Showing papers in "Reviews of Geophysics in 1989"
TL;DR: The processes that lead to charging of dust grains in a plasma are briefly reviewed in this article, where it is shown that the radial transport of dust contained in the spokes may be responsible for the rich radial structure in Saturn's rings.
Abstract: The processes that lead to charging of dust grains in a plasma are briefly reviewed. Whereas for single grains the results have been long known, the reduction of the average charge on a grain by 'Debye screening' has only recently been discovered. This reduction can be important in the Jovian ring and in the rings of Uranus. The emerging field of gravitoelectrodynamics which deals with the motion of charged grains in a planetary magnetosphere is then reviewed. Important mechanisms for distributing grains in radial distance are due to stochastic fluctuations of the grain charge and a systematic variation due to motion through plasma gradients. The electrostatic levitation model for the formation of spokes is discussed, and it is shown that the radial transport of dust contained in the spokes may be responsible for the rich radial structure in Saturn's rings. Finally, collective effects in dusty plasmas are discussed which affect various waves, such as density waves in planetary rings and low-frequency plasma waves. The possibility of charged grains forming a Coulomb lattice is briefly described.
1,470 citations
TL;DR: In this article, the authors present an improved representation of cloud in numerical models, obtaining a better understanding of vertical mixing in the deep ocean, and determining the inherent variability of the ocean-atmosphere system.
Abstract: The presence of radiatively active gases in the Earth's atmosphere (water vapor, carbon dioxide, and ozone) raises its global mean surface temperature by 30 K, making our planet habitable by life as we know it. There has been an increase in carbon dioxide and other trace gases since the Industrial Revolution, largely as a result of man's activities, increasing the radiative heating of the troposphere and surface by about 2 W m -2. This heating is likely to be enhanced by resulting changes in water vapor, snow and sea ice, and cloud. The associated equilibrium temperature rise is estimated to be between 1 and 2 K, there being uncertainties in the strength of climate feedbacks, particularly those due to cloud. The large thermal inertia of the oceans will slow the rate of warming, so that the expected temperature rise will be smaller than the equilibrium rise. This increases the uncertainty in the expected warming to date, with estimates ranging from less than 0.5 K to over 1 K. The observed increase of 0.5 K since 1900 is consistent with the lower range of these estimates, but the variability in the observed record is such that one cannot necessarily conclude that the observed temperature change is due to increases in trace gases. The prediction of changes in temperature over the next 50 years depends on assumptions concerning future changes in trace gas concentrations, the sensitivity of climate, and the effective thermal inertia of the oceans. On the basis of our current understanding a further warming of at least 1 K seems likely. Numerical models of climate indicate that the changes will not be uniform, nor will they be confined to temperature. The simulated warming is largest in high latitudes in winter and smallest over sea ice in summer, with little seasonal variation in the tropics. Annual mean precipitation and runoff increase in high latitudes, and most simulations indicate a drier land surface in northern mid-latitudes in summer. The agreement between different models is much better for temperature than for changes in the hydrological cycle. Priorities for future research include developing an improved representation of cloud in numerical models, obtaining a better understanding of vertical mixing in the deep ocean, and determining the inherent variability of the ocean-atmosphere system. Progress in these areas should enable detection of a man-made "greenhouse" warming within the next two decades.
554 citations
TL;DR: Paleomagnetic data for the late Paleozoic is undergoing rapid and drastic revision as discussed by the authors, and it is now recognized that many paleozoic poles for North America that were once considered reliable are in fact the result of remagnetization.
Abstract: Although it has been known for 25 years that some Paleozoic sedimentary rock units in Europe and North America were remagnetized during Pennsylvanian or Permian time, it is only very recently that the complex and widespread nature of the late Paleozoic remagnetization phenomenon has been generally acknowledged. It is now recognized that many Paleozoic paleomagnetic poles for North America that were once considered reliable are in fact the result of remagnetization, and as a consequence the paleomagnetic data base for the Paleozoic is undergoing rapid and drastic revision. The causes of late Paleozoic remagnetization in North America are currently the focus of much interest and active research. The remagnetization can reside in either hematite or magnetite, and different remagnetization mechanisms have been important in different settings. Chemical remagnetization processes, some related to specific diagenetic events, are dominant in hematite-bearing sandstones and carbonates. In magnetite-bearing carbonates both chemical and thermoviscous remagnetization processes appear to have been important, but it is difficult to determine which process is the dominant one in some settings. Some of the observed remagnetizations can be linked to the migration of chemically active and perhaps hot fluids during the mountain-building events that affected much of the continent during the late Paleozoic. Paleomagnetic studies promise to be important in assessing the role of orogeny in driving fluid migrations within sedimentary basins and in constraining the age of the migrations and the nature of the fluids.
