a thundercloud (Gish and Wait, 1950; Stergis et al., 1957a; Vonnegut et al., 1966; Imyanitov et al., 1969; Kasemir, 1979~. The currents range from 0.1 up to 6 A, with an average between about 0.5 and 1 A per thunderstorm cell. Gish and Wait (1950) flew an aircraft over a thunderstorm in the central United States and found an average upward current of 0.8 A at an altitude of about 12 km. They measured electric fields of up to 70 kV/m. Stergis et al. (1957b), in a series of bal loon flights at altitudes of about 25 km in central Flor ida, measured an average upward current of 1.3 A. The electric fields that they measured at this altitude were on the order of a few hundred volts per meter. Holzworth (1981), with a balloon near 20 km over a large thunder storm at Fort Simpson, NWT, Canada, on August 15, 1977, measured a vertical upward electric field of more than 6.7 V/m (the instrumentation threshold) for longer than 2 hours. These few data indicate that a positive current flows toward the ionosphere above thunder storm regions that is of sufficient magnitude to account for fair-weather conduction current. Is it possible to re late current output to such factors as frequency of cloud to-ground strokes, charge structure and separation dis tances, and cloud-top height? Both Pierce (1970) and Prentice and Macherras (1977) presented relationships for a latitudinal variation in the ratio between cloud-to ground and cloud-to-cloud flashes. This ratio is about 0.1 in the equatorial region, increasing to about 0.4 near

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THE GLOBAL ATMOSPHERIC-ELECTRICAL CIRCUIT 50 latitude. Do lightning-intensive tropical thunder- storms deliver more or less current than thunderstorms at higher latitudes, even though the ratio of cloud-to- ground to cloud-to-cloud flashes is smaller? A consider- able amount of research is necessary to determine the processes responsible for regulating the current flow from thunderstorms into the global circuit. This re- search is important for understanding the role of world- wide thunderstorm activity as the generator for the global circuit. Electrical Conductivity, Columnar Resistance, and Global Resistance Galactic cosmic rays are the main source of ionization that maintains the electrical conductivity of the atmo- sphere from the ground to about 60 km in altitude. Near the ground, however, there is additional ionization due to release of radioactive gases from the soil, and above about 60 km solar ultraviolet radiation becomes impor- tant. During geomagnetic storms, ionization due to en- ergetic auroral electron precipitation and to auroral x- ray bremsstrahlung radiation and proton bombardment during solar proton events can all be significant sources for the high-latitude middle atmosphere. The galactic cosmic rays that bombard the Earth's at- mosphere are influenced by the Earth's geomagnetic field, which produces a magnetic latitudinal effect in the incoming cosmic-ray flux. The full cosmic-ray spec- trum is only capable of reaching the Earth at geo- magnetic latitudes higher than about 60. At lower geo- magnetic latitudes, the lower-energy particles are successively excluded by the Earth's geomagnetic field, and only particles with energies greater than about 15 GeV reach the equator. The cosmic-ray spectrum thus hardens with a decrease in geomagnetic latitude, with the height of the maximum ion production rate decreas- ing from about 20 km at high latitudes to about 10 km near the equator. The cosmic-ray ion production rate profile for various geomagnetic latitudes during solar- cycle minimum using the data from Neher (1967) is shown in Figure 15.4. The ionization rate increases with geomagnetic latitude, and both the height of the peak and the slope of the ionization rate above the peak also increase. Near the ground there is about a 20 percent variation between the equatorial region and higher lati- tudes (Israel, 1973~. The cosmic-ray ionization undergoes a regular solar- cycle variation with maximum values near solar mini- mum. At high latitudes the ionization rate may vary by about 50 percent at 15 km and about 75 percent at 20 km. At 20 km the ionization-rate variation through the solar cycle is about 40 percent in mid-latitudes and 40 10~ '~\''"~\'\1111l - 23 38 53 0~463 213 O 101 1o2 1 o3 QO (ion pairs/cm3 sec atm) FIGURE 15.4 Cosmic-ray ion-production rate vertical profiles at various geomagnetic latitudes (Neher, 1967; R. Williamson, Stanford University, personal communications, 1981~. about 20 percent near the equator. In addition to the solar-cycle variation of galactic cosmic-ray flux, there are shorter-term variations that are associated with some magnetic storms, a 27-day quasi-periodic varia- tion, variations due to solar flares, a diurnal variation, and a variation in the amplitude of the diurnal variation with time. All these variations have been reviewed by Forbush (1966~. The diurnal variations are generally small. The solar-flare and magnetic-storm variations of cosmic-ray fluxes are larger (about 2 to 20 percent) and are more important for understanding solar-terrestrial electrical coupling mechanisms (Roble and Hays, 1982). The full impact of all these variations on the electrical conductivity and the properties of the global circuit has not yet been evaluated. Over land, the natural radioactivity of the solid ground adds to the cosmic-ray ion-production rate, not by direct radiation from the solid surface but in the re- lease of gaseous intermediaries from rocks and soil on the surface and from soil capillaries. These gaseous in- termediaries are then carried upward by vertical mass transfer, and radiation from them can affect the ion- production rate within the first kilometer above the Earth's surface (Israel, 1973~. Above about 60 km the solar ultraviolet radiation ion- ization exceeds the ion production owing to galactic cos- mic rays. The major source of ionization within the mesosphere is nitric oxide, which can be ionized to NO + by solar Lyman-alpha radiation at 121.6 nm. Above about 80 km, the EUV and soft x-ray radiation from the Sun produces ionization in the ionospheric E and F re

