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OVERVIEW AND RECOMMENDATIONS 16 common for the potential of an aircraft to be raised by several million volts, and most planes have discharger wicks to control the interference in radio communications when the aircraft goes into point discharge. Lightning will not usually present a hazard to commercial aircraft as long as the present design practices are continued and the standard practice of avoiding large thunderclouds is maintained. However, since many new aircraft are being developed with composite materials instead of aluminum and with the increased use of computers and microcircuit technology, the decreased electrical shielding on the outside and increased sensitivity inside means that there will be an increasing vulnerability to lightning disturbances (see Chapter 5). Thus, cloud electricity and lightning will have to be considered carefully in the design and operation of future aircraft systems. Natural telluric currents can significantly disturb man-made systems such as communication cables, power lines, pipelines, railways, and buried metal structures. The largest natural disturbances are associated with the intense auroral current systems that flow at high latitudes during geomagnetic storms. There have been frequent reports of these disturbances, inducing currents on long telephone and telegraph wires that are large enough to generate sparks and even permanent arcs. When this occurs, there can be outages and shutdowns in both land and sea cables and fires can be started by overheating the electrical systems. Currents of up to about 100 A are sometimes induced in power transformers at northern latitudes and cause power blackouts and system failures. During the large geomagnetic storm of February 11, 1958, the Toronto area suffered from an induced power blackout. Long pipelines are also affected by telluric current disturbances. The Alaskan pipeline has been the subject of careful investigation, principally because of its location across the auroral zone. One of the concerns has been the rate of corrosion of the pipeline, which is enhanced by telluric currents. However, telluric currents appear to affect electronic equipment related to operational monitoring and corrosion control rather than to produce specific serious corrosion problems. A relationship between the expected current flow and geomagnetic activity has been derived and suggests that the pipeline is a large man-made conductor that is capable of significantly affecting the local natural regime of telluric currents. There is also a concern that the long, power-transmission lines planned for future arctic development will be subject to larger induced currents by auroral activity than was previously considered. This would require new protection equipment development for high-latitude applications. Telluric currents have also been used in the search for natural resources with two different approachesâ magnetotellurics and geomagnetic depth sounding. Telluric currents can also be used to study long-period tidal phenomena and water flows and the Earth's astronomical motion and as possible precursors for earthquakes and volcanic eruptions. It has also been suggested that a natural waveguide for telluric currents in the Earth's crust, consisting of an insulating layer of dry rocks sandwiched between an upper hydrated conducting layer and an underlying conducting hot layer, could be used for communications. There are also investigations to determine the feasibility of using the natural resonances in the earth-ionosphere waveguide, Schumann resonances, as a means for long-distance communications. Recommendations An increased interest in understanding the Earth's electrical environment has resulted from recent advances in different disciplines, along with the recognition that many of man's modern technological systems can be adversely affected by this environment. This understanding appears to be on the threshold of rapid progress.
OVERVIEW AND RECOMMENDATIONS 17 There should be a concerted effort of coordinated measurement campaigns, supported by critical laboratory experiments, theory, and numerical modeling of processes, to improve our understanding of the Earth's electrical environment. Because the study of the electrical environment is commonly divided into three major componentsâlightning, cloud electricity, and the global circuit (including ion chemistry and physics and ionospheric, magnetospheric, and telluric currents)âthe specific needs in these areas are detailed below. However, it should not be forgotten that there are interactions among these components and that the understanding of these interactions may be fundamental to an understanding of an individual component. 1. More needs to be known about the basic physics of the lightning discharge and its effects on structures in order to design proper protection systems. Most lightning begins within a thundercloud, but the initiation and subsequent development of a flash within the cloud are poorly understood. The physics of electrical breakdown over distances of 10-10,000 m is not understood, nor is the relationship between the channel geometry and the fields and charges that existed before the discharge. The fields and currents that are produced by most of the important lightning processes have large submicrosecond variations, but how the discharge currents develop as a function of space and time and what the ranges of variability of the maximum I and dI/dt parameters are need to be determined. The power and energy balances within the lightning channel and many other important lightning parameters also need to be determined. There should be a comprehensive and carefully coordinated effort to understand the basic physics of intracloud and cloud-to-ground lightning discharges and their effects on our geophysical environment . This new knowledge should be applied to the development of improved lightning-protection methods. Several new techniques are now capable of providing much insight into the complex and varied physical processes that occur during a lightning flash. For example, radio interferometry and time-of-arrival methods can be used to trace the three-dimensional development of lightning channels with microsecond resolution. Rockets can be used to trigger, lightning under a thunderstorm, so that many of the physical properties of the discharge and its interactions with structures can be studied in a partially controlled environment. 2. The question of how thunderclouds generate electricity has been a fascinating scientific problem for over two centuries, but only in the past decade have cooperative experiments using new experimental techniques provided valuable insights into the complex and varied electrical processes that occur within clouds. Unfortunately, there are no sensors that can determine the charge-size relationship on the smaller cloud particles inside a thunderstorm; thus the data are not adequate to determine which of the many possible mechanisms dominate the generation and separation of charge. In addition to not knowing the charge-size relationship for various cloud particles, it is not known how this relationship evolves with time when there is lightning. The electrical forces on individual elements of precipitation can be several times larger than gravity, but further research is needed to determine whether (and how) these and other electrical effects play a significant role in the formation of precipitation. In view of the successes of recent research, significant new understanding of cloud electricity and lightning can be made by continuing to develop new instruments and by making coordinated in situ and remote measurements of selected thunderclouds. These studies should be complemented by measurements of cloud microphysics and dynamics, by comprehensive laboratory studies, and by theory and numerical modeling. The complexity of the processes that produce both precipitation and lightning makes