331 citations
TL;DR: A short survey of results related to solitary internal waves in the ocean is presented in this paper, where observations of solitary waves in shallow seas and deep ocean regions accumulated over the last two decades.
Abstract: A short survey of results related to solitary internal waves in the ocean is presented. We discuss observations of solitary waves in shallow seas and deep ocean regions accumulated over the last two decades. Work containing more or less complete hydrodynamical data that allow one to construct adequate theoretical models and to compare theory with experimental results is considered in more detail. The relation between the features of observed solitary formations and those of solitons known in theory is also developed. A summary of theoretical models describing nonlinear internal wave propagation, including the basic evolution equations and their soliton solutions, is presented in an appendix.
310 citations
TL;DR: A review of state-of-the-art research into the implications of climatic changes for the hydrologic cycle and for water resources can be found in this article, where the authors discuss the potential of such changes for future water planning and management.
Abstract: Growing atmospheric concentrations of carbon dioxide and other trace gases are leading to climatic changes with important implications for the hydrologic balance and water resources. These “greenhouse gases” are expected to alter the radiative balance of the atmosphere, causing increases in temperature and changes in many other climatic variables. Recent hydrological research strongly suggests that this so-called “greenhouse effect” will alter the timing and magnitude of runoff and soil moisture, change lake levels, and affect water quality. Such changes raise the possibility of environmental and socioeconomic dislocations, and they have important implications for future water resources planning and management. This paper reviews state-of-the-art research into the implications of climatic changes for the hydrologic cycle and for water resources and discusses the implications of such changes for future water planning and management.
304 citations
TL;DR: The early climate of Mars was characterized by liquid water on the surface and a dense atmosphere composed predominantly of CO2, and it is known that life had originated on earth and reached a fair degree of biological sophistication as discussed by the authors.
Abstract: There is considerable evidence that the early climate of Mars was very different from the inhospitable conditions there today. This early climate was characterized by liquid water on the surface and a dense atmosphere composed predominantly of CO2. The duration of these warm initial conditions on the surface of Mars is uncertain, but theoretical models suggest that they could have persisted for hundreds of millions up to a billion years. From studies of the earth's earliest biosphere, it is known that, by 3.5 Gyr ago, life had originated on earth and reached a fair degree of biological sophistication. If Mars did maintain a clement environment for longer than it took for life to originate on earth, then the question of the origin of life on Mars follows naturally. Since over two thirds of the Martian surface is more than 3.5 Gyr old, the possibility exists that Mars may hold the best record of the events that led to the origin of life, even though there may be no life there today.
228 citations
TL;DR: The history of El Nino research is traced from its beginnings through the key innovations of Bjerknes and Wyrtki to the unusual 1982-1983 event as discussed by the authors, with detailed discussions of two important processes: instability growth and vacillation between climate states.