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;; 214 , .. "ions. These EUV radiations all have a large d~y-to-day variability as well as periodic 27-day solar rotation and solar-cycle variations. During solar flares the solar EUV and x-ray radiation can be greatly enhanced, thereby increasing the ionization source and electrical conduc- tivity of the ionosphere. The ionization sources mentioned above all contrib- ute to the electrical conductivity of the atmosphere through the production of positive and negative ions and electrons. The electrical conductivity is governed by the number of positive and negative ions and elec- trons as well as by their respective mobility. The num- ber densities of various ionic species~are controlled by complicated chemical reactions between ions and neu- tral species. Many of the neutral constituents important for ion chemistry, such as water vapor and NO, are fur- thermore transported by atmospheric motions, thereby increasing the complexity of determining the global dis- tribution of electrical conductivity. All these various processes are discussed by Reid (Chapter 14, this vol- ume). Above about 80 km, the electrical conductivity is gov- erned by collisions between neutrals, ions, and electrons and by the influence of the geomagnetic field on the mo- bility of the plasma components. The electron gyrofre- quency is greater than the electron-neutral collision fre- quency above about 80 km, and therefore the electrons are restricted by the geomagnetic-field line. For the ions this occurs above about 140 km. The differential motion of the ions and the electrons in the dynamo region be- tween about 80 and 200 km in altitude gives rise to an anisotropic behavior of the conductivity. The conduc- tivity parallel to the geomagnetic-field lines is not af- fected by the field, and it increases rapidly with alti- tude, limited only by collisions between electrons and neutrals and ions. The Pedersen conductivity is parallel to an applied electric field and orthogonal to the mag- netic field. It is smaller than the parallel conductivity, and it has a maximum near 140 km, where the ion-neu- tral collision rate equals the gyrofrequency of ions. The conductivity orthogonal to both an applied electric field and the magnetic field is the Hall conductivity, which maximizes near 105 km. The Pedersen conductivity is carried by electrons below 105 km and by ions above that altitude, whereas the Hall current is mainly due to electrons. A typical profile of electrical conductivity through the daytime atmosphere is shown in Figure 15.5. Near the Earth's surface the electrical conductivity is about 10- ii mho/m. It increases exponentially with altitude, having a scale height of about 6 km until about 60 km, where the effects of free electrons become important and there is an abrupt increase in the conductivity. RAYMOND G. ROBLE and ISRAEL TZUR \ ~ M chemosphere \ 7L rm \ \~ ah\ \ \ ~ Dynerno R - 'on / K)C /a, E y _ ~ :r Dower Atmosphere l -1 2 -10 -8 ~ -4 Earth ~ Interior -low _ I I _ O 2 log a (A m ) FIGURE 15.5 Altitude variation of the electrical conductivity in the Earth's atmosphere and ionosphere from the ground to 200 km. The electrical conductivity within the Earth is shown for comparison (Vol- land, 1982~. Above about 80 km the geomagnetic field introduces an- isotropic conductivity components. The parallel con- ductivity continues to increase with altitude, whereas the Hall and Pedersen conductivities peak near 105 and 140 km, respectively, before decreasing with altitude. The electrical conductivity of the Earth is about 10-3 mho/m, and therefore the Earth's atmosphere can be considered a leaky dielectric sandwiched between two highly conducting regions the Earth's surface and the ionosphere. A calculation of the latitudinal variation of the verti- cal and horizontal electrical conductivity components in the daytime atmosphere during equinox by Tzur and Roble (1983) is shown in Figure 15.6. The electron den- sities above about 10 km are calculated using the model of Reid (1976, 1977) and the properties of the neutral atmosphere are specified from the model of Solomon et al. (1982a, 1982b). Below 10 km, the electrical conduc- tivity is represented by the Gish formula, with a latitu- dinal variation as determined from Israel (1973~. The electrical conductivity increases abruptly above about 60 km because free electrons are present in the daytime ionosphere, and the effect of the geomagnetic-field line becomes apparent above 80 km. The electrical conduc- tivity parallel to the geomagnetic-field line is larger than either the Hall or the Pedersen conductivity, so the verti- cal component of the electrical conductivity is small in the equatorial region, where the magnetic-field line is horizontal; and the horizontal component of the electri- cal conductivity is small in the polar region, where the magnetic-field line is vertical. This calculation for the global distribution of electri- cal conductivity represents an idealized case considering fair-weather conditions. In reality, the conductivity is