OVERVIEW AND RECOMMENDATIONS 18 it impossible to construct or validate theories of cloud electrification from simple field experiments. It is only through the complementary efforts of comprehensive field observations, laboratory experimentation, and numerical modeling that we can hope to understand the physical processes that are important in thunderstorms. An improved understanding of the major processes that create strong electric fields and their interactions with cloud particles and precipitation might lead to better forecasting of electrical hazards to aviation, forestry, and other outdoor activities. The first goal of the in-cloud measurements should be to determine the charge-size relationship for various cloud and precipitation particles and the role of screening layers in the upper and lower regions of the storm. The electric current densities that flow above and below the cloud should also be monitored as a function of time. Since the natural storm environment is complicated, laboratory experiments should focus on the detailed physics of mechanisms that appear to be important on the basis of both the incloud measurements and the numerical models. Laboratory experiments should also determine the effects of electric fields on drop coalescence efficiencies and the ability of electrified drops to scavenge charged constituents of atmospheric aerosols. Analyses of the in-cloud and laboratory data could be accelerated through the establishment of a common data base, particularly for theory and numerical modeling efforts. 3. Even in fair weather the solid earth and atmosphere are electrified. Thunderstorms have been identified as the dominant generator in the global electric circuit, but many details remain concerning storms as electrical generators and their electrical interactions with their neighboring environment. Lightning and the steady currents above and below thunderclouds play an important role in maintaining an electrical potential between the upper atmosphere and the surface, but the amount and type of lightning and the values of cloud currents that flow to the surface and the upper atmosphere are not well known. The lightning phenomenology and cloud currents may depend on many factors, such as the geographical location of the storm, the season, and the meteorological environment; these dependencies have yet to be determined. The charge transports to the surface under a storm are due to linear and nonlinear field-dependent currents, precipitation and other forms of convection currents, and lightning. Unfortunately, the values of each of these current components and their dependence on the stage of the storm, the lightning-flash frequency, or the local terrain are poorly known. The charge transports to land and ocean surfaces that occur in fair weather, and also to mountainous terrain, need to be determined. With recent progress in the development of satellite lightning sensors and the technology for measuring the electrical effects of storms with rocket-, balloon-, aircraft-, and ground-based sensors, a new attack on this fascinating problem of atmospheric electricity is needed. There should be an effort made to quantify further the electrical variables that are acting in the global electric circuit and to determine their relationship to the various current components that flow within and near thunderstorms. There is also an important need for theoretical and numerical studies to quantify further the role of thunderstorms as generators in the global circuit. The establishment of the ionospheric potential, or some other globally representative parameter, as a geoelectrical index that gives an indication of the state of the global circuit would be extremely useful. This index would be the electrical equivalent of the geomagnetic index that has been used for many years to characterize geomagnetic phenomena. The effects of stratiform clouds and large-scale cyclones on the global circuit also need to be quantified. Once a globally representative parameter that describes the state of the global circuit has been obtained, it can then be related to other remotely observed
OVERVIEW AND RECOMMENDATIONS 19 quantities such as the global lightning flashing rate or directly observed quantities such as the air-earth current or surface electric field. 4. Electromagnetic and optical sensors, both on the ground and on satellites, can be used to (1) detect and map lightning on a regional, national, and global scale and (2) determine, for the first time, how much lightning actually occurs and its geographic distribution as a function of time. With ground-based sensors, it should be possible to determine whether and how the characteristics of individual lightning flashes depend on their geographical location and the storm structure. If a global detection capability were implemented, it would be possible to map and monitor the intensity of lightning storms and to examine the effects of lightning on the global circuit, the ionosphere, and the magnetosphere. When combined with simultaneous spectroscopic measurements, the satellite data could also be used to determine when and where lightning produces significant concentrations of trace gases in the atmosphere. A lightning sensor, capable of measuring lightning flashes during both day and night, should be flown on a geosynchronous satellite at the earliest possible date. The resulting data when combined with those from other sensors and data from ground-based detection networks will provide information that could be used to relate lightning to storm size, intensity, location, rainfall, and other important meteorological parameters. 5. Electrical processes in the lower atmosphere and, in particular, within the planetary boundary layer, are important because these, together with global variations, determine the electrical environment of man and the biosphere. Galactic cosmic rays and various radioactive decays produce atmospheric ions that undergo a complex and still only partially understood series of ion-chemical reactions. The composition of the ions is poorly known between the surface and about 50 km, and profile measurements are needed. How the ion characteristics relate to atmospheric aerosols and various trace gases needs to be determined before the bulk electrical properties of the atmosphere can be understood. A significant fraction of the ions attach to atmospheric particles; therefore, smoke and other forms of particulates can significantly affect the electrical properties of the lower atmosphere. Turbulence and convection in the planetary boundary layer play an important role in establishing the vertical distributions of ions, trace gases, and particles. These processes also transport space charge and drive convection currents that alter the electrical properties of the planetary boundary layer. The clarification of the chemistry of atmospheric ions, their mobilities, and the physics of electrical processes in the troposphere and stratosphere will require further measurements, particularly in determining how these processes are affected by man's activities and natural events . There is also a need for further laboratory measurements and modeling to determine the important chemical reactions and ion composition in the atmosphere. A number of meteorological research stations in a variety of geographic locations should begin to measure electrical parameters routinely to determine the relationships between electrical and meteorological processes. Vertical profile measurements of electrical properties should be continued in an attempt to determine their relationships to aerosols and trace-gas chemistry. Provisions should be made for the expansion of such synoptic measurements during planned international programs (e.g., the Global Change Programme of the International Council of Scientific Unions, which is currently in the planning stages). 6. Recent research has indicated that the mesosphere may not be electrically passive but may, in fact, contain active electrical generators that are not understood. In addi