Space Plasma Physics: The Study of Solar-System Plasmas (1978)

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IMPACTS OP IONOSPHERIC/MAGNETOSPHERIC PROCESSES ON TERRESTRIAL SCIENCE AND TECHNOLOGY Edited by L. J. Lanzerotti Bell Laboratories Murray Hill, New Jersey 0797^ CONTENTS 1, 2, 3 4, 5, 6, 7 Introduction 10 Scintillation of Communication Satellite Signals at GHz Frequencies Magnetic Storms and Cable Communications Magnetosphere Impacts on Ground- Based Power Systems The Space Radiation Environment: Effects on Space Systems Electrostatic Charging on Spacecraft Space Power Systems: Ionosphere Impacts Deep Earth Induction Studies Making Use of Magnetospheric-Ionospheric Current Systems Considerations of Telluric Current Effects on Pipelines 9.1 Pipelines at Middle to Low Latitudes 9.2 The Alaskan Pipeline Problems of Magnetic Fluctuations in Geophysical Exploration R. R. Taur C. W. Anderson D. J. Williams G. A. Paulikas S. E. DeForest B. K. Ching D. I. Gough A. W. Peabody W. H. Campbell M. S. Reford 1177

1178 1. INTRODUCTION Naturally-occurring plasma processes in the space environment around the earth have produced visual spectacles for mankind in the form of the aurora since the polar regions were first visited and populated. With the growth of a technological civilization the auroral phenomenon assumed a more important role than that of a merely awesome display in the heavens. Its importance occurred because of the fact that many inventions and technical advances made use of the auroral- producing natural environment for their operation and/or were designed to function in an environment whose physical state is determined by little-understood processes. Early in this century, of course, it was the exis- tance of the ionosphere that made successful Marconi's wireless transmissions over large distances. It was whistler waves, produced by lightning and propagating in the magnetosphere, that caused interference on early communications systems in the first 'vorld War. It has long been known that drastic changes in the ionospheric conductivity produced by particle ejections from solar flares and by large magnetic storms would affect radio communications over a large portion of the globe. As the technical needs of society have begun to require the utilization of larger portions of the space around the

1179 planet, including the placing of sophisticated systems into this space, the impacts of the environment on the systems have become ever more important considerations. In addition, new ground-based technologies can be influenced and affected by space plasma processes. It is not unreasonable to expect, as past experience shows, that as technology changes and advances, subtle and not-so-subtle physical processes in the environment (probably unthought of at present; see chapter by Sturrock, this volume) will be of importance in determining how the technology will be adapted for implementation. This chapter contains ten brief reports written by engineers and scientists who have been concerned with the implementation of science and technology under the constraints of the terrestrial space environment as it is presently known and understood. The subject areas were selected both because of their present technological and scientific importance as well as for their possible future interest in technological planning, Common themes occur in several of the reports. For example, whenever society has needed long conductors to accomplish a technical task, the waves and current systems of the ionosphere and magnetosphere have been found to produce undesired effects by inducing currents in the conductors. These effects may consist of disruptions of the conductors in power lines or of the cables used in long-haul communications. The effects may take the form of unwanted background that hampers corrosion engineering survey work on pipelines.

1180 Ionospheric and magnetospheric current systems may produce unwanted effects on long conductors, but they are needed scientifically as the "source" systems for carrying out deep induction studies of the earth's crust and upper mantle. However, the scale size of these source systems must be known, and the dynamical nature of the ionosphere and magnetosphere must be considered during such studies. These same current systems, so necessary for scientific deep-induction studies, provide an unwanted background for geophysical prospecting work in the search for petroleum and mineral resources. The particles in the magnetosphere can affect space systems in a variety of ways. A concern since the discovery of the radiation belts has been the radiation damage of semi- conductor components and systems by high energy particles of magnetospheric and solar origin. However, the low energy particles also affect spacecraft systems by producing differential electrical charging, with subsequent electrical breakdowns and discharges on satellite surfaces. Satellite logic and control systems can i be drastically altered by such discharges. The following reports do not, by any means, touch upon all of the technological and science areas that presently are impacted by magnetospheric and ionospheric processes. They do, however, provide representative examples of the wide range of topics of current interest. Each individual section is identified as to its major contributor.

1181 2. SCINTILLATIONS OF COMMUNICATION STATELLITE SIGNALS AT GHz FREQUENCIES (R. R. Taur, COMSAT Laboratories, Clarksburg, MD 20734) 2.1 Introduction Rapid fluctuations (scintillations) of radio frequency signals propagated through the ionosphere have long been a concern to all designers of communication systems, civilian as well as military. The scintillations occur at all frequencies, from the HF(3-30 MHz ) on up. It was believed that by going to microwave frequencies (GHz and higher), in addition to increased communication bandwidth, the ionosphere would no longer be a problem because it is essentially non-absorbing at such frequencies. However, it was realized from the initial implementation of the INTELSAT network (using 4 and 6 GHz) that scintillations of the signal amplitudes (with peak-to-peak excursions ranging from 1 to 10 dB) are frequently reported from some earth stations located at low latitudes. Studies of the characteristics and patterns of the fluctuations led to the suggestion that they are produced by irregularities in the F-region (200-300 km) of the ionosphere (Craft and Westerlund, 1972; Taur 1973). Since that time, extensive experimental and theoretical investigations have been conducted to further understand and explain this phenomenon. The morphology of the satellite -signal characteristics is now reasonably, well mapped out as a result of the observational work. However, there is not yet an adequate theory that can satisfactorily relate ionospheric scintillations at GHz frequencies

1182 to ionospheric scintillations at VHP, (30-350 MHz) which have been studied for many years. This situation may be attributed to the difficulty of calculating the multiple scattering effects as well as a lack of detailed knowledge concerning the structure of ionospheric irregularities, whose production mechanism remains largely unknown. Nevertheless, it appears that with the current knowledge of the morphological be- havior and statistics of the ionospheric scintillationsi reasonable design margin requirements can be estimated for inclusion in satellite communications systems in order to ensure satisfactory performance of the systems (Taur, 1973). 2.2 Characteristics of Ionospheric Scintillations 2.2.1 Patterns of Occurrence Scintillations in the GHz frequency range occur principally between ±25° geomagnetic latitude near the years of maximum solar activity (Taur, 1973). Small amplitude signal fluctuations in the auroral zone have been reported by Pope and Fritz (1970). The boundary of the geomagnetic equatorial zone that produces scintillations appears to shrink with decreasing solar activity (Taur, 1974). Measurements were made in southeast Asia during 1975-1976 to investigate the equatorial scintillation "boundary" during near-minimum solar activity. Preliminary results of data analyzed to date indicate that, during periods of minimum solar activity, the boundary is located at about ±20° geo- magnetic latitude. The variations of scintillations with latitude and longitude have not been established as yet due to the limited number of observing stations and satellites.

1183 The scintillations are observed almost exclusively shortly after local ionospheric sunset during the months near equinox (Taur, 1973). As the number of sun spots de- creases, it has been observed that the total number of days on which scintillation is observed decreases accordingly. There is apparently no change in the diurnal pattern as the level of solar activity changes. 2.2.2 Spectral Properties Figure 2.1 contains a typical power spectrum of ionospheric scintillation at 4 GHz. These data were taken from a scintillation event measured at the Hong Kong earth station in September 1973. It is seen that, for frequencies > 0.125 Hz, the spectrum rolls off with an average slope of f . This implies that under weak scattering conditions the 3-dimensional spatial power spectrum of the ionospheric electron density fluctuations should follow a power law proportional to f~5 (Taur, 1976). The roll-off of the scintillation spectrum starting at about 0.125Hz indicates that the relative perpendicular velocity between the ionospheric irregularities and the signal wave path is about 50 m/s. If it is assumed that the irregularities are mainly moving horizontally, the corresponding drift velocity should be about 150 m/s for a path elevation angle of about 20°.

1184 During the weak scintillations, the scintillation index (defined as the normalized rms deviation in carrier power and which varies with wavelength X between 1.5 and 4 GHz (Taur, 1976)) changes to a X variation between 4 and 6 GHz (Craft and Westerlund, 1972). Older theories would predict a frequency dependence of X ' under the assumption of a power-law size distribution of the ionospheric electron density under weak scattering conditions (Crane, 1975). 2.3 Conclusions The effect of ionospheric scintillations on communi- cations satellite signals at GHz frequencies is morpho- logically defined to a reasonable extent with the exception of a few areas, such as frequency dependence and directional variations, for which more measurements should be made to verify existing postulates. The solar activity dependence and the seasonal and diurnal patterns of GHz scintillations have suggested that the irregularities are more likely to be produced when the ionosphere is dense and unstable. These findings, together with some reported differences between weak scintillation activities on easterly and westerly communications links, have led to the hypothesis of a sharp wedge-like irregular region in the ionosphere near the sunset line. However, there is as yet no theory that can satisfactorily explain the production mechanism of the ionospheric irregulari- ties that produce the scintillations in this frequency range.

1185 In addition, there is little information pertaining to the thickness and spatial distribution of these irregularities. Such information is necessary for the development of a theory to correlate VHP and GHz scintillations (Warnik and Liu, 1974) . The recent experimental and theoretical work on the Rayleigh-Taylor instability as related to spread-F conditions (Kelly et al., 1976; Scannapieco and Ossakow, 1976) could prove important in the ultimate understanding of GHz scintillations in the equatorial regions. 2.4 References Crane, R. K., 1975, Spectra of Amplitude and Phase Scintil- lation, Proc. of the IES Symposium, Naval Research Laboratory, Arlington, Va., 1975. Kelley, M. C., G. Haerendal, H. Kappler, A. Valenzuela, B. B. Balsley, P. A. Carter, W. Ecklund, C. W. Carlson, B. Hausler, and R. Torbert, Evidence for a Rayleigh-Taylor type instability and upwelling of depleted density regions during equatorial spread-F Geophys. Res. Letters, _3, 448, 1976 Pope, I. H. and R. B. Fritz, Observation of Simultaneous Scintillation on VHP and S-Band Satellite Trans- missions at High Latitudes, NOAA TR-ERL-207-OD-6, November 1970. Scannapieco, A. J., and Ossakow, S. L. Nonlinear equatorial spread F, Geophys. Res. Letters, 3, 448, 1976.

1186 Taur, R. R., Ionospheric Scintillations at 4 and 6 GHz, COMSAT Technical Review, 3_, 145, 1973. Taur, R. R. Gigahertz Ionospheric Scintillation, COMSAT Technical Review, £, 1974. Taur, R. R., Simultaneous 1.5- and 4-GHz Ionospheric Scintillation, Radio Science, 1976. Warnik, A. W., and C. H. Liu, Ionospheric Irregularities Causing Scintillation of GHz Frequency Radio Signals, J. Atmos. Terr. Phys., 871, 1974. i tc LU -10 - -20 - -30 - 0.012S 0.125 FREQUENCY (Hz) 1.25 FIGURE 2.1 Power spectrum of 4 GHz ionospheric scintillation event measured at the Hong Kong earth station in September 1973.