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THE GLOBAL ATMOSPHERIC-ELECTRICAL CIRCUIT 90 80 70 - Y 60 50 5 40 30 o 80 60 40 20 0 -20 -40 -60 -80 LATITUDE probably quite variable depending on variations of ion- ization rates, ion and neutral chemical reactions, aero- sol and cloud interactions, and a host of other meteoro- logical factors. The electrical columnar resistance that determines the local air-earth current flow and electric field is determined by the height integral of the recipro- cal of the electrical conductivity distribution. The bulk of the columnar resistance resides in the troposphere, which can be strongly controlled by electrical conduc- tivity variation owing to factors such as clouds, fog, aerosols, and pollution. Boeck (1976) showed that changes in the global conductivity may result in from 85Kr being released into the atmosphere. The low-fre- quency radiation from power lines (Vampola, 1977) may also affect the precipitation of electrons and conse- quently affect ionization in the stratosphere. The co- lumnar resistance derived from the Gish formula is about 1.3 X 10~7 Q/m2. This fair-weather value proba- bly varies considerably in place and time as determined by aerosol and weather conditions. The global variation of the columnar resistance is not well known, yet it is an important property of the global electrical circuit. The global resistance is the parallel circuit resistance ob- tained by adding the various columnar resistance val- ues. Muhleisen (1977) estimated a global resistance of 230 Q without mountains and 200 Q when the Earth's orography is considered. The magnitude of the variabil- ity of global resistance is poorly known, and it is an im- portant parameter that needs to be determined in order to improve our understanding of the Earth's global elec- trical circuit. Fair-Weather Vertical Current Density and Total Current The total current flowing in the global circuit is not well known, and only crude estimates have been made. 215 80 60 40 20 0 -20 -40 -60 -80 LATITUDE FIGURE 15.6 Latitudinal distribution of the (a) vertical and (b) horizontal components of logo ~ (mhos/m), where ~ is the electrical conductivity (Tzur and Roble, 1983). Its value is generally estimated by integrating the mea- surable fair-weather vertical current density over the fair-weather area of the globe. A recent estimate by Muhleisen (1977) used 10-12 A/m2 for inhabited and in- dustrialized areas, 2-4 X 10-12 A/m2 for vegetated ground and for deserts, 2.5 X 10-12 A/m2 over the At- lantic Ocean to derive a total global current of about 1000 A. About 750 A is derived for the current flow over oceans and 250 A for the current flow over the conti- nents. Muhleisen (1977) pointed out that the columnar resistance over mountains is much smaller than over flat land near sea level, and he estimated that as much as 20 percent of the global vertical current streams toward mountains. Gathman and Anderson (1977) showed that the air-earth current also has a latitudinal variation due to the effect of the Earth's geomagnetic field on the cos- mic-ray ionization rate throughout the troposphere. Another means of estimating the total current flow in the circuit is to estimate the total number of thunder- storms working simultaneously and multiply that value by the average current output determined from mea- surements over thunderstorms, as was discussed previ- ously. Estimates of the number of global thunderstorms range from 1500 to 2000. If these numbers are multi- plied by the average current output of thunderstorms (0.5 to 1.0 A), the total current is about 750 to 2000 A, which is nearly the same magnitude as the total current derived by air-earth current estimates, about 1000 A. Blanchard (1963) indicated that currents as large as hundreds of amperes flow from the ocean surface into the atmosphere as a result of electrified droplets that are ejected from the bursting of small bubbles. It also should be mentioned that intense electrification is associated with some volcanic eruptions. When such phenomena occur, they may have a significant input into the global electrical circuit. There is considerable uncertainty with these estimates that needs to be resolved. It is un

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216 likely that the global current can be accurately deter- mined from only a few localized measurements, and some other means must be used to determine its value. Localized measurements, however, are needed to cleter- mine the range of variability, especially from globally representative stations such as on a mountaintop (Rei- ter, 1977a). Ionosphere Potential An important parameter for determining the electri- cal state of the global circuit is the ionospheric potential, which specifies the potential difference between the ground and the ionosphere assuming that the ground is arbitrarily referenced to zero. This quantity can be esti- mated by using the values of the global resistance, about 200 Q. and the values of the total current flowing in the circuit, 750 to 2000 A, to get 150 to 400 kV. The ionospheric potential is of fundamental impor- tance for the global circuit because it is one of the few measurable global parameters. It can be determined by integrating the altitude profile of the electric field mea- sured from ascending or descending balloons or aircraft. This technique uses the facts that the electrical conduc- tivity of the ionosphere is so large that any horizontal potential difference is small and the entire ionosphere has a uniform potential difference with respect to the Earth. At high latitudes, however, it is necessary to con- sider the horizontal potential differences that are gener RAYMOND G. ROBLE and ISRAEL TZUR ated by the solar-wind/magnetospheric generator dis- cussed later. Another factor is that the electric field decreases exponentially with altitude in such a manner that the product of the electrical conductivity and elec- tric fielc] remains constant with altitu~le. Therefore, the bulk of the ionospheric potential drop occurs within the first few kilometers above the Earth's surface, a region that is easily attainable by balloons and aircraft. The ionosphere potential can also be determined from measurements of the air-earth current density and electrical-conductivity profiles. Muhleisen (1977) sum- marizec] the distribution of his ionospheric potential measurements made from balloons during the period 1959 to 1970. The minimum measured potential was 145 kV, the mean 278 kV, and the maximum 608 kV. The measurements also showed that the diurnal UT var- iation of ionospheric potential is similar to the Carnegie curve, shown in Figure 15.3(top), and that there is an 11-year variation in ionospheric potential that is out of phase with the solar sunspot cycle. Markson (1976, 1977) made aircraft measurements of the ionospheric potential that are shown in Figure 15.7. These measure- ments have a diurnal UT variation similar to the Carne- gie curve and also show the magnitude of day-to-day variability that is associated with the inherent variabil- ity of the worldwide thunderstorm generator and, to a smaller extent, the global resistance. Measurements of the ionospheric potential are important for understand- ing the global electric circuit and should be continued. FIGURE 15.7 Summary of ionospheric po- ~. ~ . . . . . . tential measurements as a function of time 330 _ A (UT) of aircraft soundings made by Markson 320 _ / \ _ /: 160 ~\ 1 SO 7 ~2 4 3 ~O ~O ~2 4 ~'4 34 ' 27 27 30 27 24~ not2r~r ~nterwo. i I ~ I l I I I I I I I I I I I ~ I I I I ~ I 0 1 2 3 4 5 6 7 8 9 10 it 12 13 14 1S 16 17 18 19 20 21 22 23 0 HOUR (UT)