Abstract: El Nino events—anomalous warmings of the tropical Pacific with associated climatic and economic impacts around the globe—have occurred at several-year intervals since before written records began with the logs of Francisco Pizarro in 1525. In this review, the history of El Nino research is traced from its beginnings through the key innovations of Bjerknes and Wyrtki to the unusual 1982–1983 event. Recent research is then reviewed, with detailed discussions of two important processes: instability growth and vacillation between climate states. Throughout the paper there are adjunct discussions of extraregional teleconnections, ecological impacts, and research on El Nino in the ancient record. The final section discusses the present paradigm for vacillations between El Nino and non-El Nino states and speculates on the possibly chaotic nature of El Nino. El Nino and its atmospheric counterpart, the Southern Oscillation, appear to occur as an internal cycle of positive and negative feedbacks within the coupled ocean-atmosphere climate system of the tropical Pacific, although hypotheses based on external forcing also exist. All events are preceded by westerly wind anomalies on the equator near the date line. Baroclinic equatorial Kelvin waves are generated, propagating eastward toward South America where they depress the thermocline and raise sea level, while the deep, upper ocean reservoir of warm water in the western Pacific is depleted. Sea surface temperature (SST) anomalies in the cool eastern Pacific occur primarily because the normal source of cold water is depressed below the reach of mixing and upwelling processes. In the central equatorial Pacific, eastward advection by anomalous zonal flows is the principal mechanism. Nonlinear heat transfer to the lower atmosphere creates a positive ocean-atmosphere feedback resulting in the unstable growth of anomalies along the equator. Much of the present research aims at determining how the ocean-atmosphere system vacillates between the El Nino and non-El Nino states. Coupled models suggest that a longer time scale, negative-feedback process produces the transitions: at the apex of an El Nino development an anomalous atmospheric convection above the areas of maximum SST produces areas of reduced upper layer thickness in the off-equatorial ocean, which slowly propagate westward to the western boundary as Rossby waves and back to the central equatorial Pacific as upwelling Kelvin waves, reestablishing the normal cooling process. A similar negative feedback of opposite sign completes the second half of an oscillation, returning again to the El Nino state. However, the notion that El Nino-Southern Oscillation variability results only from an internal feedback process is still highly contentious, and a number of external forcing mechanisms have been proposed.
198 citations
TL;DR: In this paper, the principles of modeling fluid flow in systems containing up to three fluid phases (namely, water, air, and organic liquid) are described, and a number of hypothetical numerical problems are presented to illustrate the physical behavior of systems in which multiphase flow and transport arise.
Abstract: Multiphase flow and transport of compositionally complex fluids in geologic media is of importance in a number of applied problems which have major social and economic effects. In petroleum reservoir engineering, efficient recovery of energy reserves is the principal goal. Unfortunately, some of these hydrocarbons and other organic chemicals often find their way unwanted into the soils and groundwater supplies. Removal in the latter case is predicated on ensuring the public health and safety. In this paper, principles of modeling fluid flow in systems containing up to three fluid phases (namely, water, air, and organic liquid) are described. Solution of the governing equations for multiphase flow requires knowledge of functional relationships between fluid pressures, saturations, and permeabilities which may be formulated on the basis of conceptual models of fluid-porous media interactions. Mechanisms of transport in multicomponent multiphase systems in which species may partition between phases are also described, and the governing equations are presented for the case in which local phase equilibrium may be assumed. A number of hypothetical numerical problems are presented to illustrate the physical behavior of systems in which multiphase flow and transport arise.
176 citations
TL;DR: A review of hazards mitigation approaches and techniques indicates that significant advances have been made in hazards assessment, volcano monitoring, and eruption forecasting as mentioned in this paper, but a predictive capability for more voluminous and explosive eruptions still has not been achieved, and the greatest advances in volcanic hazards mitigation in the near future are most likely to be achieved by wider application of existing technology to poorly understood and studied volcanoes, rather than by refinements or new discoveries in technology alone.
Abstract: At the beginning of the twentieth century, volcanology began to emerge as a modern science as a result of increased interest in eruptive phenomena following some of the worst volcanic disasters in recorded history: Krakatau (Indonesia) in 1883 and Mont Pelee (Martinique), Soufriere (St. Vincent), and Santa Maria (Guatemala) in 1902. Volcanology is again experiencing a period of heightened public awareness and scientific growth in the 1980s, the worst period since 1902 in terms of volcanic disasters and crises. A review of hazards mitigation approaches and techniques indicates that significant advances have been made in hazards assessment, volcano monitoring, and eruption forecasting. For example, the remarkable accuracy of the predictions of dome-building events at Mount St. Helens since June 1980 is unprecedented. Yet a predictive capability for more voluminous and explosive eruptions still has not been achieved. Studies of magma-induced seismicity and ground deformation continue to provide the most systematic and reliable data for early detection of precursors to eruptions and shallow intrusions. In addition, some other geophysical monitoring techniques and geochemical methods have been refined and are being more widely applied and tested. Comparison of the four major volcanic disasters of the 1980s (Mount St. Helens, U.S.A. (1980), El Chichon, Mexico (1982); Galunggung, Indonesia (1982); and Nevado del Ruiz, Colombia (1985) illustrates the importance of predisaster geoscience studies, volcanic hazards assessments, volcano monitoring, contingency planning, and effective communications between scientists and authorities. The death toll (>22,000) from the Ruiz catastrophe probably could have been greatly reduced; the reasons for the tragically ineffective implementation of evacuation measures are still unclear and puzzling in view of the fact that sufficient warnings were given. The most pressing problem in the mitigation of volcanic and associated hazards on a global scale is that most of the world's dangerous volcanoes are in densely populated countries that lack the economic and scientific resources or the political will to adequately study and monitor them. This problem afflicts both developed and developing countries, but it is especially acute for the latter. The greatest advances in volcanic hazards mitigation in the near future are most likely to be achieved by wider application of existing technology to poorly understood and studied volcanoes, rather than by refinements or new discoveries in technology alone.