1187 3. MAGNETIC STORMS AND CABLE COMMUNICATIONS (C. W. Anderson, American Telephone and Telegraph Company, Basking Ridge, N.J. 07920) 3.1 Introduction The correspondence between geomagnetic storms and difficulties on cable communications systems has been observed for many years, both on oceanic systems and land-based systems. The "great" geomagnetic storm of March 24, 1940 (Germaine, 1940) was reported to have rendered inoperative eighty percent of all long-distance telephones out of Minneapolis, Minnesota. During another "great" geomagnetic storm on February 10, 1958, (Winckler, 1959) Western Union experienced severe interruptions on its North Atlantic telegraph cables. At the same time the Bell System transatlantic cable from Clarenville, Newfoundland, to Oban, Scotland, had induced voltages estimated to be 2,650 volts. Although the transatlantic cable system was never entirely out of service, the effect of the time- varying earth potentials was to have voices transmitted in the eastward direction at alternately loud squawks and faint whispers while the westbound signal strengths remained near normal. Another "great" geomagnetic storm in terms of communication cable considerations occured on August 4, 1972 (World Data Center Report, 1973) and provided the opportunity for a detailed study of ground induction effects on cable systems.

1188 3.2 Geomagnetic Data Correlations The existence of the Solar Forecasting Center maintained by the Space Environmental Laboratory, NOAA, in Boulder, Colorado, makes attractive the notion of forecasting magnetic storms which will cause difficulties on cable communications systems. Forecasts of the geo- magnetic disturbance A index and its related Kp index, while useful for many activities, has not been found useful for predicting those large magnetic storms which will induce electric fields at the earth's surface greater than "blO V/Km, the fields required for severe communications impairments (Anderson et al., 1974). For example, an investigation by Anderson (1974) showed that unexplained outages of the L-4 long haul cable system occurred on only 8 of 24 days between January, 1969 and August, 1971 during which the Kp index was ±7 £in a quasilogarithmic scale of 0 to 9) . The attempts to correlate the L-4 cable system outages with large Kp values have probably been frustrated because Kp is only a measure of the magnitude of the largest geomagnetic variation in a three hour interval and is not a measure of the time rate of change of the field. For example, Lanzerotti and Surkan (1974), through a statistical analysis of geomagnetic power spectra and Kp, concluded that there "exist no one-to-one relationships between power level

1189 and K indices" and that "no simple, single-valued function can relate the power level to the K index." A geomagnetic index based on geomagnetic fluctuation levels (AB/At) would greatly assist engineers in making estimates of the effects of geomagnetic field-induced currents on a system- wide basis. 3.3 Geomagnetic Disturbances The geomagnetic disturbances that are normally associated with power system problems are believed to be manifestations of the intensification of the auroral electro- jet giving rise to polar magnetic substorms (Albertson et al., 1973; Kisabeth and Rostoker, 1971). From a lack of sufficient geomagnetic data coverage it is not known if these types of disturbances have induced earth potentials large enough to produce the cable system problems reported in the past. However, such currents were not the cause of the problems during the August 4, 1972 storm when an extended O30 minute) shutdown of a link in the Bell System long-haul coaxial cable (L-4) system occurred between Piano, Illinois, and Cascade, Iowa. A summary of the study by Anderson et al., (1974) is given here to demonstrate that for this occurrence, when reasonably good data were avail- able, the induced currents in the earth were produced by magnetospheric disturbances as opposed to solely ionospheric currents.

1190 One minute scalings of ground-based magnetometer chart-recorded data were made from 14 geomagnetic observa- tories in Canada and the United States. These data were used to examine the development of the geomagnetic dis- turbance around the time of the L-4 cable outage O2242UT on August 4, 1972). The rate of change of the magnetic field intensity and direction over North America for the one-minute interval 2241 to 2242UT is shown in Figure 3.1. The contours, -9 plotted at 200Y/min levels (1 y = 10 " Tesla), were con- structed on the basis of linear interpolation between magnetic observatory-measured values. A large change in the field magnitude was recorded over western Canada at this time. The change and the development of the disturbance did not resemble an ionospheric current system (Anderson et al., 1974). At the time of the L-4 outage, two near-equatorial U. S. satellites, Explorer 45 and ATS-5, were over the Western Hemisphere and recorded magnetic field changes that signified the compression of the magnetopause to inside the spacecraft orbits at %4 R altitude (Explorer 45) and ^5.5 t? R altitude CATS-5). The crossings of the magnetopause C occurred about two minutes apart at the two spacecraft, signifying large distortions in the boundary. A sketch of the equatorial plane view of the earth and the magnetospheric boundary at 2240UT and 2242UT is shown in Figure 3.2 together with the locations of the two spacecraft and four of the ground magnetic observatories

1191 3.4 Induced Currents The estimated direction of the field change at the time of the L-4 outage O700 with respect to the Piano- Cascade route) and its magnitude at Piano (^700y), both derived from Figure 3.2, were used together with a three- layer earth conductivity model to calculate the potential difference along the 242 km route. It was found that the calculated surface electric field of 7V/km along the Piano-Cascade cable was more than sufficient to cause a shutdown in the system, which was designed for earth potentials of 6.5 V/km ± 20%. The design limit arose from the basic powering scheme for the cable. The power system is grounded at one end and the voltages of the DC-DC converters are balanced so that the voltage to ground at the "floating ground" is zero. In the presence of slowly varying voltages, the floating ground has a threshold of 370V, above which, for protection, it becomes automatically grounded. The floating ground must be restored to its normal condition manually. 3. 5 Summary It is not known as yet exactly what type of mechanism produced the large magnetic variations on August 4, 1972, but it appears they were associated with large assymetric distortions of the earth's magnetosphere, and hence large magnetopause currents, rather than the classic ionosphere currents. This area of investigation is

1192 relatively undeveloped at present and, if pursued vigor- ously, should be beneficial to further understanding of solar wind-magnetospheric interaction processes. The susceptibility of cable communication systems to geomagnetic storms has decreased over the years due to the phasing out of ground return cable systems. The major systems that would be (or might possibly be) affected are those that are DC powered from an earth ground. Efforts in the Bell System have now produced power regulation systems that can withstand earth potential variations up to 7.5 V/km without any noticeable transmission impairments. However, further work needs to be done on investigating the scale sizes of large geomagnetic disturbances and how these are related to the powering systems of transcontinental and transoceanic cables. 3.6 References Albertson, V. D., J. M. Thorson, Jr., R. E. Clayton, and S. C. Tripathy, Solar-Induced currents in power systems: cause and effects, IEEE Trans. Power Apparatus and Systems, PAS-92, 471, 1973. Anderson, C. W., III, Effects of the August 4, 1972, Magnetic Storm on Bell System Long-Haul Com- munication Routes, Corrosion '74, National Association of Corrosion Engineers, March 1974.

1193 Anderson, C. W., L. J. Lanzerotti and C. G. MacLennan, Outage of the L-4 System and the Geomagnetic Disturbances of August 4, 1972, Bell System Technical Journal, 53^ 1817, 1974. Germaine, L. W., Magnetic Storm of March 24, 1940 - Effects in Communication Systems, EII Bulletin, May 7, 1940. Lanzerotti, L. J., and A. J. Surkan, ULF Geo- magnetic Power Near L=4,4 Relationship to the K-Index, J. Geophys. Res., 79, 2413, 1974. Winckler, J. R., Auroral X-Rays, Cosmic Rays and Related Phenomena during the Storm of February 10, 1958, J. Geophys. Res., 64, 1959. World Data Center A, Collected Data Reports on the August 1972 Solar-Terrestrial Events, UAG-28, 1973.

1194 118 § 5 .5 I I •o i IC s> u -s i I o u.

1195

1196 4. MAGNETOSPHERE IMPACTS ON GROUND-BASED POWER SYSTEMS (D. J. Williams, Space Environment Laboratory, NOAA, Boulder, Colorado, 80302). 4.1 Introduction One of the most dramatic manifestations of large scale magnetospheric plasma dynamics is the geomagnetic storm - a phenomenon identified as an intense worldwide decrease of the earth's surface magnetic field intensity produced by the sudden inward motion, and subsequent electric and magnetic field-produced drift, of the high altitude magnetospheric plasma. The impact of the geo- magnetic storm on an important operational ground-based system - the ground-based power distribution system - is discussed in this section. The relevance here of magneto- spheric research lies in the fact that our improved knowledge of the geomagnetic storm has allowed the power-distribution engineers to much better understand the reasons for their problems and thereby has guided future design work towards the most appropriate operational solutions. It has been known since at least the Second World War that geomagnetic storms can cause severe disturbances in the operations of communications and power-distribution systems. The power-distribution effects can vary from reduced operating efficiency and interrupted service to users to equipment damage. Such effects have been well-documented in the literature (Davidson 1940; Brooks, 1959; Slothower and Albertson, 1967; McKinnon, 1972; Albertson, et al., 1973).

1197 For example, during the geomagnetic storm of March 24, 1940, transformer banks in several northeast United States power stations were taken out of service due to incorrect operation of differential relays. Electric service was temporarily halted in portions of New England, New York, Eastern Pennsylvania, Minnesota, Quebec, and Ontario. During the February 9, 1958, magnetic storm Toronto, Canada, was plunged into a temporary blackout because of the tripping of circuit breakers in an Ontario transformer station. The development of the geomagnetic storm during the August 1972 solar flare activity (see also Section 3, above) was held responsible for the failure of a 230 KV power transformer at the British Columbia Hydro and Power Authority. 4.2 Cause of Problem The cause of these disruptions of power-distribution systems is the induction of earth currents and resultant earth surface potentials during a geomagnetic storm. Cal- culations (Kellogg, 1966; Albertson and van Baelen 1970) have shown (and observations validate) that induced voltages up to 10 volts per mile can easily be generated during a geo- magnetic storm. Induced voltages of this magnitude can disturb systems containing long conductive lines with equip- ments which are interconnected and grounded at large separation distances.