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THE GLOBAL ATMOSPHERIC-ELECTRICAL CIRCUIT (a) 1 500 LLI 1000' He L, 500' - x9 1500 50C o x9 o - 20,000 Lo zag 1 5,000 I1J ~90 O X Q OCR for page 206
222 FIGURE 15.13 Contours of calculated (a) ground potential gradient in volts per meter along the Earth's surface and (b) ground cur- rent density in amperes per square meter `~hen multiplied by 10- Id. (c) and (d) are per- spective illustrations of the ground potential gradient and ground current density, respec- tively. > a As ~ 150 o ~ SO o o 'Jo creasing geomagnetic latitude to about 160 V/m above about 60. The electric field is greatly disturbed in re- gions under thunderstorms, indicating an upward-di- rected field. The electric field is not greatly modified by the mountains because of the large grid spacing (about 5 in latitude and longitude). The ground electric cur- rent, however, is strongly influenced by the mountains, as shown in Figure 15.13(b). The contours of the en- hanced fair-weather current flow nearly outline the continental regions, with the largest current flowing into the high mountain areas (e.g., Tibet, Andes, Ant- arctica, Rocky Mountains). This is primarily due to the larger electrical conductivity, with respect to sea level, that exists on the high mountain peaks and to the de- creased columnar resistance over mountains. A compar- ison with a similar calculation made without mountains reveals that about 20 percent of the total current flows into the high mountain areas. Other features of the model calculations are described in detail by Hays and Roble (1979) and Roble and Hays (1979~. Model Improvements The global models of atmospheric electricity that have been constructed are primarily analytical models that have considerably simplified mathematical pre- scriptions. These models, nonetheless, provide consider- able insight into the electrodynamics of the global cir- cuit. There is a clear need to develop numerical models that allow a more realistic prescription of physical pro RAYMOND G. ROBLE and ISRAEL TZUR (a) 9O I c, in, , , I , , , , , , 70 50 30 10 -10 30 50 70 90 0 60 120 180 240 300 360 LONGITUDE (c) x90 (d) ~0 , ~ o (b) ~2 n i) By J em 60 120 180 240 300 360 LONGITUDE E ~: 15 o x 10 at 5 it,, O o x90 LATITUDE 90 cesses. For example, the analytic model of Hays and Ro- ble (1979) assumed an ionosphere with a uniform con- ductivity and electrical vertical profiles that are represented by two exponential functions and simulates the latitudinal variations of cosmic-ray ion production rates only crudely. A numerical model could adopt a more realistic cal- culation with latitude and longitude, for example, such as that shown in Figure 15.6, and also allow for a day- night variation of electrical conductivity. Realistic per- turbations to the global pattern due to solar-terrestrial influences could then be modeled to determine the mag- nitude of the global response to such events (Tzur et al., 1983~. A numerical model could also be expanded to in- clude the total Maxwell current instead of only electrical conduction currents. Such modifications are important, especially for the middle atmosphere where ambipolar diffusion can alter the distribution of currents and fields as discussed by Tzur and Roble (1983), and for the tro- posphere, where convection, precipitation, conduction, lightning, and displacement currents can be important in disturbed regions. The main improvements in any global model will come primarily by more accurate parameterizations of electrical processes within the troposphere. The numeri- cal model should include the electrical charge structures and conductivity modifications due to clouds and fog, a prescription of turbulent convective processes within the planetary boundary layer, physical charge-transfer process due to the nature of the Earth's surface (e.g.,

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THE GLOBAL ATMOSPHERIC-ELECTRICAL CIRCUIT forests, deserts, glaciers), aerosol and smoke generation and dispersion, and anthropogenic processes (Anderson, 1977~. A global-scale representation of these lower-at- mospheric processes is necessary to evaluate their im- pact on the global circuit. One means of obtaining infor- mation on such global processes is to use output from the various general circulation models (GCMs) that have been developed to study the dynamic meteorology of the Earth's atmosphere. The GCM-calculated winds, tem- perature, humidity, cloudiness, turbulence, and other meteorological phenomena could be used to develop the electrical parameterizations for use in global models of atmospheric electricity. A coupled interactive electri- cal-dynamic GCM may greatly improve our under- standing of various electrical processes within the global circuit. Regional Modeling Global models of atmospheric electricity are gener- ally constrained by computer size to a grid that is on the order of 5 in latitude and longitude (about 500 km). The electrical processes need to be parameterized on that scale for insertion into the global models. There is a clear need for the development of regional and local electrical models that use appropriate boundary condi- tions