164 citations
TL;DR: Hole 504B is the most complete hole yet drilled into the oceanic crust in situ, and it therefore provides the complete "ground truth" now available to test our models of the structure and evolution of the upper Oceanic crust.
Abstract: Hole 504B is by far the deepest hole yet drilled into the oceanic crust in situ, and it therefore provides the most complete “ground truth” now available to test our models of the structure and evolution of the upper oceanic crust. Cored in the eastern equatorial Pacific Ocean in 5.9-m.y.-old crust that formed at the Costa Rica Rift, hole 504B now extends to a total depth of 1562.3 m below seafloor, penetrating 274.5 m of sediments and 1287.8 m of basalts. The site was located where the rapidly accumulating sediments impede active hydrothermal circulation in the crust. As a result, the conductive heat flow approaches the value of about 200 mW/m² predicted by plate tectonic theory, and the in situ temperature at the total depth of the hole is about 165°C. The igneous section was continuously cored, but recovery was poor, averaging about 20%. The recovered core indicates that this section includes about 575 m of extrusive lavas, underlain by about 200 m of transition into over 500 m of intrusive sheeted dikes; the latter have been sampled in situ only in hole 504B. The igneous section is composed predominantly of magnesium-rich olivine tholeiites with marked depletions in incompatible trace elements. Nearly all of the basalts have been altered to some degree, but the geochemistry of the freshest basalts is remarkably uniform throughout the hole. Successive stages of on-axis and off-axis alteration have produced three depth zones characterized by different assemblages of secondary minerals: (1) the upper 310 m of extrusives, characterized by oxidative “seafloor weathering“; (2) the lower extrusive section, characterized by smectite and pyrite; and (3) the combined transition zone and sheeted dikes, characterized by greenschist-facies minerals. A comprehensive suite of logs and downhole measurements generally indicate that the basalt section can be divided on the basis of lithology, alteration, and porosity into three zones that are analogous to layers 2A, 2B, and 2C described by marine seismologists on the basis of characteristic seismic velocities. Many of the logs and experiments suggest the presence of a 100- to 200-m-thick layer 2A comprising the uppermost, rubbly pillow lavas, which is the only significantly permeable interval in the entire cored section. Layer 2B apparently corresponds to the lower section of extrusive lavas, in which original porosity is partially sealed as a result of alteration. Nearly all of the logs and experiments showed significant changes in in situ physical properties at about 900–1000 m below seafloor, within the transition between extrusives and sheeted dikes, indicating that this lithostratigraphic transition corresponds closely to that between seismic layers 2B and 2C and confirming that layer 2C consists of intrusive sheeted dikes. A vertical seismic profile conducted during leg 111 indicates that the next major transition deeper than the hole now extends—that between the sheeted dikes of seismic layer 2C and the gabbros of seismic layer 3, which has never been sampled in situ—may be within reach of the next drilling expedition to hole 504B. Therefore despite recent drilling problems deep in the hole, current plans now include revisiting hole 504B for further drilling and experiments when the Ocean Drilling Program returns to the eastern Pacific in 1991.
157 citations
TL;DR: In this article, the authors consider the development and verification of global computer models of the F-region which simulate the interactions between physical processes in the ionosphere, focusing on the inputs to the ionospheric system such as magnetospheric electric field and auroral precipitation.