1198 4.3 Effects of Induced Currents Arbitrary differential relay operation in power- distribution systems during geomagnetic storms produces a judgmental problem in that system operators are unsure of whether or not the relay indication is of an induced trans- former effect or a real transformer malfunction. Standard operating procedure is to remove transformers from the distribution networks during cases of relay tripping in order that they may be inspected. A complete inspection takes a few days and is required in order to protect a valuable and sizeable commercial investment (power trans- formers cost in the $0.5 - 1.0 million range). Since the northeast U.S is supplied by a complex power grid stretching throughout the northeastern and northcentral United States, as well as southern Canada, the arbitrary loss of trans- formers within this network during a geomagnetic storm could conceivably result in a massive power blackout. The final understanding that geomagnetic activity is the cause of these relay perturbations has led to a reasonable oper- ational procedure to significantly reduce potential hazards. Given prior knowledge of geomagnetic disturbances, operators now do not remove transformers from the supply network in the case of a relay trip during a geomagnetic storm. A more serious effect of geomagnetic storms is the current induced in the windings of power transformers

1199 (Albertson, et al., 1973). The magnitude of these currents can be as high as 100 amps and can result in half-cycle saturation of the transformer core. Figure 4.1 shows examples of such induced currents. At the bottom of the figure is shown the world-wide magnetic activity index, Dgt, in gammas (ly = 10 tesla) for the period 13 - 16 May, 1969. A geomagnetic storm begins at the end of 14 May. In the center of Figure 1 a magneto- gram trace recorded at Great Whale River in the auroral zone is shown on an expanded scale from the shaded time interval of the D . plot. The rectangles placed above the magneto- gram trace show the time periods of induced currents in transformer windings and the magnitudes of the currents. Actual recordings of the induced currents are shown on an expanded scale in the upper right-hand corner of Figure 4.1 for the interval 2100 UT 15 May through 0200 UT 16 May for three power stations. The Philadelphia Electric Company Power Station at Delta, Pennsylvania (on nearly the same geomagnetic meridian as Great Whale River) shows an induced current of approximately 80 amps during this interval. Some direct consequences of currents causing half- cycle saturation of transformer cores are fluctuations in the distribution system voltages and internal localized heating in the transformers themselves. Excess internal local heating produced by these induced currents may thermally degrade either the insulation of the transformer core

1200 laminations or any major insulation adjacent to a structural steel path. Such thermal degradation is cumulative in nature and will contribute to an overall shorter insulation life expectancy. As lifetime considerations are a major element in the costing of electric power, this particular problem becomes very real to the everyday user of electricity. Possible means for reducing the induced current magnitudes are being studied and developed (Albertson, et al., 1973) . While future systems should have better protection features, it is unclear whether or not it will prove practi- cable to incorporate protective devices in the huge existing power network. Thus, the problems posed to ground-based power distribution systems by geomagnetic storms are quite real. While these problems can best be solved by power systems engineers, our continuing and advancing understanding of magnetospheric processes and how they effect geomagnetic storm development should be made available to operators of these systems in order that they may understand in a much more comprehensive way the input conditions defining their problems. 4.4 References Albertson, V. D., J. M. Thorson, Jr., R. E. Clayton, and S. C. Tripathy, Solar-induced currents in power systems: cause and effects, IEEE Trans- actions on Power Apparatus and Systems, PAS-92, 471, 1973.

1201 4.4 References (cont'd.) Albertson, V. D., and J. A. van Baelen, Electric and magnetic fields at the earth's surface due to auroral currents, IEEE Transactions on Power Apparatus and Systems, PAS-89, 578, 1970. Brooks, J., The Subtle Storm, A Reporter At Large, New Yorker Magazine, 39, Feb. 19, 1959. Davidson, W. F., The magnetic storm of March 24, 1940 - Effects in the power system, Edison Electric Institute Bulletin, 365, July 1940. Kellogg, P. J., Terrestrial effects of solar activity, Proceedings of Minnesota Power Systems Conference, October, 1966. McKinnon, J., The August 1972 solar activity and related geophysical effects, NOAA Space Environment Laboratory Report, December 1972. Slothower, J. C., and V. D. Albertson, The effects of solar magnetic activity on electric power systems, Journal of the Minnesota Academy of Science, 34, 94, 1967.

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1203 5. THE SPACE RADIATION ENVIRONMENT: EFFECTS ON SPACE SYSTEMS (G. A. Paulikas, Aerospace Corp., El Segundo, Calif., 90009). 5.1 Introduction When Arthur C. Clarke proposed the concept of synchronous-orbiting communications satellites, he could not have anticipated that energetic particle radiation, trapped in the earth's magnetic field, would be one of the factors that designers of synchronous-orbiting space- craft would have to face. Similarly,designers of early concepts of space stations envisioned these stations to orbit the earth at an altitude of about 1000 miles - an orbit which we now know passes through the heart of the inner Van Allen radiation belt. Interestingly enough, the space station featured during the 1960's in Disney- land's "Voyage to the Moon" was in just such an orbit and an entire generation of visitors were misled regarding the very real hazards to man and machines posed by energetic trapped radiation. Discovery of the Van Allen belts was the first major scientific result derived entirely from observations carried out on board spacecraft. Several generations of experiments, and many billions of data bits later, the scientific interest and practical applications derived from our knowledge about the trapped radiation continue

1204 to make research on energetic particles in the magneto- sphere exciting and rewarding. The phenomenology of the radiation belts and the relevant physical processes differ for different regions of space, and a full description of the practical benefits to be gained by an improved understanding of the space environment would be unduly complex for the present purpose. Accordingly, this section presents a description of the radiation environment at synchronous orbit (altitude ^5.6 earth radii above the surface) and the effects of this radiation environment on the cost and capabilities of present and future space systems. 5.2 Synchronous Altitude Environment The synchronous orbit, because of its obvious utility for terrestrial applications, is undoubtedly the single most heavily populated orbit in space. More than fifty spacecraft, spread over longitude, are flying in this orbit today, and the plans of various nations (Hearth, 1976; Bekey and Mayer, 1976) emphasize an even heavier utilization of it for future communications, earth obser- vations, meteorology, and data relay spacecraft. In ad- dition to the practical uses of the orbit, it also happens that this altitude region is the location of a fascinating space plasma physics laboratory. The plasma pause, the extraterrestrial ring current, the boundary of the zone

1205 of trapped energetic particles (and hence the approximate limits of access of solar particles) and the earthward terminus of the magnetotail plasma sheet, all meet and interact near this location. The study of the dynamics and interactions of the various plasmas with vastly different temperatures and densities and the development of an understanding of the processes which generate and transport energetic electrons and energetic protons is an area of research vital to economic utilization of synchronous-orbiting spacecraft. The trapped radiation environment at the synchronous orbit consists of electrons and protons (and higher atomic number particles) with energy spectra as exemplified in Figure 5.1. These data may be considered representative although it is knownthat order-of-magnitude changes in intensity are very common. It is this vari- ability that provides both interesting problems to scientists and significant headaches to space system designers. Some of the problems known to occur when a spacecraft is immersed in this environment are sketched below. 5.3 Impacts of the Environment The fluxes of low energy particles-electrons and protons with energies less than about 20 KeV - are sufficiently intense to cause degradation of thermal

1206 control surfaces and exterior coatings; this portion of the particle energy spectra also determines whether the phenomenon of spacecraft charging (discussed in Section 6, below) occurs. The high specific ionization of protons in the hundredsof KeV to MeV energy interval makes mandatory the complete coverage of solar cells for protection. Trapped energetic electrons are the major cause of radiation damage to solar arrays as well as to semiconductor devices contained in the deep interior of a spacecraft. For example, a typical solar cell array of a spacecraft loses a few percent of the original power output per year of exposure (Figure 5.2). The useful lifetime of radiation sensitive devices such as computer memories, although these may be deeply buried in the interior of a spacecraft, are totally deter- mined by their accumulated radiation dose from trapped energetic electrons during periods of low solar activity, and by the sum of the dosage from trapped electrons and solar cosmic ray protons (and alpha particles) near the maximum in the solar cycle. The recent trend toward the use of large-scale integrated circuit semiconductor devices aboard spacecraft has served to reinforce the appreciation that modern electronics, although capable of spectacularly efficient performance, remain sensitive to radiation damage. Thus, a considerable premium is placed

1207 on accurate knowledge of the environment so that quanti- tative predictions of component lifetime (or necessary shielding weight) can be made. (The problem is magnified because mass market pressures, rather than considerations of radiation hardness, drive the semiconductor manufacturers.) Quantitative predictions of the space radiation environment at the accuracy desired by spacecraft designers elude space scientists. Order-of-magnitude variations in the electron and proton fluxes are commonly observed (Figure 5.3). The environment at synchronous altitude resembles, in its dynamic behavior, nothing as much as terrestrial weather, with the important qualification that all quantities must be plotted on logarithmic scales. The spacecraft designer calls for 10% predictions; the space scientists offer him factors of two in accuracy. A factor of two uncertainty in the expected radiation dose may mean a considerable uncer- tainty in the expected lifetime. For a spacecraft costing $50 million, any increase in replenishment rate has formi- dable fiscal implications. As a result, the designer over- designs and, in so doing, increases the costs above the minimum in order to compensate for the inadequate knowledge about the space environment. Uncertainties of a factor of two in the environ- ment lurk everywhere. For example, the fact that the geo- magnetic equatorial plane does not coincide with the geographical equatorial plane means that some longitudes

1208 in the synchronous orbit are a more benign location than others - by about a factor of two. The radiation intensities also seem to fall off rather rapidly as one moves off the magnetic equator. It may thus be advantageous for some purposes to place a spacecraft into a slightly inclined synchronous orbit, purchasing in the process an improvement in the radiation environment at the expense of additional operational complexity. Solar activity, affecting the magnetosphere via the solar wind and the interplanetary magnetic field, causes both long term (II years) and short term (days) changes in the energetic radiation environment. Research in the physics of the trapped energetic particles has long since passed the exploratory stage. We now fully appreciate that the trapped energetic particles are but the tail end of a broad distribution (Figure 5.1) whose evolution in time and space is governed by a complex of interactions with electromagnetic waves generated by space plasmas themselves. Some of the waves are generated by the local distribution of particles; other families of waves are generated by different particle populations in other regions of the magnetosphere and reach the region of interaction via complicated paths. Studies of wave-particle interactions have reached the level of sophistication and precision that experiments - in the laboratory sense of the word - to change the energetic particlepopulations

1209 seem feasible using man-generated beams of properly tailored electromagnetic waves. While the practical effects of such experiments, if successful, may seem on the surface desirable - for example, sweeping the synchronous "corridor" free of energetic electrons - no one has yet considered how an environmental impact statement for such a necessarily global experiment would be written. The contributions that solar cosmic ray protons make to the total radiation dose accumulated by a syn- chronous orbiting spacecraft were mentioned earlier and illustrated in Fig.5.2. It turns out that solar protons have surprisingly efficient access to this region of space and the radiation damage associated with a single major solar proton event may be comparable to the damage caused in several years of exposure to the trapped energetic electron population (Figure 5.2). The discovery that even low energy solar particles can readily penetrate to the syn- chronous orbit was one of the surprising results returned by the first scientific experiment flown aboard a syn- chronous orbiting spacecraft (Lanzerotti, 1968). The earth's magnetic field is a far less effective shield than had been expected on the basis of simple calculations. It now appears that fully self-consistent calculations of solar cosmic ray trajectories, including models of the

1210 geomagnetic field that realistically account for internal and external current systems and hydromagnetic wave popu- lations, are necessary in order to arrive at an agreement between theory and the experimental observations of solar cosmic ray access to, and diffusion in, the inner magneto- sphere. It is clear that long-term studies of the radi- ation environment are necessary in order to develop a data base of sufficient spatial and temporal coverage to allow accurate specification of the radiation environment. Simultaneously, the development of an understanding of the physical processes governing the evolution of the space environment is necessary if we are ever to approach the predictive capability which we seek. 5.4 Summary As noted in the scientific discussions, there are strong similarities and analogies between the work magnetospheric physicists must do in order to carry out analyses of the plasmas encountered in space and the re- search going on in other branches of plasma physics. For example, the processes governing the loss of particles from the radiation belts are far better understood than the processes which energize and transport the particles about the magnetosphere. The former, essentially involving wave-particle interactions, is concerned with resonances