provided by a global model to resolve subgrid-scale phenomena and to investigate electrical phenomena in a more limited area. Considerably more physics can be incorporated into such ' models, and the results in turn can then be used to provide appropriate parameteriza- tions of these processes for inclusion into global models. The calculated electrostatic potential contours and vectors of current flow over a mountain plateau and mountain peak (Tzur and Roble, 1985a) are shown in Figures 15.14(a) and 15.14(b), respectively. The re- gional model employed for these calculations used a de- tailed representation of electrical conductivity through- out the atmosphere, with boundary conditions that allow free current exchange between the global and re- gional models. It is assumed that the plateau and moun- tain perturb electrical quantities locally but are small enough with respect to the globe that their feedback into the global circuit is small. Over the plateau the vertical electric current flow is about three times larger than over sea level, primarily because of the reduced colum- nar resistance over the elevated surface. The results also show considerable horizontal current flow in the upper atmosphere, because of the presence of the plateau, in- dicating a local readjustment of the current system. A similar calculation for a mountain peak is shown in Fig- ure 15.14(a). The mountain is seen to distort signifi- cantly the potential pattern in such a manner as to cause 223 ELECTRIC POTENTiAI~, LOG ~ {V, 4 12 - 10 E - c, 8 ~ 6 Is 4 2 10 S 0 5 10 100 90 80 70 60 50 40 30 20 10 DISTANCE (km) ~ I ~ r ~ I ~ I ~ I ~ I ~ I ~ I ~ I ~ I ~ - (b) _ 1 /~: ~ ~ l ~ - 1 1 1 1 1 - 1 1 1 1 1 ~ ~ ~ ~ - 1 1-~ ~ ~ l: _ ~ 1 1 1 1 l ~ l ~ ~ _ 1 - 1 1-- 1-~ ~ ~ ~ ~ 1 ~ ~ = ~ O u .e~ 0 20 40 60 80 100 120 140 160 180 200 220 FIGURE 15.14 Regional calculation of the potential distribution and current density flow over a mountain peak (a) and a mountain plateau (b). The length of the maximum arrow indicates a current density of 2 X 10- i2 A m~2 in (a) and 1.8 x 10- ii A/m2 in (b) (Tzur and Roble, 1985b.) an enhanced current to flow into the peak. These calcu- lations are numerical extensions of the analytic mathe- matical procedure that Kasemir (1977) used to calculate the electric current and field distributions around mountains. Such calculations are important to interpret various measurements of the fair-weather electric field and current in the vicinity of mountaintops (Cobb et al., 1967; Cobb, 1968) and provide a quantitative frame- work to evaluate the extent of the electrical disturbance source by mountains of various shapes and also to deter- mine the important characteristics that need to be incor- porated into global models. Another important regional problem is to investigate the electrical interaction of a thunderstorm with its im- mediate environment. Thunderstorms are considered point current sources in the global model, and regional calculations on a much smaller scale are needed to ex- amine such problems as the magnitude of the current output from thunderstorm models and its relationship to the characteristics of its electrical environment. For example, the effect of the ionospheric magnetic-field- line configuration on the vertical current output from a thunderstorm that is represented as a dipolar current source (Tzur and Roble, 1985b) is shown in Figures 15.15(a) and 15.15(b). When the geomagnetic-field lines are assumed to be vertical tFigure 15.15(a)], the upward current flow in the middle atmosphere is con- fined to the immediate vicinity of the storm, whereas when the geomagnetic-field lines are assumed to be hor- izontal there is considerably more horizontal current flow in the middle atmosphere. The calculations suggest differences between the current output from thunder- storms in equatorial regions and in high latitudes. These calculations illustrate the types of problems that need to be addressed with regional models, not only

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224 100 90 80 70 60 SO An 30 20 80 7n 60F SOL 4o L 30 lo 0 llll SOO 4S0 400 3S0 300 2S0 200 ISO 100 SO O SO 100 1S0 200 2S0 300 3S0 400 450 SOO DISTANCE ( km ) FIGURE 15.15 Contours of logic J (Aims), the regional vertical cur- rent flow toward the ionosphere from a thunderstorm model consider- ing an ionosphere where (a) the magnetic-field lines are horizontal and (b) the magnetic-field lines are vertical. (Tzur and Roble, 1985b.) to understand local phenomena but also to provide the appropriate guidance for incorporating these effects into global models. ELECTRICAL COUPLING BETWEEN THE UPPER AND LOWER ATMOSPHERE The main generators operating within the Earth's global atmospheric circuit are summarized in Table 15.2. As discussed in the previous section, thunder- storms are generators whose current output maintains a vertical potential difference of about 300 kV between the ground and ionosphere, with a total current flow of about 103 A. The classical picture of atmospheric elec- tricity assumes that the ionosphere is at a uniform poten- tial, and it does not account for either ionospheric or TABLE 15.2 Generators in the Global Electric Circuit THUNDERSTORMS-current output maintains a vertical potential difference of 300,000 V between ground and ionosphere. Current ~103A. IONOSPHERIC DYNAMO-tides at ionospheric heights maintain horizontal potential differences of 5000-15,000 V between high and low latitudes. Current ~ 105 A. MAGNETOSPHERIC DYNAMO-interaction of solar wind with Earth's geomagnetic field maintains a horizontal dawn-to-dusk potential drop of 40,000-100,000 V across polar caps. Current ~ 106 A. RAYMOND G. ROBLE and ISRAEL TZUR magnetospheric dynamos. The physics of both dynamos are discussed in the chapter by Richmond (Chapter 14, this volume). The ionospheric dynamo is driven by both tides gen- erated in situ and tides propagating upward from the lower atmosphere. These tides generate horizontal po- tential differences of 5-10 kV within the ionosphere, with