Abstract: Consideration is given to the development and verification of global computer models of the F-region which simulate the interactions between physical processes in the ionosphere. The limitations of the physical models are discussed, focusing on the inputs to the ionospheric system such as magnetospheric electric field and auroral precipitation. The possibility of coupling ionospheric models with thermospheric and magnetospheric models is examined.
TL;DR: In this article, the characteristics of the surface inferred from Lander imaging and meteorology data, physical and magnetic properties experiments, and both inorganic and organic analyses of Martian samples are analyzed.
Abstract: Data collected by two Viking landers are analyzed. Attention is given to the characteristics of the surface inferred from Lander imaging and meteorology data, physical and magnetic properties experiments, and both inorganic and organic analyses of Martian samples. Viking Lander 1 touched down on Chryse Planitia on July 20, 1976 and continued to operate for 2252 sols, until November 20, 1982. Lander 2 touched down about 6500 km away from Lander 1, on Utopia Planitia on September 3, 1976. The chemical compositions of sediments at the two landing sites are similar, suggesting an aeolian origin. The compositions suggest an iron-rich rock an are matched by various clays and salts.
TL;DR: In this paper, the authors present a review of the current state of the art in remote sensing based weather forecasting, focusing on the measurement and extrapolation up to about 2 hours ahead of fields of various weather parameters, especially rainfall.
Abstract: Digital remotely sensed observations of the atmosphere, particularly from radars and satellites, have become increasingly available over the last 15 years. Together with developments in computer technology this has stimulated the design and operation of a variety of systems to exploit these data in weather forecasting. Growing awareness of the importance of detailed site specific weather information and forecasts from zero to a few hours ahead has led to the emergence of a particular kind of forecasting called nowcasting which depends on the exceptionally detailed knowledge of the current pattern of weather that remote sensing can provide. This type of weather forecasting is reviewed, with emphasis on the measurement and extrapolation up to about 2 hours ahead of fields of various weather parameters, especially rainfall. Trends in the design of nowcasting systems are discussed, and potential benefits summarized.
TL;DR: In this article, a review of the ability to quantify climate change with respect to near-surface air temperature (measured 1.25 m above ground), sea surface temperature, precipitation, snow cover, sea ice, and vegetation measured from space and the Earth's surface is presented.
Abstract: While a great deal of climate data have been gathered over the past hundred years, there remains a number of problems limiting our ability to fully utilize these data in reconstructing the climate of the past century. This is particularly true for research demanding high precision and/or detailed local or regional-scale climate analyses. In this review we consider our ability to quantify climate change with respect to near-surface air temperature (measured 1.25–2 m above ground), sea surface temperature, precipitation, snow cover, sea ice, and vegetation measured from space and the Earth's surface. Among the data issues we discuss are calibration, observing practices, urbanization, station changes, data representativeness, data access, and areal coverage. The diversity of measurements over the past century and the new monitoring system being introduced via space-based and surface-based platforms offer an unparalleled opportunity for global monitoring; but to quantify climate change, we must tackle such issues as changing retrieval algorithms, relatively short periods of record, satellite Earth location precision, incompatibility with previous conventional historical observations, calibration, and potentially overwhelming data volumes. A new specialty within the climate field is beginning to emerge to address these problems. Despite the litany of problems the instrumented climate record can tell us a great deal about the spatial distribution and secular trends in temperature and precipitation over many areas of the world. In the future a blend of many data types and observing systems will be necessary to better quantify climate change. These large data sets will have to be made accessible to scientists in such a way that allows them an opportunity to check the veracity of their hypotheses and predictions regarding climate change.
TL;DR: The Gaia hypothesis as mentioned in this paper states that the climate and chemical composition of the Earth's surface environment is, and has been, regulated at a state tolerable for the biota, and that the evolution of organisms and their material environment is a single, tightly coupled process, with the selfregulation of climate and chemistry as an emergent property.