1211 between various families of electromagnetic waves and gyrating particles playing the key role in diffusing particles in pitch angle and hence out of the "containing" volume. The later processes - energization and transport - are more akin to stochastic or hydromagnetic problems often encountered in laboratory work. Using detailed wave- particle interaction calculations, space scientists have obtained quite satisfactory agreement between the theoret- ically calculated and the experimentally observed structure of portions of the earth's radiation belts (Coroniti and Thorne, 1973) . No comparable progress is yet visible in quantitatively calculating the generation and transport rates of energetic electrons; such an accomplishment would have clear and beneficial practical consequences. Looking toward the future, an enormous expansion is foreseen in the number of space systems designed to serve terrestrial applications and terrestrial customers. The large majority of earth-applications spacecraft are most efficient if located in geostationary orbit. A sample of the kind of synchronous-orbiting applications spacecraft which seem possible with only modest extrapolations of present technology can be found in reports edited by Hearth (1976) and in Berkey and Mayer (1976) . It is clear that the investment in such space systems is likely to run into the tens of billions of dollars. For example, studies

1212 of a Space Solar Power Station (SSPS, see Section 7, below) envisions solar arrays 50 square kilometers in area, costing $2 - $3 billion! The premium on a precise, quantitative understanding of the space environment and the impact of the environment on space systems is enormous. Even savings of fractions of a percent, derived as a result of better information regarding the energetic radiation (for example, extending the life of a spacecraft and thus decreasing the replenishment rate) translate directly into savings which run into the tens of millions of dollars. 5.5 References Bekey, I., and H. Mayer, 1980-2000: Raising our Sights for Advanced Space Systems, Aero- nautics and Astronautics, 14, Number 7/8, 34, 1976. Coroniti, F. V., and R. M. Thorne, Magneto- spheric Electrons, Ann. Rev. Earth and Planet. Sci.,1, 107, 1973. Goldhammer, L. J. and S. W. Gelb, Synchronous Orbit Performance of Hughes Aircraft Company Solar Arrays, paper presented at the llth Intersociety Energy Conversion Engineering Conference, September 1976. Hearth, D. P., ed., Outlook for Space, National Aeronautics and Space Administration Report NASA SP-386, January 1976.

1213 10'1 1 10 102 103 104 105 106 PARTICLE ENERGY leV) FIGURE 5.1 Energy spectrum of trapped electrons and protons as observed in the synchronous orbit by experiments on ATS-6. AUGUST 1972 SOLAR PROTON FLARE 20 30 40 50 60 MONTHS IN SYNCHRONOUS ORBIT FIGURE 5.2 Performance of a solar cell array as a function of time after launch. The long-term decrease in performance is attributable to damage by energetic electrons; the step function decrease is the effect of one large solar proton event (from Goldhammer and Gelb, 1976).

1214 0.1 30 40 50 60 70 80 DAY NUMBER, 1975 90 100 FIGURE 5.3 Variations in the energetic electron flux (n, A = electrons > 1.6 MeV; 0 = electrons > 3.9 MeV) at the synchronous orbit (bottom traces) correlated with the direction of the interplanetary magnetic field and two geomagnetic disturbance indices (from Paulikas and Blake, 1976).

1215 6. ELECTROSTATIC CHARGING ON SPACECRAFT (S. DeForest, University of California, San Diego, La Jolla, Calif., 92037) 6.1 Observations and Effects 6.1.1 Scientific Problems An electrically isolated probe inserted in a plasma will generally assume a negative electrostatic potential with respect to the undisturbed plasma. The magnitude of this potential is of the order of the electron temperature expressed in electron volts. This is true for spacecraft inserted into natural plasmas as well as for probes in laboratory plasmas. Thus, potentials of a few volts are commonly observed on spacecraft in the plasmasphere(Whipple, 1974) but potentials greater than 10,000 volts can be seen on spacecraft in the plasma sheet or in plasma cloud regions of the magnetosphere (DeForest, 1972). The record observed to date is 19,000 volts measured on the synchronous orbit ATS-6 satellite during an eclipse of tne sun on tne satellite by the earth. All uncontrolled spacecraft charging compromises or ruins the measurement of low energy particles of all species. When the complication of photoelectrons and other locally-produced secondary particles are considered, a spacecraft can assume a slightly positive potential, but the plasma sheath surrounding the vehicle has a potential minimum (Whipple, 1975, Grard, 1972). Therefore, natural

1216 plasma particles of both signs are excluded from direct measurement. This is a very serious problem since the low-energy particles that do reach a badly mounted instru- ment are locally-produced secondaries which reflect from the sheath potential minimum. These secondaries are only indirectly linked to the ambient natural flux. Their densities will vary, therefore, in a way that mimics the behavior of the ambient particles. Nearly all spacecraft have exteriors consisting of various insulators. This type of outer surface is usually desired for thermal control. However, the charge stored on different parts of the spacecraft can cause differential potentials of thousands of volts between in- sulating surfaces. In general, the sunlit side of a space- craft willbe held to near zero potential while the dark side charges negatively. For spacecraft at the important synchronous orbit, (see Section 5 above) this means that locally-produced electric fields of magnitude few volts per meter can distort both particle and field measurements. 6.1.2 Operational Problems The overall charge state of a spacecraft probably has minimal impact on operations. However, if the vehicle is charged negatively for long periods some increased sur- face degradation due to ion bombardment will be noticed. The major operational effect arises from differ- ential charging. Differential charging is produced by

1217 exposed insulators or electrically floating conductors. Discharges from external vacuum-deposited aluminum (VDA) can remove the outer surface and thus degrade the thermal properties. Similarly, the outer surfaces of optical elements can be seriously affected. McPherson (1975) and Rosen (1975) have given convincing evidence that several different classes of spacecraft operating in synchronous orbit experience operating anomolies due to discharges produced by electro- static charging. A frequent occurrence is false commands being given by noise on the spacecraft command lines. In at least one case, total loss of an Air Force spacecraft most probably occurred because of extreme differential charging prolonged by an intense geomagnetic substorm. 6.2 Understanding and Controlling Spacecraft Charging 6.2.1 Understanding Theoretical and experimental studies are currently under way (McPherson, 1976) under sponsorship of several agencies to understand and control spacecraft charging. The level of understanding needed varies with the user. Scientifically, some investigators want to understand the phenomenon completely as an interesting plasma physics problem with far-reaching cosmological implications. Magnetospheric physicists would like to be able to measure uncontaminated natural plasmas. Practically, many systems

1218 offices are trying to run operational spacecraft, and they simply want to fix the problem of extraneous commands - with as little redesign of existing vehicles as possible. In general, an approach to understanding must begin with a reasonable model of the environment in which the vehicle is to operate. For many common orbits (syn- chronous in particular) insufficient data presently exist to specify the environment completely. For successful modeling and prediction, a complete distribution function from less than an electron volt to about 100 KeV is needed for all ion species and electrons. This must be known as a function of time and space. This requirement also implies a knowledge of local electric and magnetic fields. For example, McIlwain (1976) has only rather recently demon- strated the existence of strong magnetic field-aligned ion fluxes. No mass spectrometers have yet flown at synchronous orbit (although both the European Space Agency GEOS and the U. S. Air Force SCATHA spacecraft will carry them). An understanding also involves knowledge of the response of various materials to the environment. This response includes photoelectron emission, particle back- scatter coefficients, and secondary electron production. Differences in these properties can create potential differ- ences between adjacent materials even when they are exposed to an identical environment. From the spacecraft charging

1219 point of view, the best outer layer is a conductor, but frequently the spacecraft thermal constraints cannot be met with pure conductors. For other spacecraft parts, such as solar cells, making the outer layer conducting is prohibitively expensive, and is of dubious value if the conductor cannot withstand the radiation environment. 6.2.2 Controlling Once the mechanism of charging is understood, it can be controlled by a careful selection of the space- craft external surfaces and their placement. This care will insure the health and design life of a spacecraft. However, design problems with the outer surfaces of space- craft optics may still exist. Scientific spacecraft which have as a prime mission the measurement of low energy particles will most probably have to carry some means of active control of the spacecraft potential. This control could be achieved by an electron gun or some other type of active device. In a few cases, there is the possibility of simply enclosing the entire spacecraft within a continuous conducting surface. 6.3 Present and Future Impacts Just as the need for spacecraft magnetic cleanli- ness specifications has been known for some time, so now electrostatic cleanliness specifications are being proposed for spacecraft. The need for these specifications is

1220 evident for scientific observations as well as for the reliable operation of many classes of space vehicles. Such a specification has been proposed for the NASA ISEE program and the Dynamics Explorer. In the future it can be expected that spacecraft construction will take into consideration the protection of electronic devices from environmentally-produced dis- charges. The principal way in which this will be accom- plished is through careful attention to details of grounding and to the development of new materials which will have thermal and electrical properties tailored to suit the needs of both the thermal design and the electrostatic specifications. McPherson (1976) has recently reviewed the work being done in this field. Another prospect for the future is active control of the potential by electron guns, ion accelerators, or other means. One current suggestion is to shine an ultra- violet light on the dark side of a spacecraft in order to eliminate differential charging. 6.4 Other Spinoffs Cosmological Studies As mentioned above, spacecraft charging has cosmological implications. Many investigators are interested in the electrostatic charging of bodies in space by particles because of the implications for planet formation. There is obviously much carry-over from the study of spacecraft to celestial objects.

1221 For large bodies, the effects of natural charging might be readily evident. For instance, dust migration by charging has been proposed as an explanation for the differ- ence in surface features between the front and back side of the moon (Grard, 1972). 6.5 Summary Spacecraft can now be designed to withstand the effects of charging - after all, most of them do quite well most of the time anyway. For a scientific payload, great care must be taken to avoid spurious noise in the measured particle fluxes and the electric and magnetic field data. However, for many operational spacecraft the question be- comes one of probability and economics. What construction techniques will have the highest expected return (e.g., communications, weather observation, etc.) for the lowest price? An answer to that question requires a better under- standing of the environment and materials response before specific recommendations can be made to the spacecraft designer. 6.6 References DeForest, S. E. Spacecraft Charging at Synchronous Orbit, J. Geophys. Res., 77, 751, 1972. Grard, R. J. L., Photon and Particle Interaction with Surfaces in Space, P. Reidel Publishing Co., Dordrecht-Holland, 1973.

1222 6.6 References (cont'd.) Mcllwain, C. E., Auroral Electron Beams Near the Magnetic Equator, in Physics of the Hot Plasma in the Magnetosphere, edited by B. Hulquist and L. Stenflo, Plenum Publishing Corp., New York, N. Y., 1976. McPherson, D. A., D. P. Cauffman, and W. Schober, Spacecraft Charging at High Altitude -- the SCATHA Satellite Program, AIAA paper 75-92, presented at AIAA meeting in Pasadena, Ca. 1975. McPherson, D. A., Spacecraft Charging Modeling Status, prepared for SAMSO, Los Angeles Air Force Station, Los Angeles, Ca., 1976. Rosen, A., Spacecraft Charging Environment In- duced Anomolies, AIAA paper 75-91, presented at Pasadena, Ca., January 1975. Whipple, E. C., J. M. Wapnock, and R. H. Winkler, Effect of Satellite Potential on Direct Low Density Measurements Through the Plasmapause, J. Geophys. Res. 79, 179, 1974. Whipple, E. C., Observation of Photoelectrons and Secondary Electrons Reflected from a Potential Barrier in the Vicinity of ATS-6, EOS 56. 408, 1975. (Also to be published by AIAA, 1975).