a total current flow on the order of 105 A. The mag- netospheric dynamo, on the other hand, is driven by the interaction of the solar wind with the Earth's geomag- netic field and generates a horizontal dawn-to-dusk po- tential drop of typically 40-100 kV across the magnetic conjugate polar cap and a total current flow of 106 A. The magnetospheric convection pattern is Sun-aligned relative to the geomagnetic poles (north geomagnetic pole 78.3 N and 291 E, south geomagnetic pole 74.5 S and 127 E), and therefore the pattern remains fixed relative to the Sun but moves in a complex fashion over the Earth's surface as the Earth rotates about its geo- graphic pole. Several empirical models that describe the horizontal ionospheric potential distribution about the magnetic polar cap have been constructed (Volland, 1975, 1978; Heppner, 1977; Sojka et al., 1979, 1980; Heelis et al., 1983~. The magnetospheric convective potential distri- butions are all similar, with a positive perturbation on the dawn side and a negative perturbation on the dusk side of the magnetic polar cap. The main differences between the various models are due to small horizontal . . scale structure variations for various levels of geomag- netic conditions. The calculated average potential pat- terns over the southern hemisphere polar cap for four different universal times (0000, 0600, 1200, and 1800 UT) using the model of Sojka et al. (1980) are shown in Figures 15.16(a)-15.16(d). Satellite observations have shown that the instantaneous magnetospheric convec- tion pattern is highly variable with considerable small- scale deviations from the mean structure, indicating a turbulent plasma flow. The dawn-to-dusk potential drop across the polar cap varies from about 30 kV for quiet geomagnetic activity, to about 60 kV for average geomagnetic activity, and to about 150-200 kV during geomagnetic storms. In addition, for greater geomag- netic activity the convection pattern expands equator- ward by about 5 from its normal quiet-time position. The magnetospheric convective electric field is gener- ally confined to the vicinity of the polar cap by shielding charges in the Alfven layer of the magnetosphere. How- ever, during rapid changes of magnetospheric convec- tion a temporary imbalance in these shielding charges can occur, and the high-altitude electric fields can cause immediate effects at the magnetic equator at all longi- tudes (Gonzalez et al., 1979) . The observed propagation

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THE GLOBAL ATMOSPHERIC-ELECTRICAL CIRCUIT UT=OO O -30~ >~0 -60 / {,.. I ... ,. . s i;. | PA;,' :' -90 ~ UT= 12 -so 1 '; \ i \ ~ -120 it ~~ 15 '$ / -60 / A,... / - ,. x; f 'A'. ,: it, -120\~` -180 14490 _gc ~ 120 _ ~ 150 LONGITUDE ~ a o o o -30 a 30 .... ,,.,, ;.\ \ 'N / ,_ `N 90 0-90 :'22'''II~60 i \ -120~` I \ t`60 -60~ I ..... .... . .. / ,/ 120 LONGITUDE (b) UT= 18 0 - -\ ,.. . . , ... ... ... :.!. ':,',. 2 ;: , .. G . i. .d .\ -120\\ \~60 ;\ ~,~490 . .. ., . / 'I <~5420 ~ 150 -180 LONGITUDE (c ) LONGITUDE (d ) FIGURE 15.16 Contours of electric potential (kilovolts) associated with magnetospheric convection at high latitudes in the southern hemisphere for four universal times (a) 0000 UT, (b) 0600 UT, (c) 1200 UT, and (d) 1800 UT. The symbol G represents the geographic pole and GM the geomagnetic pole.

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226 may be due to currents either within the magnetosphere or ionosphere (Nopper and Carovillano, 1978) or through the lower-atmospheric electrical waveguide (Kikuchi et al., 1978~. The downward mapping of the ionospheric electric fields toward the lower atmosphere has been considered previously by a number of authors (Mozer and Serlin, 1969; Mozer, 1971; Atkinson et al., 1971; Volland, 1972, 1977; Chiu, 1974; Park, 1976, 1979~. These stud- ies have all shown that large horizontal-scale electric fields within the ionosphere map efficiently downward in the direction of decreasing electrical conductivity, and that downward electric-field mapping is much more efficient than upward mapping. Both Chin (1974) and Park and Dejnakarintra (1977b) showed that the anisotropy of the electrical conductivity can have an im- portant influence on the mapping properties of electric fields. Horizontal electric fields of small-scale size (~ 1- 10 km) are rapidly attenuated as they map downward into the atmosphere from ionospheric heights, but elec- tric fields of larger horizontal scales (~500-1000 km) map effectively right down to the Earth's surface, as shown in Figure 15.17. Since the electrical conductivity of the Earth's surface is large, horizontal electric fields can usually be ne- glected, and a vertical electric-field variation results to accommodate horizontal variations of ionospheric po- tential. Calculations by Park (1976, 1979) and Roble and Hays (1979) showed that the magnetospheric gener- ator can produce perturbations of +20 percent in the 140 40 ~ ~ Skm .^ \ 20 _ o QUIET Nl<;HT 10 - sooJ? 0.2 04 06 MAPP I NG FACTOR 08 1.0 FIGURE 15.17 Downward mapping factor of the horizontal ionos- pheric electric field as a function of altitude indicating the magnitude of the attenuation of the electric-field strength for various horizontal scale sizes, X, in kilometers. RAYMOND G. ROBLE and ISRAEL TZUR air-earth current and ground electric field at high lati- tudes during quiet geomagnetic periods and larger vari- ations during geomagnetic storms and substorms. A cal- culation that shows the downward mapping of a 100-kV dawn-to-dusk potential drop superimposed on a 300-kV ionospheric potential is given in Figure 15.18(a). The potential perturbation penetrates from the ionosphere down to the tropopause with little attenuation but rap- idly decreases within the troposphere to zero at the Earth's surface. The calculated electric field at the Earth's surface is 190 V/m on the dusk side of the polar cap, as shown in Figure 15.18(b) . Changes in electrical conductivity caused by varia- tions in cosmic-ray ionization