Abstract: The Gaia hypothesis postulates that the climate and chemical composition of the Earth's surface environment is, and has been, regulated at a state tolerable for the biota. This notion was introduced in 1972 and 1973 (Lovelock, 1972; Margulis and Lovelock, 1974; Lovelock and Margulis, 1973). The wording of these early papers was sometimes poetic, rather than scientific, but Gaia has matured and might be better stated as a theory that views the evolution of the biota and of their material environment as a single, tightly coupled process, with the self-regulation of climate and chemistry as an emergent property. It is a theory that makes “risky” predictions, for example, that oxygen is and has been regulated during the existence of land plants, within ±5% of its present level; it is therefore falsifiable. Numerical models are used to illustrate the potential for stable self-regulation of tightly coupled systems of organisms and their environments.
TL;DR: The Gaia hypothesis as mentioned in this paper states that biological processes homeostatically maintain, on a planetary scale, geochemical and climatic conditions favorable for life, which is the central theme of the Gaia hypothesis.
Abstract: The Gaia hypothesis' central theme is that biological processes homeostatically maintain, on a planetary scale, geochemical and climatic conditions favorable for life. A number of distinct hypotheses have been proposed, spanning a range from the self-evident to the highly speculative. The self-evident forms of Gaia reiterate the well-documented fact that biological processes are critical to biogeochemical cycles, adding the straightforward (though important) point that the coupling between biotic and physical processes should create feedback loops. The speculative forms of Gaia assert that biological processes regulate the physical environment, keeping Earth's climate and surface geochemistry stable and favorable for life. As metaphors, these versions of Gaia are intriguing, untestable, and, if taken literally as a basis for research, potentially misleading. As hypotheses, they are ill-defined, unparsimonious, and unfalsifiable.
TL;DR: In this paper, a review of the underlying principles and interrelationships among the above flux mechanisms is presented, and the constitutive equations relating these flux components are available from the "dusty gas" model of Mason et al.
Abstract: The increasing use of natural unsaturated zones as repositories for landfills and disposal sites for hazardous wastes (chemical and radioactive) requires a greater understanding of transport processes in the unsaturated zone. For volatile constituents an important potential transport mechanism is gaseous diffusion. Diffusion, however, cannot be treated as an independent isolated transport mechanism. A complete understanding of multicomponent gas transport in porous media (unsaturated zones) requires a knowledge of Knudsen transport, the molecular and nonequimolar components of diffusive flux, and viscous (pressure driven) flux. The constitutive equations relating these flux components are available from the “dusty gas” model of Mason et al. (1967). This review presents a brief discussion of the underlying principles and interrelationships among each of the above flux mechanisms. Some aspects of these transport mechanisms are, to our knowledge, generally unrecognized in the Earth science literature. The principles underlying the transport mechanisms are illustrated with binary systems; the constitutive equations are then cast in forms thought to be most useful for the study of natural unsaturated zones. The viscous and diffusive fluxes are coupled in the constitutive equations through the Knudsen diffusivities; a knowledge of Knudsen diffusivities is necessary to calculate the viscous component of flux and pressure gradients. The Knudsen diffusivities can be calculated from measurements of the Klinkenberg effect. Two examples are presented showing that in natural systems, very small pressure gradients (1 Pa/m or less) can produce viscous fluxes greater than or equal to diffusive fluxes and that, conversely, pressure gradients of this magnitude can be generated by diffusive processes. The example calculations show that major concentration gradients can be developed for stagnant (zero flux, nonreactive) gases. A method is presented for approximating the viscous and diffusive flux components of gases in a multicomponent system from a knowledge of the concentration profiles of stagnant gases. In subsoil environments, argon and nitrogen are considered to be stagnant gases. Fick's laws are essentially, by definition, inadequate to deal with stagnant gases. In the examples presented, the error associated with estimating the total fluxes of nonstagnant gases by Fick's law, relative to stationary coordinates, ranges from a few percent to orders of magnitude.
TL;DR: In this paper, an acceptable false alarm rate for misidentifying noise as a spectral peak is set, and a normalized periodogram is then evaluated, and the highest peak is tested against the limit set by the false alarm criteria.