1223 7. SPACE POWER SYSTEMS: IONOSPHERIC IMPACTS (B.iCChing, The Aerospace Corporation, El Segundo, Ca., 90245) 7.1 Introduction Among the prospects for meeting our country's energy needs in the 21st century, perhaps the boldest, and certainly the most exciting, are the space power systems. As currently envisioned, huge power stations, each capable of serving a large city, would orbit at geosynchronous altitude and transmit space-generated microwave power to earth. The impact of space power systems on the ionosphere, and conversely, of the ionosphere on the power systems, has not been treated with any depth in the many reviews that have been written on the power satellite concept (e.g., Glaser, 1977). Indeed, the microwave spectral region was selected for power transmission largely because of the atmosphere's transparency to such frequencies. Thus, it would seem that interactions between the beam and the medium would be of little or no consequence. Preliminary systems studies (Maynard et al., 1975), however, have iden- tified potential ionospheric problems related to microwave transmission and vehicle exhaust products. Since such problems could have an Impact on communications systems and other microwave transmission, they must be considered in the design and development of space power systems.

1224 7.2 Microwave Transmission Baseline designs of power satellites call for 2 an average flux through the atmosphere of about 100 W/m in a beam approximately 7 km in diameter. Only about a thousandth of one percent of the energy in the beam will be absorbed in the ionosphere, and thus there will be virtually no impact on the transmitted power level. On 2 the other hand, 0.001* corresponds to 1 mW/m , which is of the same order of magnitude as the solar energy con- tained in wavelengths shorter than 100 A (the extreme ultraviolet, EUV) that represents the major source of heat and ionization in the thermosphere and ionosphere. A comparison of solar EUV and microwave energy absorption rates in the upper atmosphere is shown in Figure 7.1. The microwave absorption profile does not reflect possible anomalous absorption, which might occur if the response of the medium to the additional energy input were to trigger further enhanced absorption and plasma instabilities. Microwave and EUV radiations interact in completely different ways with the atmosphere. The microwave photons are much less energetic than the solar radiation and are unable to produce ionization or dissociation. In fact, the solar EUV photons interact initially with the atmos- pheric molecules and atoms, whereas the microwave radiation reacts with the ambient electrons. Despite this fundamental

1225 difference, energy considerations alone suggest a potentially significant effect on the thermal state of the atmosphere-ionosphere system, at least in the vicinity of the microwave beam. Concern over potential microwave-induced perturbations of the ionosphere may be attributed in large part to the results of recent ground-based radio- frequency ionospheric modification experiments (see the special issue of Radio Science, £ (11), 1974). These experiments, conducted at input heater frequencies of a few to ten MHz, utilized power outputs of the order of a few 2 tens of yW/rn . Assuming that the level for interactions scales as the square of the frequency, microwave-induced ionospheric effects would be expected to occur at power levels similar to those being considered for the space power systems. Although it is not presently known exactly what effects might be stimulated by high power microwave trans- missions, some indication of what might result can be provided by the results of the recent RF ionospheric experiments. It is found that the energy absorbed from the heater beam by the ionospheric electrons raises the elec- tron temperature. This electron heating results in a spatial redistribution of the electron density due to thermal expansion along the earth's magnetic field lines.

1226 Since the electrons diffuse up field lines, the expansion results in a depletion in the electron density in the upper ionosphere. In contrast, in the lower ionosphere, the enhanced electron temperature lowers the rate of electron-ion recombination, resulting in an increase in the electron density. With increased power levels, plasma instabilities are excited that produce additional nonlinear absorption of the heater beam that results in the generation of field-aligned density irregularities. These irregularities give rise to various scattering phenomena, the outstanding example being spread-F. Of course, any modification of the ionosphere by the microwave beam of the space power system will likely be localized in extent and thus will probably not have an impact on a global-scale. The absorption of energy is ini- tially confined to the beam width (roughly 7 km), although subsequent interactions and the excitation of plasma instabilities will increase the sphere of influence beyond the limits set by the beam width. In the RF ionospheric heating experiments the recovery of the ionosphere to its normal state occurred fairly rapidly following the heater turn-off. For space power transmission systems, however, the power will be "on" for the lifetime of the satellite (perhaps 30-100 yrs.) and the beam position will be essentially fixed in geographic space (due to the use of

1227 geosynchronous orbits). Thus, any perturbations induced by the beam will not be very effectively diluted as might be the case with a non-geostationary beam. Because of the coupling between the thermosphere and ionosphere, some changes may be induced in the neutral structure which would provide some "feedback" to the ionosphere. For example, enhancement of the neutral temperature within the beam would lead to changes in neutral composition, which, in turn, would affect the electron-ion recombination rate and hence the electron density. The heated column of air, which presents a discontinuity in the medium, could also give rise to atmospheric gravity waves. The wave features would be impressed on the ionized component. The possible effects of microwave transmission in the thermosphere and ionosphere are summarized in Table 7.1. Whether any or all of these phenomena would occur as a result of space power systems is a question yet to be answered. A mitigating factor is that microwave frequencies are far-removed from ionospheric plasma frequencies. How- ever, the high flux levels could be counter-active, par- ticularly if they are sufficient to trigger instabilities in the plasma. 7.3 Vehicle Exhaust 7 8 The sheer size of a power satellite, 10-10 kg, is indicative of the high level of vehicle activity that

1228 will be needed to lift and assemble and then service and maintain a fleet of 50-100 power satellites. It is estimated that the equivalent of 10 shuttle launches per day will be necessary during the peak of the construction period (Williams, 1975). Advanced heavy lift launch vehicles will probably expel only water vapor when at ionospheric altitudes (Beichel, 19710- However, the expulsion of large quantities of water molecules can produce significant changes in the local ionospheric structure, as was strikingly demonstrated during the launch of Skylab 1 (Mendillo et al., 1975). Water molecules interact efficiently with atomic oxygen ions, resulting in increased rates of electron-ion recombination. In the case of Skylab 1 there was a substantial depletion (—50$) of the total electron content that persisted for several hours and encompassed a region on the order of 2000 km in diameter. Water vapor from booster engines could also affect the D-layer of the ionosphere, which is composed mainly of water cluster ions. The continual injection of water vapor in substantial amounts is likely to affect the ion formation rates and, hence, the properties of the layer. In addition to the launch vehicles, which ascend only to low earth orbit, orbit transfer vehicles will be needed to tug the assembled satellites to geosynchronous altitude. These transfer vehicles require a high-performance propulsion system, with the likely candidate being the solar-

1229 electric propulsion system. Such a system operates basically by the acceleration, and then expulsion, of a heavy, ionized metal. Preliminary studies have not indicated serious pollution problems in the ionsphere or magnetosphere; however, further investigation using realistic traffic models is necessary. 7.4 Conclusions Considerations of ionospheric effects will no doubt influence decisions related to transmission power levels of space power systems as well as the propellants and propulsion systems necessary for placing the systems in orbit. Any modification of the ionosphere must be kept at levels tolerable toother uses, such as communica- tions and radar systems. Realistic traffic models, including allowance for growth on an international basis, will be crucial for a sound assessment of the potential impacts. Since the power levels and the volume of space traffic will both be far greater than that encountered in present day operations, a vigorous program of theoretical and experimental investigation is required in order to provide the proper guidance in the design and development of space power systems.

1230 7.5 References Beichel, R., Propulsion Systems for Single-Stage Shuttles, Astron. and Aeron. , 12_ (Nov.), 32, 1971i. Glaser, P. E., Solar Power from Satellites, Phys. Today, 30., 30, Febr. 1977 Maynard, 0. E., W. C. Brown, A. Edwards, J. T. Haley, G. Meltz, J. M. Howell and A. Nathan, Microwave Power Transmission System Studies, Vol. II, NASA-CR-13^886, 1975. Mendillo, M., G. S. Hawkins and J. Klobuchar, A Sudden Vanishing of the Ionospheric F Region Due to the Launch of Skylab, J. Geophys. Res., 80, 2217, 1975. Williams, J. R., Geosynchronous Satellite Solar Power, Astron. and Aeron., 13, 46, Nov. 1975.

1231 Table 7.1 Possible Effects of Microwave Propagation IONOSPHERE Electron temperature increase Electron density decrease in D-region, increase in P-region Modification of electron energy distribution Anomalous absorption and heating, leading to field-aligned irregularities and radio scattering phenomena THERMOSPHERE Neutral temperature increase Modification of relative composition Modification of airglow characteristics Excitation of atmospheric gravity waves MUTUAL COUPLING EFFECTS Neutral composition affects the electron-ion recombination rate Ion density affects the neutral wind system Neutral winds and gravity wave structure affect the ion distribution

1232 100 10 11 10 POWER ABSORBED, J-m FIGURE 7.1 Daily average absorption of solar radiation at middle latitudes compared with the energy that would be absorbed from a microwave beam of uniform flux 100 W/m2 at a frequency of 2.5 GHz.

1233 8. DEEP EARTH INDUCTION STUDIES MAKING USE OF MAGNETOSPHERIC-IONOSPHERIC CURRENT SYSTEMS (D. I. Gough, University of Alberta, Edmonton, Alberta, Canada) 8.1 Source Current Systems Time-varying electric currents in the magneto- sphere and ionosphere induce secondary currents in the solid earth. The fields of the internal currents can be used in the study of conductive structures within the earth. From the standpoint of such induction studies, the sources of the current systems external to the solid earth are in the magnetosphere and ionosphere. Useful current systems, in order of increasing period, include those of magnetic pulsations, magnetospheric substorms, the daily geomagnetic variation, and the ring- current decay field D . in magnetic storms. S u An important parameter in induction studies is the skin depth, the depth at which a field of period T falls to 1/e of its incident amplitude. In convenient units the skin depth is 30.2x /fp" km where T is expressed in hours and the resistivity p is expressed in ohm-m. Resistivities of rocks in the crust and upper mantle range 4 from less than 1 ohm-m to 10 ohm-m. Source fields can be characterized: (a) by their spectra; that is by the periods T at which they apply useful energy, and the corresponding effective depths of penetration in the earth; (b) by their spatial geometries. The spatial geometries

1234 of the magnetospheric and ionospheric sources can be compli- cated and are different for the Individual sources. Magnetic pulsations produce fields with periods up to a few minutes and with amplitudes of a few nanoteslas CnT). They are useful mainly in magnetotelluric study of the sedimentary rocks of the crust to depths of a few kilometers. These sources can be highly localized in the ionosphere/magnetosphere. Magnetospheric substorms are very important in carrying out induction studies by magnetometer arrays and by the magnetotelluric method. In low and middle latitudes substorms provide transient magnetic fields of order 20 nT in the period range 15 < T < 150 minutes. The geometry of the currents is complicated and is still undergoing active exploration CRostoker, 1972, Kisabeth and Rostoker, 1977). Present indications are that a westward ionospheric current in the auroral oval joins magnetic field-aligned currents at its ends, which connect the ionospheric segment to a closure in the magnetosphere. This transient current system can be regarded as superimposed upon steady-state "soleonoidal" currents involving a southward ionospheric current in the morning sector auroral zone and a northward ionospheric current in the evening sector, with sheets of field-aligned current at the northern and southern limits. In middle and low latitudes the magnetic storm source field may change only smoothly and moderately across

1235 a magnetometer array, and anomalous fields within the array can be identified with induced currents and in some cases can be quantitatively modeled. However, at the present time, in latitudes near the auroral zone the complicated source-field geometry encourages the use of arrays for the study of the external current systems rather than for induction studies of the solid earth. The daily variation dynamo (Matsushita, 1975) and its associated magnetic field produce a travelling wave with respect to the solid earth. At middle latitudes the S (quiet day) variation has spectral peaks at periods of 24, 12, 8 and 6 hours; on distributed days shorter periods are added to the spectrum. Even in a medium of resistivity 10 ohm-m, typical of the upper mantle, the skin depth at T=24h is 470 km, so the daily variation fields penetrate deeply into the upper mantle. At the dip equator the concentration of S currents in the equatorial electro- jet provides a narrow, linear localized current source use- ful in induction studies at crustal and uppermost mantle depths. The spectrum of the D _ field, associated with ST/ the decay of the ring-current in the late stages of magnetic storms, contains useful energy at periods up to 3 days and has been used in induction studies to depths of order 700 km (Lahiri and Price, 1939; Matsushita, 1975).