during solar-terrestrial events can also change the downward mapping charac- teristics as discussed by Roble and Hays (19794. In addi- tion, they also have shown that because the magneto- spheric potential pattern is Sun-aligned in geomagnetic coordinates, a ground station, balloon, or aircraft at a given geographic location should detect variations or- ganized in magnetic local time. For early magnetic local times the ionospheric potential perturbations of the Earth's potential gradient are positive, and for later magnetic local times the perturbations are negative. At high geomagnetic latitudes these variations are super- imposed on the diurnal UT variation of potential gradi- ent maintained by worldwide thunderstorm activity. Kasemir (1972), using data obtained at the South Pole and Thule, Greenland, noted a departure of the diurnal UT variation measured at these stations from the oce- anic diurnal electric-field variation measured during the cruises of the ship Carnegie, which is generally ac- cepted as the UT variation due to worldwide thunder- storm activity. The polar curves have a similar shape to the curve derived from the Carnegie cruises but at a much reduced amplitude. From these results Kasemir concluded that another agent besides worldwide thun- derstorm activity may modulate the global circuit at high latitudes. The position of the magnetospheric potential pattern over the Earth's surface is shown in Figure 15.16 for four different UTs. It can be seen that the downward map- ping of this potential pattern to the Earth's surface gives rise to a complex UT variation due to the displacement of the geographic and geomagnetic poles. The calcu- lated UT variation of the ground electric field at South Pole Station due to the downward mapping of magneto- spheric potential pattern is shown in Figure 15.19; the Carnegie UT variation and the Kasemir (1972) mea- surements are also shown. It is seen that the positive po- tential perturbation maps down over South Pole Station from about 0200 to 1400 UT, and the negative potential perturbation from 1400 UT to 0200 UT. When this pat

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THE GLOBAL ATMOSPHERIC-ELECTRICAL CIRCUIT 100 80 E 60 - I CD I 40 20 POTENTIAL ( kV ) 1~ 1 1 (a) 1 ~ O ~ 40 50 60 70 80 90 80 70 60 50 40 LATITUDE (degrees) 195 190 ~ 185 > 180 c, J 175 170 con `t 165 <3 ~ 160 LLJ ~ 155 As O 150 ~45 1 1 1\J1 1 1 40 50 60 70 80 90 80 70 60 50 40 LATITUDE (degrees) I I I I I I - (b) tern is superimposed upon the Carnegie UT variation, it can be seen that the magnetospheric potential tends to supress the amplitude of the Carnegie variation, a result similar to Kasemir's (1972) observations. A similar situa- tion exists at Thule, Greenland, although the predicted amplitude of the magnetospheric potential variation is reduced somewhat because the station is so near the northern geomagnetic pole. There is considerable day- to-day variability associated with the magnetospheric convection potential pattern; however, in the time-av- erage sense the positive and negative variations are both out of phase with the Carnegie UT variation at these stations and may be responsible for the suppressed am- plitude of the UT diurnal variation observed by Kasemir (1972~. Stratospheric balloon measurements of magneto- spheric convection electric fields have been made for a 1.2 1.0 0.8 0.6 'S.P Convection \Ocean 130 Days , Arctic 295 Days 1 1 1 1 1 1 1 1 1 1 1 O 6 12 18 GMT 1 24 FIGURE 15.19 Normalized diurnal variation of the ground poten- tial gradient as measured in the Arctic and Antarctic by Kasemir (1972) (solid curve), the diurnal potential gradient variation from the Carnegie cruise (dashed curve), and the calculated potential gradient at South Pole Station due to the downward mapping of the ionospheric potential pattern (long/short dashed curve) shown in Figure 15.16. 227 FIGURE 15.18 (a) Calculated contours of potential (kilovolts) in a dawn-to-dusk cross section across the magnetic polar cap. A 100- kV dawn-to-dusk potential drop is superim- posed upon a 300-kV ionospheric potential background. (b) Calculated latitudinal varia- tion of the ground potential gradient (volts per meter) over the magnetic polar cap. number of years (e.g., Mozer and Serlin, 1969; Mozer, 1971; Holzworth and Mozer, 1979; Holzworth, 1981). Recently D'Angelo et al. (1982) processed over 1200 hours of stratospheric balloon data and correlated the vertical electric field with magnetic activity parame- ters. During quiet geomagnetic conditions the classical Carnegie curve was reproduced. During more active geomagnetic conditions, however, the dawn-dusk po- tential difference of the magnetospheric convection pat- tern was shown clearly to influence the fair-weather field. The above measurements suggest an electrical coupling between the magnetospheric dynamo and the global electrical circuit and indicate a need for more measurements. With the move of the incoherent-scatter radar from Chatanika, Alaska, to Sonderstrom Fjord, Greenland, and the operation of the EISCAT radar from Tromso, Norway, a unique opportunity exists to examine the high-latitude electrical coupling between the upper and lower atmosphere. SOME OUTSTANDING PROBLEMS Component processes within the global circuit have been studied for nearly a century. The ground electric field is the most common measurement, although nu- merous measurements of the electrical conductivity and air-earth current have also been made. These measure- ments form the basis of much of our knowledge concern- ing the influence of local processes on the electrical structure of the global electrical circuit. The measure- ments of currents and fields over land stations are highly variable, being subject not only to variations of the global generator but also to local meteorological and an- thropogenic influences that at times dominate the global electrical variations. Over the oceans the local influences can be less, but considerable averaging is still necessary to derive the global variations.

