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OVERVIEW AND RECOMMENDATIONS 12 ated. The larger-scale horizontal electric fields that do map to the surface become vertical at the surface because of the high surface conductivity. The solar-wind/magnetosphere field can alter the surface fields at high latitudes by 20 to 50 V/m depending on the level of geomagnetic activity and the magnitude of the dawn-to-dusk potential drop across the magnetic polar cap. Telluric Currents Telluric currents consist of both natural and man-induced electric currents flowing in the solid earth and oceans. The fundamental causes of the natural currents are electromagnetic induction resulting from a time-varying, external geomagnetic field or the motion of a conducting body (such as seawater) across the Earth's internal magnetic field. These telluric currents, in turn, produce magnetic fields of their own that add to the external geomagnetic field and that produce a feedback on the ionospheric current system. The complexities associated with telluric currents arise from the complexities in the external current sources and the conductivity structure of the Earth (see Chapter 16). The external inducing field also has various scale sizes that contribute to the complexities in the telluric current systems. The ionospheric dynamo currents that are associated with the solar diurnal and lunar tides have a planetary- scale size. The ionospheric current variations, however, also have smaller-scale features that are associated with auroral and equatorial electrojets. At low frequencies, the external inducing sources can be approximated by a planetary-scale field that is occasionally altered by strong spatial gradients during geomagnetically disturbed conditions. At higher frequencies (magnetic storms, substorms, or geomagnetic pulsations), the source can often be quite localized and highly time dependent. Electromagnetic induction caused by ionospheric and magnetospheric current variations has a pronounced effect on telluric currents and on man-made systems. These effects have been detected by a number of investigators, and it is now well recognized that there is a direct electromagnetic coupling from the ionosphere to the telluric currents. The large variation in conductivity of the solid earth can give rise to various channeling effects within the Earth, thereby considerably complicating the flow patterns of the telluric currents. The current patterns are different for different frequencies of external induction. The longer the period of the time-varying field, the deeper into the Earth the induced currents are expected to flow. For example, a signal with a period of about 24 hours is generally believed to have a skin depth of 600 to 800 km. The distribution of sediments, the degree of hydration, differences in porosity, and other properties of the Earth all have an influence on the signal response. Properly interpreted, telluric currents can be a tool to study both shallow and deep structures within the Earth. Techniques for Evaluating the Electrical Processes and Structure The cornerstone of our understanding of the Earth's electrical environment is an integration of measurements, theory, and modeling. The new instruments and techniques that have been developed in recent years are diverse, and various chapters in this volume contain details on techniques beyond those illustrated in this Overview. Remote measurements of electric and magnetic fields can now be used to infer many properties of lightning and lightning currents. Also, the amplitude and time characteristics of thunder and various radio-frequency (rf) noise emissions can be used to trace the geometrical development of lightning channels within clouds as a function of both space and time. There are now large networks of ground-based lightning detectors that
OVERVIEW AND RECOMMENDATIONS 13 can discriminate between in-cloud and cloud-to-ground lightning and accurately determine the locations of ground- strike points. With such a detection capability, it should now be possible to determine whether and how the characteristics of individual cloud-to-ground discharges depend on their geographic location, the local terrain, and/or the meteorological structure of the storm. The rf noise that is generated by lightning in the hf and vhf bands appears in the form of discrete bursts, and within a burst there are hundreds to thousands of separate pulses. If the difference in the time of arrival of each pulse is carefully measured at widely separated stations, the location of the source of each pulse can be computed, and the geometrical development of the rf bursts can be mapped as a function of time. Satellite observations of lightning have provided rough estimates of the global flashing rates and the geographic distribution of lightning as a function of season. Optical detectors, such as those now in orbit on the Defense Meteorological Satellite Program (DMSP) satellites, are limited in their temporal and spatial coverage, but they have provided data that show a progression of lightning activity toward the summer hemisphere and notable absences of lightning over the ocean during the observing intervals (see Chapter 1). The data to date are only for local midnight, dawn, and dusk; there is a need to obtain data at other times. Measurements of hf radio noise by the Ionosphere Sounding Satellite-B have also been used to estimate a global lightning flash rate. Global detection of lightning is necessary to determine the global flashing rate and how this rate relates to other parameters in the global circuit. In recent years, the National Aeronautics and Space Administration has developed new optical sensors that could be used to detect and locate lightning in the daytime or at night and with continuous coverage by using satellites in geosynchronous orbits. These sensors are capable of measuring the spatial and temporal distribution of lightning over extended periods with good spatial resolution and offer significant new opportunities for researchâwithout the inherent sampling biases of low-altitude orbiting satellitesâand for many applications. Artificial triggering of lightning now provides the capability of studying both the physics of the discharge process and the interactions of lightning with structures and other objects in a partially controlled environment (see Chapter 2). When a thunderstorm is overhead and the surface electric field is large, a small rocket is launched to carry a grounded wire rapidly upward. When lightning is triggered by the wire, the first stroke is not like natural lightning, but subsequent return strokes appear to be almost identical to their natural counterparts. Triggered lightning is now being used to investigate the luminous development of lightning channels, the characteristics of lightning currents, the velocities of return strokes, the relationships between currents and electromagnetic fields, the mechanisms of lightning damage, the performance of lightning protection systems, and many other problems. The main benefit of this triggering technique is that it can be used to cause lightning to strike a known place at a known time, thus enabling controlled experiments to be performed. Although lightning cannot be reproduced in full in the laboratory, several lightning simulators have been developed and have provided some quantitative information on the generation of thunder. Cloud electrification and charge-separation processes are closely coupled to the cloud microphysics and the storm dynamics. The natural storm environment is extremely complicated, and its quantification involves a host of electrical and meteorological parameters. Many of these parameters and their measurements are treated in Chapters 7 and 8 and the three-volume publication, Thunderstorms: A Social, Scientific, and Technological Documentary, edited by Kessler (1982). One of the greatest needs is for an in-cloud instrument that can measure in a thunderstorm environment the charge on the smaller cloud particles as a function of particle size and type (see Chapter 8).