1236 8.2 Techniques of Induction Studies Data from the world-wide network of geomagnetic observatories are used in the study of the radial varia- tion of electrical conductivity, the principal planetary- scale feature of the earth's electrical structure. The source fields are principally the daily variation, Dg., and fields of longer periods (Bailey, 1973). In the magnetotelluric method, sources such as magnetic pulsations and magnetospheric substorms provide -1 4 input in the period range 10 to 10 seconds or more, to a single station at which two orthogonal horizontal compo- nents of magnetic field, H (T) and H (T), and the corresponding electric field components E (T) and E (T) are recorded simultaneously. The magnetic field detectors are commonly coils or fluxgate magnetometers; the electric fields are detected as potential differences between separated grounded electrodes. At the surface of a half-space made of iso- tropic plane layers, the apparent resistivity at a particular period T is pa = 0.2 T (Cagniard, 1953) and p (T) can be fitted to layered models 3, of p(depth) which are non-unique but may approximate the true structure. The real earth seldom resembles a set of isotropic plane layers, and in general magnetotelluric data 2 E T*1 X = 0.2 T Iy_ H H y X

1237 define a resistivity tensor whose maximum and minimum values are interpreted in terms of resistive structure. A typical depth range for magnetotelluric soundings is from the sur- face to 100 km depth. Contemporary magnetotelluric techniques are reviewed by Hermance (1973). In magnetometer array studies, an array of three- component magnetometers records magnetic fields in the period range 10-1440 minutes, at a number of points on the earth's surface. In studies reported to date, the number of magneto- meters has ranged from four to eight, often placed along a straight line, or from 15 to 46 covering an area. The area 6 2 covered was 10 km in the largest array yet used (Camfield et al., 1971). Large,two- dimensional arrays record a single magnetic event over the whole area, and highly con- ductive structures can be mapped in terms of amplitudes and phases of spectral peaks supplied by the source field. In some cases the fields of internal, induced currents can be separated from those of the external source, and the internal fields can be fitted to calculated fields from induction in model structures (Porath et al., 1970; Jones and Pascoe, 197D• In all cases, anomalous conductors can be mapped with a precision limited by the magnetometer spacing and can be assigned maximum depths. An alternative approach to interpretation expresses the electromagnetic response of the earth, at

1238 each station, as a transfer function from the horizontal components (in which the external source field is largest) to the vertical component (in which the fields of internal, induced currents often predominate). Such transfer functions, calculated at several periods in which several variation events supply the inputs, allow adjacent small linear magnetometer arrays to be combined into a partial approxi- mation to a two-dimensional array. The problems of the quantitative interpretation of magnetometer arrays are severe (Gough, 1973a) ; one of the most serious is discussed here. This most intractable difficulty affects all types of investigation of the earth by electromagnetic induction, and therefore deserves mention. Anomalous currents in the region under study often flow in a long, highly conductive channel in the crust or upper mantle which connects large, unknown con- ductive volumes of the earth elsewhere to the area being studied. In such cases the induction may occur mainly in the large, distant conductors where the source field may differ greatly from that recorded by the array. Such channelled-current anomalies are more numerous than those in which induction occurs mainly in the structures covered by the array, and are extremely difficult to interpret beyond the point of locating the conductive channel. An example is shown in the next section.

1239 8.3 Some Representative Results The principal feature of the electrical resistivity within the Earth is a very rapid fall to values below 1 ohm-m in the depth range 400-800 km (Banks 1972). This increase in conductivity is certainly related to temperature but may be related also to pressure and to seismically observed phase changes at depths near 390 km and 650 km. Since the skin depth at p = 1 ohm-m for the 24 hour term in the daily variation is only 150 km, the fields of external current systems penetrate negligibly beyond a depth of 'v-lOOO km. In effect, electrical sounding using induction by magnetospheric-ionospheric currents is limited to the depth range 0-800 km. In the top 100 km the resistivity varies through four orders of magnitude, from 0.25 ohm-m for 4 sea-water to 10 ohm-m for dry crystalline crustal rock. Local anomalies in geomagnetic variation fields represent currents induced in conductive structures within 300 km of the earth's surface. Many of them can be classi- fied as continental-edge anomalies, subduction-zone anomalies, the anomalies of western North America, rift valley anomalies or crustal anomalies. A review with many references up to 1973 has been given elsewhere (Gough, 1973b). Only later references are quoted here. Large anomalies, mainly in the vertical components of fields with periods Ih _< T _< 24h, appear near continental edges. Largely, and in some cases wholly, these can be

1240 ascribed to induced electric currents in the sea-water. There may also be a contribution due to a step in the conductive structure of the upper mantle below the continen- tal edge, but existing information is indecisive on this point. Prominent conductivitiy anomalies in zones of active subduction are known from magnetometer array studies in Honshu Island, Japan, and in the Peruvian Andes. These probably result from the ascent of molten material with ionic conductivity, from partial melting of the downgoing plate. No such conductor is found beneath the Andes of central Chile. Large magnetometer arrays have revealed complex conductive structures in the upper mantle under western North America: the results are summarized in Figure 8.1. Highly conductive mantle material estimated to be at least 100 km thick underlies the Basin and Range Province, with still greater development below the Wasatch Fault Belt and the Southern Rockies. North of the line XX (near 43°N in the Figure) the conductive layer is much thinner ^10-20 km for p = 5 ohm-m. The electrical conductivity, the heat flow from beneath the crust, and seismic wave velocities in the upper mantle are correlated (Gough, 19710, and are all consistent with anomalously high temperatures producing partial melting in the upper mantle.

1241 Local anomalies associated with induced currents in the upper mantle and crust have been discovered beneath the eastern Rift Valley of Kenya and have been studied by a magnetometer array and by the magnetotelluric method. High conductivities rise to crustal depths in Iceland and it is highly probable that an array study across a mid-ocean ridge would contribute to identifying the location of molten material. The technical problems of ocean-floor magneto- meter arrays have yet to be overcome, however. Local induction anomalies within continents are sometimes associated with sedimentary basins (Porath and Dziewonski, 1971) in which the conductivity is probably due to saline pore-water. In other cases long, very narrow anomalies of the channelled-current type seem to be related to fracture zones, in which graphite is commonly present. An example of a crustal anomaly of this type is illustrated in Figure 8.2. This North American Central Plains conductive structure was discovered by a magnetometer array near the Black Hills of South Dakota and it was suggested that the currents might be channelled by graphite in metamorphic rocks of the basement (.Camfield et al., 1971). A later array study (Alabi et al., 1975) mapped the structure from the Canadian Shield of Saskatchewan, where graphite sheets are common in exposed metamorphic rocks in a large fracture zone, to another major fracture zone 1800 km to the south in Wyoming. The

1242 conductive structure is believed to lie in a great fracture zone which may mark a continental collision 1.7 billion years old. Figure 8.2 shows how the induced currents perturb the field of a magnetospheric substorm, in the Fourier spectral term of period 68 minutes. Further references to the literature of induction studies using the current systems of the magnetosphere and ionosphere can be found in recent reviews by Frazer (197M and Gough (1973b, 197*0. 8.4 References Alabi, A. 0., P. A. Camfield and D. I. Gough, The north American central plains conductivity anomaly, Geophys. J. R. Astr. Soc. , 43_, 815, 1975. Bailey, R. C., Global magnetic sounding - methods and results, Phys. Earth Planet. Interiors, 7_, 23^, 1973- Banks, R. J., The overall conductivity distribution of the Earth, J. Geomagn. Geoelectr., 24, 337, 1972. Banks, R. J. and P. Ottey, Geomagnetic deep sounding in and around the Kenya Rift Valley, Geophys. J. R. Astr. Soc., 36, 321, 1974. Cagniard, L., Basic theory of the magnetotelluric method of geophysical prospecting, Geophysics, 18, 605, 1953- Camfield, P. A., D. I. Gough and H. Porath, Magnetometer array studies in the northwestern United States and southwestern Canada, Geophys. J. Roy. Astr. Soc., 22, 201, 1971.

1243 Prazer, M. C., Geomagnetic deep sounding with arrays of magnetometers, Rev. Geophys, and Space Phys., 12, 401, Gough, D. I., The interpretation of magnetometer array studies, Geophys, J. Roy. Astr. Soc . , 35, 83, 1973a. Gough, D. I., The geophysical significance of geomagnetic variation anomalies, Phys. Earth and Planetary Int., ]_, 379, 1973b. Gough, D. I., Electrical conductivity under western north America in relation to heat flow, seismology and structure, J. Geomag. Geoelect., 26, 105, 1974. Hermance, J. P., Processing of magnetotelluric data, Phys . Earth and Planetary Int., 7_, 349, 1973- Jones, P. W. and L. J. Pascoe^ A general computer program to determine the perturbation of alternating electric currents in a two-dimensional model of a region of uniform conductivity with an embedded inhomogeneity, Geophys. J. R. Astr. Soc., 24, 3,19 71. Kisabeth, J. L. and G. Rostoker, Modeling of the three di- mensional current system associated with magnetospheric substorms, Geophys, J. Roy. Astr. Soc., 49, 1977- Lahiri, 3. N. and A. T. Price, Electromagnetic induction in non-uniform conductors and the determination of the conductivity of the Earth from terrestrial magnetic variations, Phil. Trans. Roy. Soc. London, Ser. A., 237, 509, 1939. Matsushita, S., Morphology of slowly-varying geomagnetic external fields, Phys. Earth and Planetary Int., 10, 299, 1975

1244 Porath, H., Magnetic variation anomalies and seismic low- velocity zone in the western United States, J. Geophys. Res. , 76., 2643, 1971. Porath, H. and A. Dziewonski, Crustal resistivity anomalies from geomagnetic deep-sounding studies, Rev. Geophys. and Space Phys. , 9_, 891, 1971 Porath, H., D. W. Oldenburg and D. I. Gough, Separation of magnetic variation fields and conductive structures in the western United States, Geophys. J. R. Astr. Soc., !£, 237, 1970. Rostoker, G., 1972. Polar magnetic substorms, Rev. Geophys. and Space Phys., 10, 157-211.