Abstract: The theory of nonlinear dynamics can assist in the selection of appropriate models to guide the analysis of observational time series. Weak and moderate nonlinearities lead to periodic or quasi-periodic functions having discrete spectral lines. These functions are best analyzed by using the classical periodogram for equally spaced data and Lomb's extension of the periodogram for unequally spaced data. Strong nonlinearities can lead to aperiodic functions that have the continuous spectra characteristic of chaos. Power spectral analysis alone cannot be used to distinguish stochastic input to the time series from contributions to the continuum caused by deterministic nonlinearities. The evaluation of higher-order spectra (bispectra, etc.) provides one means of distinguishing between those parts of the continuum due to hidden determinism and those due to stochastic input. This paper outlines a procedure for the analysis of mixed spectra modeled by a nonlinear system. First, an acceptable false alarm rate for misidentifying noise as a spectral peak is set. A normalized periodogram is then evaluated, and the highest peak is tested against the limit set by the false alarm criteria. The phase and amplitude of the maximum peak are estimated by least squares, provided the peak is judged to be significant. These parameters, together with the periodogram-determined frequency, are used for a point-by-point subtraction of the sinusoid from the record. The prewhitening procedure is continued until all significant peaks are removed, leaving the estimated continuum part of the spectrum. In order to illustrate the procedure and general concepts, three examples are analyzed. A calculated time series showing the long-term variation of Earth's obliquity demonstrates the difficulty of analyzing a quasi-periodic function when the length of the record is insufficient to separate nearby lines in the spectrum. Quasi-periodic behavior is shown by the sunspot record, which has 15 significant peaks at the 0.05 level; 12 of the peaks are simple combination tones of the other three. Analysis of unequally spaced observations is illustrated by the Vostok ice record of CO2 variation. The detected peaks of CO2 variation include the 100,000-, 40,000-, and 20,000-year peaks detected in oxygen isotope records.
TL;DR: In this article, a review of the progress in making measurements of wavelength and scale sizes is presented, and different techniques for inferring wavelength are described, and then a series of case studies are considered to demonstrate applications.
Abstract: A plasma wave can be described by its frequency and wave vector, while a nonlinear plasma structure such as a solitary wave can be described by its temporal signature and scale size. Time series or frequency measurements are relatively easy to perform from spacecraft, but wavelength or scale size measurements are difficult. Nevertheless, direct measurements of wavelength have contributed to understanding plasma waves and fluctuations; for example, such measurements have established the widespread existence of spatial irregularities in the magnetosphere, determined the characteristics of waves upstream of the earth's bow shock, and allowed the size and motion of double-layer-like structures along auroral field lines to be inferred. This paper reviews progress in making measurements of wavelength and scale sizes. The different techniques for inferring wavelength are described, and then a series of case studies is considered to demonstrate applications.
TL;DR: X ray absorption spectroscopy (XAS) has proven to be a versatile structural probe for studying the local environments of cations in a variety of earth materials ranging from crystalline solids, silicate glasses, and high temperature silicate melts to aqueous sorption systems which involve metal complexes associated with (or sorbed at) mineral/water interfaces as mentioned in this paper.
Abstract: Since its development in the early 1970s, synchrotron-based X ray absorption spectroscopy (XAS) has proven to be a versatile structural probe for studying the local environments of cations in a variety of earth materials ranging from crystalline solids, silicate glasses, and high-temperature silicate melts to aqueous sorption systems which involve metal complexes associated with (or sorbed at) mineral/water interfaces. The structural information provided by XAS includes average interatomic distances and the number and chemical identities of neighbors within 5–6 A of a selected cation. In many cases (e.g., short-range ordering of cations in minerals; cation environments in gels, glasses, melts, and metamict (radiation damaged) minerals; and the structure and composition of sorbed species at mineral/water interfaces), XAS provides unique structural data not duplicated by other methods. This paper presents a tutorial review of the physics underlying XAS and data analysis methods and a selective review of applications of XAS to earth materials.
TL;DR: Early research on the earth's magnetic environment is reviewed in this article, with attention given to the period when only ground-based observations were possible, and the possibility of electron beams from the sun and the Chapman-Ferraro cavity.
Abstract: Early research on the earth's magnetic environment is reviewed, with attention given to the period when only ground-based observations were possible. Early work on geomagnetism is discussed as well as the sunspot cycle, solar fares, the possibility of electron beams from the sun, and the Chapman-Ferraro cavity. Consideration is also given to the ring current, Alfvens theory and electric fields, interplanetary plasma, and polar magnetic storms.