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228 It is unlikely that enough ground-based measure- ments of electric currents and fields can be made simul- taneously to define the instantaneous properties of the global circuit and its temporal variations. A globally representative single measurement is needed to define the characteristics of the global circuit. A measurement that has been used by a number of investigators to define the state of the global circuit is the ionospheric poten- tial. This value is derived from the height integral of the vertical electric-field profile that is measured either by ascending and descending balloons or by aircraft. An inherent property of the measurement is that the iono- spheric potential is nearly uniform over the globe be- cause of the highly conducting Earth's surface and iono- sphere. The ionospheric potential is equivalent to the product of the worldwide thunderstorm current output and the total global electrical resistance. It is generally assumed that changes in the global electrical resistance are small and that variations in ionospheric potential reflect the variations in worldwide thunderstorm cur- rent output. Only occasional electric-field soundings have been made over the years, and there is a clear need to increase the frequency of such measurements. An- other means of obtaining the ionospheric potential is from tethered balloons, as described by Vonnegut et al. (1966) and Ho~zworth (19843, that have the capability of providing high-time-resolution measurements for global studies. Markson (1978) called for the establish- ment of the ionospheric potential as a geoelectric index that gives an indication of the state of the global circuit. This index would be the electrical equivalent of the geo- magnetic index that has been used over the years to study geomagnetic phenomena within the Earth's at- mosphere. The main problem impeding progress in understand- ing the global circuit is the determination of the current output from thunderstorm generators. There are only a few measurements of the total current flow from storms, and there is a clear need for more measurements to de- fine the current output properties in terms of thunder- storm size, duration, lightning flash frequency, charge separation distance, and other parameters. Recent bal- loon measurements at stratospheric heights over thun- derstorms (Holzworth, 1981) showed prolonged periods (1-3 hours) of do electric fields reversed from fair- weather conditions, indicating a current flow toward the ionosphere. In addition to single thunderstorm measurements, it is important to be able to obtain information on the global distribution of thunderstorm occurrence. Pre- vious information has been derived from weather sta- tions, when thunder was heard from an observing site (Crichlow et al., 1971), from Schumann resonance RAYMOND G. ROBLE and ISRAEL TZUR (Polk, 1982), and from radio and spheric measurements (Volland, 1984~. More recently, lightning detection from satellites has been used to derive information on the global distribution and flash-rate frequency from space, as discussed in the chapter by Orville (Chapter 1, this volume). Krider et at. (1980) designed a ground de- tection network that detects cloud-to-ground lightning flashes and deployed the detectors in regions that cover large parts of North America. And finally, Davis et al. (1983) examined the feasibility of detecting lightning from a satellite in synchronous orbit. They estimated that three such satellites could provide worldwide cov- erage. These measurement techniques provide a capability of making progress in the century-old problem of under- standing the role of thunderstorms as generators within the global circuit. Simultaneous measurements of the ionospheric potential (either from vertical electric-field soundings or from a tethered balloon), along with mea- surements of lightning flash frequency (either cloud-to- ground flashes from a ground network or total flash rate from satellites), may determine the degree of synchroni- zation between the two phenomena. Such measure- ments can be used to monitor the electrical state of the global circuit and also can provide a global indicator to help in understanding various local and regional mea- surements. These measurements would also be useful for improving our understanding of the role of solar-ter- restrial perturbations in altering the properties of the global circuit (Reiter, 1969, 1971, 1972; Markson, 1971, 1978; Cobb, 1978; Herman and Goldberg, 1978; Roble and Hays, 1982~. There is also a need to determine the electrodynamic processes operating within the middle atmosphere. Ac- cording to the classical picture of atmospheric electric- ity, the middle atmosphere should be passive, yet cer- tain rocket measurements indicate the existence of large electric fields of unknown origin (Bragin et al., 1974; Tyutin, 1976; Hale and Croskey, 1979; Hale et al., 1981; Maynard et al., 1981; Gonzalez et al., 1982~. These large electric fields are not understood, and it has been suggested that instrumental effects may be in- volved (Kelley, 1983; Kelley et al., 1983~. It is important to resolve this issue because of the fundamental implica- tions involved in understanding electrodynamic pro- cesses within the middle atmosphere. Finally, progress in understanding the global circuit and possible solar-terrestrial coupling mechanisms re- quires a collaborative effort between observations and theoretical modeling. The measurements are needed to verify model predictions and guide model development, and the modeling results provide a physical constraint for understanding measurements and suggesting various

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THE GLOBAL ATMOSPHERIC-ELECTRICAL CIRCUIT key experiments. The technology and models are cur rently available to make progress in resolving the funda mental problem of global atmospheric electricity (Dole zalek, 1972~. REFERENCES Anderson, F. J., and G. D. Freier (1969). Interactions of the thunder- storm with a conducting atmosphere, J. Geophys. Res. 74, 5390- 5396. Anderson, R. V. (1977~. Atmospheric electricity in the real world (use- ful applications of observations which are perturbed by local ef- fects), in Electrical Processes in Atmospheres, H. Holezalek and R. Reiter, eds., Steinkopff, Darmstadt, pp. 87-99. Atkinson, W., S. Sundquist, and U. Fakleson (1971~. The electric field existing at stratospheric elevations as determined by tropospheric and ionospheric boundary conditions, pure Appl. Geophys. 84, 46- 56. Blanchard, D. C. (1963~. Electrification of the atmosphere by parti- cles from bubbles in the sea, Frog. Oceanog. 1, 71. Boeck, W. L. (1976~. 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