1245 50- 45- 40- 35- FIGURE 8.1 Distribution of anomalously high conductivity in the upper mantle of western North America as mapped by three large magnetometer arrays. The density of stippling indicates variations of the thickness- conductivity product in an upper mantle layer. The inset shows a vertical section through a model by Porath (1971) which fits observed fields along the east-west profile^ at 38°N, with conductivities expressed in siemens per meter (1 Siemens unit « 0.94 ohm).

1246 28 August I972 T-68.3min IB. 95' 28 BUG 1972 68.3 KIN -55' -50' -45' -40f i 4 X RMPLIT'O'DEl 105' 115 -55 -50' -45' -40' 95' -55' -SO1 -45' Y RMPLITUDEi -40. 95* -55' -50' -45' Z RMPLITUDE -55 X PHPSE -40' 95' -55 34 -SO' -45' 15' 105 FIGURE 8.2 Perturbation of the field of a magnetospheric substorm by the North American Central Plains conductivity anomaly. In each map the crosses indicate the locations of magnetometers in an array. The polarization of the horizontal field at a period of 68.3 min. is mapped on the left. The other six maps show amplitudes and phases of the Fourier transforms of the substorm field components at the spectral period of 68.3 min. The northern border of the array was close to the ionospheric part of the source current, as shown by the X amplitude and polarization maps. The conductivity anomaly is best seen in Y and Z amplitudes and Z phase. Amplitudes are in nT, phases in minutes. After Alabi et al. (1975).

1247 9. CONSIDERATIONS OF TELLURIC CURRENT EFFECTS ON PIPELINES 9-1 Pipelines at Middle to Low Latitudes (A. W. Peabody, Ebasco Services, Inc., New York, N.Y., 10006) 9.1.1 Introduction The rapid expansion of the transmission pipeline industry in the decades following the nineteen thirties caused increasing attention to be devoted to controlling the corrosion of pipeline steel. One of the problems that faced pipeline corrosion engineers involved an unusual form of variable "stray" direct currents and potentials on the pipelines. These could not be correlated with stray direct currents and potentials of a man-made nature. The phenomenon became particularly apparent on long cross- country electrically-continuous steel pipes which normally had a protective coating of an insulating material used to assist in the mitigation of corrosion effects. These pipelines were, in effect, long insulated electrical con- ductors subject to inductive effects from variable magnetic fields (see, for example, Gideon et al., 1970). The nature of the non-man-made stray effects on pipelines was found to be similar to effects on long distance telegraph lines during magnetic storms associated with severe sunspot activities. As a result, any otherwise-unexplainable stray effects observed on pipelines by pipeline corrosion engineers came to be known as telluric current effects.

1248 9.1.2 Corrosivity of Telluric Currents The frequency and severity of occurrence of telluric currents are quite erratic with, often, long "quiet" periods when there is no significant effects at all. Further, the pipeline locations where the telluric currents discharge to earth (with the resulting possible consumption of pipeline steel) tend to move from place to place with no long-term concentration of a corrosive effect at any one location. Both of these effects arise from the variable nature of geomagnetic activity (see, for example, Campbell, 1976). The pipeline corrosion effect arises from the discharge of current from the pipe to the ground through small holes in the non-conducting protective coating of the pipe. These holes develop from damage during the laying of the pipe. A short-term study of the peak values of induced currents on pipelines in several states made by Gideon et al. (1970) showed typical values in the range of one ampere. Using extreme values of the exposed pipe area in the pipe coating, they found a negligible 10~ m/yr. corrosion for 0.5-0.75m diameter pipes (see also Campbell, 1977). In addition to their erratic occurrence pattern, telluric currents can reverse direction from time to time as well. This also operates to reduce their corrosivity

1249 compared to a steady, uninterrupted, unidirectional flow of direct current. McCollum and Ahlborn (1916) reported the corrosion rate as a function of period and their results are shown in Fig. 9.1 (this work was carried out in connection with the operation of street railway systems). t It can be seen from Pig. 9.1 that for telluric currents with periods in the range of 5 mins. to several hours (typical of geomagnetic variations) that the conclusions of Gideon et al. (1970) would be reduced by ^75%. There has been some evidence to support the possibility of a concentration of telluric current dis- charge at a specific location where subsequent severe damage will result. One such instance, studied by the writer's associates in connection with an early 1940's examination of corrosion damage on a new pipeline system extending from the Atlantic to the Pacific across the Isthmus of Panama, found that currents reaching values in the order of 30 amperes would at times flow from ocean to ocean, with the direction of flow changing from time to time. Current discharge to earth occurred at the ocean terminals with corrosion failures developing on some terminal facili- ties prior to being placed in service. The currents could not be related to man-made effects. Although the writer does not have specific docu- mentation to the effect, he has been advised that there has

1250 been at least one other location where known telluric current discharge effects at an ocean pipeline terminal have caused corrosion problems. From this very limited knowledge base, it does appear reasonable to ecpect a con- centration of the corrosion problem where a pipeline terminates at a location where the resistance to the flow of discharge currents would be very low. An ocean terminal is most apt to fulfill this condition. Because of the erratic nature of the telluric current effect, essentially no active corrective measures, such as the active "cathodic protection" techniques (Peabody, 196?), are taken that are specifically directed towards the effects of telluric current activity. In the rare instances where a problem is identified (such as at an ocean terminal on a pipeline particularly subject to telluric effects), the telluric current can be drained from the pipe to earth through a separate ground connection designed for the purpose. 9.1.2 Telluric Currents and Pipeline Corrosion Control When making field measurements to ascertain the level of cathodic protection on a pipeline, the cognizant corrosion engineer must make observations of pipeline-to- earth potential and direct current flew measurements accurate to at least ±5 millivolts and ±100 milliamps, respectively. The inability to make observations to such levels of accuracy could result in misleading or completely meaningless data. Telluric current effects, when active,

1251 have the capability of completely screening the true steady-state conditions. The writer has observed both pipeline potentials and currents of a telluric current origin which were several times the magnitude of the normal steady-state conditions. It is obvious that without advance warning of possible telluric current activity, pipeline corrosion survey work could be scheduled and performed at a time when the results obtained are meaningless insofar as interpretation of normal conditions are concerned. Recognizing this, the NOAA Space Environment Services Center in Boulder, Colorado, initiated regular publication of predictions for pipe-induced current activity (Campbell and Doeker, 19710. Pipeline corrosion engineers can obtain this information (updated daily) by telephone in order to plan their survey activities. Additionally, in order to make it possible to evaluate past incidents, the Space Environment Services Center periodically publishes past activity data in Materials Performance, a publication of the National Association of Corrosion Engineers. 9.1.3 Summary The effect of telluric currents on pipelines at low and middle latitudes is more of a nuisance than a serious problem for corrosion. The nuisance arises from the telluric current interference with normal Dioeline

1252 corrosion survey engineering work. This problem is a sufficient nuisance to have caused the implementation of a regular service for predicting pipe-induced current activity.

1253 9.2 The Alaskan Pipeline* (W. H. Campbell, U. S. Geological Survey, Denver, Colo., 80225) 9.2.1 Introduction Associated with every occasion that society finds reason to build long lines of conducting material there is a reawakened interest in the induced currents in these lines associated with geomagnetic disturbances (Burbank, 1905). Typically such interest mounts near the 11-year maxium in geomagnetic activity (Bartels, 1963). The Alaska oil pipeline, a colossus in size with respect to those within the lower 48 states, traverses the auroral regions, regions in which the geomagnetic disturbance energy could be at least 100 times that troubling engineers in the mid-U.S. (Campbell, 1973). The principal geomagnetic disturbances seen at high lati- tudes arise from currents within the ionized layers of the atmosphere about 100 km above the earth. The surface manifestations of these ionospheric currents are electric and magnetic fields which induce current flow in the conducting earth. Such current is channeled into regions of the highest conductivity, either of geologic origin or of man-made origin, as in the case of a pipeline. The Alaska oil pipeline (Raymer, 1976) is essentially a long, surface-grounded conductor extending from about 69.3° geomagnetic latitude at Prudhoe Bay on *Abstracted by the editor from Campbell (1

1254 the Arctic Ocean, generally southward near the region of auroral zone maximum, to about 61.6° geomagnetic latitude at Valdez on the Pacific Ocean (Pig. 9.2). Assuming the pipe steel to have a resistivity of _y about 1.4 x 10 ohm-meters, the length to be 796.4 miles (1.28 x' lO^km), the mean diameter 48 inches (1.22 m) , and the mean wall thickness 0.512 inches (1.30 cm), then the end-to-end resistance of the pipeline is 3.6 ohms. This assumes that the insulating connections at the 12 pumping stations are shorted by conducting jumpers. The resistance per unit length of pipe, Rm, is thus 2.8l x 10~ ohm/m. 9.2.2 Effects of the Auroral Currents Because of the high pipe conductivity with respect to the ground to which it is electrically connected via zinc grounding cables at regular intervals (^150m) and because the pipeline is long with respect to the scale of the inducing fields, the currents flowing in the pipeline will be just of that magnitude necessary to eliminate the induced electric field parallel to the pipeline. The Alaskan pipeline route runs generally in a geomagnetic north-south direction for the northern and southern sections and east-west in the central section (Fig. 9.2). The ducted induced electric fields that are of concern are those parallel to this pipe- line route. At maximum they would represent the horizontal electric component E = E esc 9, where 9, the angle

1255 between the horizontal electric field component and geomagnetic North, varies between about 1.0 and 1.6. The scale to the right side of Fig.9-3 gives the computed equivalent pipeline current for a geomagnetic east-west electric field parallel to the pipeline (left- hand scale) as a function of the period of the electric variations (Campbell, 1977). To obtain the equivalent horizontal components such values should be multiplied by esc 9. The electric field values are deduced from analyses of magnetic field variations measured at several places along the pipeline route and then using a three-layer model of the earth to compute the induced electric field. The magnetic variation studies are carried out for different levels of geomagnetic disturbance as represented by the geomagnetic disturbance index Ap. The statistics of occurrence of Ap over several solar cycles show that 50% of the index values are ^7 or less (see, for example, Campbell, 1977). An Ap value of 7 would correspond to currents near a 1-hr, period of ^2.7 amps for the sample conditions of Fig. 9.3. Multiplied by esc 6 = 1.07 (for the average pipeline angle with respect to the auroral current system) a value of ^2.9 amps is obtained. This value represents a daily average current of ^1 amps (the average daily pipeline current level is about 1/3 the magnitude at the maximum auroral latitude