TL;DR: In this paper, the authors present results from magnetovariation fields recorded by two-dimensional arrays of magnetometers and discuss the conductive structures mapped and studied and their tectonic implications.
Abstract: This review presents results from magnetovariation fields recorded by two-dimensional arrays of magnetometers. The emphasis is on the conductive structures mapped and studied and their tectonic implications. Eleven arrays were operated in North America between 1967 and 1985. A region of highly conductive uppermost mantle and/or lower crust, extending at least from 33°N to 54°N, has been shown to extend beneath much of the continent west of the Rocky Mountains. Two recent investigations are discussed in more detail: those of conductive structures under the Canadian Rockies and EMSLAB (Electromagnetic Sounding of the Lithosphere and Beyond) array results for Washington and Oregon, including a conductive strip beneath the Cascades volcanoes. Correlations with high heat flow and seismic parameters make it reasonable to attribute these regional conductors to partial melting and/or hot saline water. Within the craton the North American Central Plains conductor is discussed; this narrow, crustal feature is associated with a major fracture zone and age boundary, which may be a plate boundary of Proterozoic time. An array beneath the auroral electrojet, for study of the external currents, is noted. Results from four array studies in southern Africa are considered. Two of these relate to the Southern Cape Conductive Belt, which correlates not with high heat flow but with a large static magnetic anomaly. The magnetovariation, magnetic and gravity anomalies, and geological data can be accounted for if an accumulation of ophiolitic rocks including serpentine is present in the lower crust. Such an accumulation may have formed at a subduction in Proterozoic time. Two other arrays delineated a conductive belt associated with the southwestern end of the African rift system, in Namibia. Subsequent resistivity soundings located a conductor of resistivity 10 ohm m at less than 4 km depth. Two array studies in India follow, one in the north and one in the south. The array in the Himalayan foothills and Indo-Gangetic plain mapped a conductor transverse to the regional strike; this may be a fracture zone produced in the interplate collision, with a rise of asthenospheric material such as fluids into the fractures. The southern Indian array reveals crustal conductors under the Indo-Sri Lankan graben and another under the Comorin Ridge southwest of the tip of the subcontinent. Array studies in central and southeastern Australia and in Fennoscandia, in regions without large local conductivity anomalies, have been used to study conductivity variation with depth. A note on the Carpathian crustal anomaly is included as an example of combination of response parameters secured at different times to form a “virtual” array.
TL;DR: In this paper, a review of advances in the mathematical physics of unsaturated flow in soils and porous media is presented. But the main focus of the review is on the optical analog of the unsaturated flows and the scattering of plane pulses.
Abstract: This review takes the form of a set of Chinese boxes. The outermost box gives a brief general account of modem developments in the mathematical physics of unsaturated flow in soils and porous media. This provides the necessary foundations for the second box, which describes the quasi-linear analysis of steady multidimensional unsaturated flow, which is an essential prerequisite to the analog. Only then can we proceed to the innermost box, devoted to our major theme. An exact analog exists between steady quasi-linear flow in unsaturated soils and porous media and the scattering of plane pulses, and the analog carries over to the scattering of plane harmonic waves. Numerous established results, and powerful techniques such as Watson transforms, far-field scattering functions, and optical theorems, become available for the solution and understanding of problems of multidimensional infiltration. These are needed, in particular, to provide the asymptotics of the physically interesting and practically important limit of flows strongly dominated by gravity, with capillary effects weak but nonzero. This is the limit of large s, where s is a characteristic length of the water supply surface normalized with respect to the sorptive length of the soil. These problems are singular in the sense that ignoring capillarity gives a totally incorrect picture of the wetted region. In terms of the optical analog, neglecting capillarity is equivalent to using geometrical optics, with coherent shadows projected to infinity. When exact solutions involve exotic functions, difficulties of both analysis and series summation may be avoided through use of small-s and large-s expansions provided by the analog. Numerous examples are given of solutions obtained through the analog. The scope for extending the application to flows from surface sources, to anisotropic and heterogeneous media, to unsteady flows, and to linear convection-diffusion processes in general is described briefly.