1256 and about 1/5 the magnitude for an average of all stations along the pipeline). Individual surges in current amplitude between about 30 min. and 2 hr. could be about 5 to 10 times the size indicated in Fig. 9.3 (see Campbell, 1977), corresponding, in the above example, to individual fluctuations reaching M.4 amps. Similar calculations for small and large magnetic disturbance conditions (Ap ^ 52 and Ap > 150, values that can statistically occur approximately once every two months and every two years; see Campbell, 1977) give current surges — 95 and £ 280 amps, respectively. Such large transient currents could cause problems at electronic monitoring and control points along the pipeline. The increased pipe area of the 1.2m Alaska pipe would reduce corrosion to ^0.4 to ^0. 7 the values quoted in Section 9-1.2 from Gideon et al (1970) for a one-ampere current. Increasing the current by a factor of ten would mean that the comparable Alaskan corrosion should be ^ 10 " m/yr., a truly negligible amount for a 1.3 cm wall pipe thickness. In addition, the computed corrosion rate should be reduced by the appropriate factor shown in Fig. 9.1. 9.2.3 Summary Corrosion in the Alaskan pipeline from the auroral current system appears to be a negligible concern. However, it appears that large transient currents can be

1257 established under geomagnetic disturbance conditions. These currents could greatly disturb, or prevent, corrosion survey engineering studies on the pipeline. They could also produce severe problems for the pipeline monitoring and control electronics. 9.3 References Bartels, J., Discussion of time-variations of geomagnetic activity indices Kp and Ap, 1932-1961, Ann. Geophys., 19_, 1, 1963. Burbank, J. E., Earth currents and a proposed method for their investigation, Terr. Mag., 10, 23, 1905. Campbell, W. H., Spectral composition of geomagnetic field variations in the period range of 5 min. to 2 hr. as observed at the earth's surface, Radio Sci. , 8_, 929, 1973- Campbell, W. H., An analysis of the spectra of geomagnetic variations having periods from 5 min to 4 hrs., J. Geophys. Res., 81, 1369, 1976. Campbell, W. H., Induction of auroral currents within the surface conductivity anomaly presented by the Alaskan pipeline, Geophysics, submitted, 1977• Campbell, W. H., and R. B. Doeker, Pipe induced current activity indices, Materials Performance, 13_, 9, Gideon, D. N., A. T. Hopper, and R. E. Thompson, Earth current effects on buried pipelines; analysis of observations of telluric gradients and their effects, Am. Gas Assn., cat. no. L30570, 77 pp., Apr., 1970.

1258 " ,0' 10 50 PERCENT DIRECT CORR0SION RATE 100 FIGURE 9.1 Percentage of direct current corrosion rate as a function of the period of the induced current (from McCollum and Ahlborn, 1916). ALASKA PIPELINE ROUTE FIGURE 9.2 Alaska oil pipeline shown as a dark line connecting Prudhoe Bay to Valdez. Geomagnetic latitudes along this route are indicated by line segments every degree from 60° to 70°. Location of geomagnetic observatories at Barrow, College, and Sitka are appropriately marked.

1259 PERIOD (mm or hr) 10mm ISm1n 30fn1n Ihr -I 1 JOO 300 700 1000 2000 3000 9000 7000 10000 15000 PERIOD T (setond!) FIGURE 9.3 Electric fields (10~6 V/m) as a function of period T (sec) obtained from a layered earth conductivity model and a spectral analysis (four hour sam- ples) of high latitude geomagnetic dis- turbances. Separate curves represent values for geomagnetic activity level indices, Ap, shown at the right. On the right margin is the equivalent pipeline current, I (amperes). Momentary surges of field and current at the time of max- imum disturbance reach amplitudes 5 to 10 times the magnitudes shown in this figure.

1260 10. PROBLEMS OF MAGNETIC FLUCTUATIONS IN GEOPHYSICAL EXPLORATIW (M'. S. Re ford, Geoterrex Ltd., Ottawa, Ontario, Canada) 10.1 Introduction In exploration geophysics, magnetic surveys are conducted to obtain information about subsurface rocks. Most of the surveys involve measurements of the total intensity of the earth's magnetic field across the survey area. Of necessity, the measurements include time fluctua- tions of the field, and one of the problems that must be dealt with is the separation of the unwanted time varia- tions from the desired spatial variations. The nature and seriousness of the problem, and its solution, depend on the nature of the geophysical survey. Most magnetic surveys are made with airborne magnetometers, and probably more than one million line-kms are flown every year all over the world. The most common objectives of a survey are to assist geological napping and mineral exploration. Typically, a survey may involve flying lines one km apart, measuring the field intensity in units of one gamma (ly = 1 nanotesla), at intervals of one second, and compiling the results as contour maps with a basic contour interval of ten gammas. Greater precision than this is required in petroleum exploration surveys, which are aimed at determining the depth to the magnetic basement rocks

1261 (buried beneath several kms of almost non-magnetic sedi- mentary rocks) and at detecting local structures on the basement rock surface. High-resolution magnetometers measuring in units of 0.01 gammas are often used for such surveys, and local anomalies of one gamma relief may be important. Another type of survey is common over the oceans, where the structure of the oceanic crust is revealed by the magnetic anomaly patterns recorded in data obtained from towing magnetometers behind ships, and sometimes near the ocean bottom. Finally, there are ground magnetometer sur- veys, usually restricted today to local detailing of individual mineral prospects. Figure 10.1 shows a sample magnetic contour map from a high-resolution airborne survey in the Canadian Arctic. The contour interval is one gamma, with half-gamma contours in places, and the survey was flown with east-west lines one mile apart and north-south tie lines three miles apart. Magnetic surveys such as this are interpreted in geological terms, both from contour maps, and from profiles along the survey lines. 10.2 Acquiring Survey Data The problems of time variations may be tackled at several stages of a survey. For a start, the problems are minimized by collecting data which includes the least

1262 possible effects of fluctuations. Two very different approaches are used at present. One of these approaches is the measurement of the vertical magnetic gradient as well as the total field intensity. Gradiometers, measuring the difference in intensity between two high-resolution magnetometers, have been in regular survey use for ten years. Both magnetometers are affected equally by time fluctuations, leaving the measured gradient unaffected. However, the resolution of.such a gradiometer is a function of the vertical distance between the sensors, which cannot be increased much beyond 70m without introducing serious operational problems; i.e., noise effects from relative movement between the sensors in turbulent air. Within these limits gradiometer measurements solve the problem of time fluctuations, but do introduce other difficulties in its place. One of the major of these is the increased costs. Thus, gradiometer surveys represent a very small proportion of the total volume of magnetic survey work presently being done, and have only partially replaced total intensity surveys, even in petroleum exploration. A second approach is to use a fixed, ground-based magnetometer to record time fluctuations, and to accept survey data only when these fluctuations fall within specified limits. Since variations which are linear for a number of minutes can be corrected in leveling the data, the

1263 specifications are keyed'to short periods. For instance, a specification in high-resolution petroleum surveys might be to reject data obtained when the ground magnetometer trace showed departures exceeding 2 gammas from any chord two minutes long. This type of specification works well in most parts of the world, but can be very difficult to apply where irregular fluctuations are common (for example, near the auroral zones) or in areas affected by the equatorial electrojet. If the application of such a specification is too rigid, an aircraft can wait forever on the ground for the ideal conditions which never arrive. It is better to obtain data, even though compromised, and re-fly certain lines to check for possible errors by comparison of results from both flights. A third approach might be suggested—to schedule surveys for periods when fluctuations are forecast to be minimal. Unfortunately, this is not very practical. Ex- ploration and budget flows cannot be modified to fit the sunspot cycle. Also, many surveys involve a data collection period of one to three months, which is usually fixed by the optimum flying conditions. Forecasting of magentic fluctua- ions is no more certain than forecasting weather, and an unpredicted period of several weeks excessive magnetic activity is simply one of the hazards of working near the auroral zone.

1264 10.3 Processing Survey Data Having obtained the survey data, fluctuations must be removed, as far as possible, in the subsequent data processing. Two approaches are generally used. First, the fixed ground magnetometer measurements may be subtracted from the airborne magnetometer measurements. For success, this pre-supposes certain conditions. Obviously, time synchronization between ground and mobile units must be good, and the ground magnetometer must be carefully sited to avoid man-made interference, such as trucks moving near the sensor. Second, the fluctuations at both places must be nearly identical. This makes it diffi- cult, if not impossible, to apply such a subtraction in many situations. The survey area may be several hundred kms away from the base station where the ground magneto- meter is established. Differences in the earth electrical conductivity at the base station and at the survey points are probably the most critical factor. For instance, fluctuations over resistive basement rocks are generally stronger and contain higher frequency components than those over a nearby sedimentary basin, or ocean. Additional com- plications occur along coastlines, where the conductivity contrast can increase the amplitudes of the fluctuations relative to points off the shore or island (Vacquier, 1972). At the present time, it does not seem practical to predict how successful subtraction may be in all circumstances,

1265 nor how many ground magnetometers would be needed at what locations for optimum results. Establishing independent recording ground magnetometers at remote locations can be very expensive. Hence, the method is applied usually on a limited and empirical basis, and not in areas such as the auroral zones, where it is likely to introduce more problems than it solves. As a rule of thumb in middle latitudes, ground stations may be located so that the mobile magnetometer is not taken more than 200 kms away. The effect of subtraction is then tested on the mobile data: if recognizable fluctuations are removed, or at least reduced, by subtraction, then it may be applied at an early stage in the data processing. An example is shown in Figure 10.2, from a test aeromagnetic survey in the Gulf of Mexico aimed at detecting anomalies with a relief of less than two gammas. Both ground and airborne magnetometer (traces 1 and 3) showed "mlcropulsations" of about 0.5Y relief, which were removed by subtracting ground from air measurements (trace M. Even if subtraction is applied, and more importantly if it is not, the survey data must be leveled together before it can be contoured. "Tie lines" are run for this purpose, usually perpendicular to the lines. At each intersection of a line and tie line, the difference between the two magnetic measurements is calculated. These differences are analyzed, and adjustments are applied to reduce them to zero, at the

1266 same time keeping the pattern of adjustments smooth along each line. In effect, the adjustments represent an attempt to reconstruct the time fluctuations. Study of the adjust- ments can show any lines of data which might be badly affected by erratic time fluctuations. The distance between tie lines is partly set with regard to errors that could arise from unrecognized time fluctuations (Reford and Summer, 1964). However, the tie lines improve the definition of magnetic gradients which are nearly parallel to the lines, and this consideration also affects the choice of tie line spacing. 10.4 Discussion The problems of time fluctuations are more severe for marine than for airborne surveys for several reasons. Because of the differences in speed, the same fluctuation will be spread over a greater distance along the airborne measurements and will affect the true shape of magnetic anomalies proportionately less. Also, as a result of speed, the time elapsed between intersections is much smaller for airborne than for marine profiles, so that fluctuations will be better corrected in leveling the data together. Finally, as a practical consideration, marine magnetometers are most often used as auxiliary instruments, not the main survey tool, so that there may be little chance of repeating sur- vey lines affected by magnetic disturbances.

1267 O •e « _ O t/5 e -a 2 ra O O. O — I I O I o •2 T3 05 O „ E 5

1268 i AIR MAG WITH ALTITUDE CORRECTION ®GROUND MAG 20 SECS AIR MAG WITH ALTITUDE CORRECTION MINUS GROUND MAG FIGURE 10.2 Aeromagnetic survey over the Gulf of Mexico and ground "base" station data taken to help eliminate the magnetic fluctuations.