Showing papers in "Reviews of Geophysics in 1976"

Some basic stress diffusion solutions for fluid-saturated elastic porous media with compressible constituents

Abstract: This is a study of the formulation, some basic solutions, and applications of the Biot linearized quasistatic elasticity theory of fluid-infiltrated porous materials. Whereas most previously solved problems are based on idealizing the fluid and solid constituents as separately incompressible, full account is taken here of constituent compressibility. Previous studies are reviewed and the Biot constitutive equations relating strain and fluid mass content to stress and pore pressure are recast in terms of new material parameters, more directly open to physical interpretation as the Poisson ratio and induced pore pressure coefficient in undrained deformation. Different formulations of the coupled deformation/diffusion field equations and their analogues in coupled thermoelasticity are discussed, and a new formulation with stress and pore pressure as basic variables is presented that leads, for plane problems, to a convenient complex variable representation of solutions. The problems solved include those of the suddenly introduced edge dislocation and concentrated line force and of the suddenly pressurized cylindrical and spherical cavity. The dislocation solution is employed to represent that for quasi-static motions along a shear fault, and a discussion is given, based on fracture mechanics models for fault propagation, of phenomena involving coupled behavior between the rupturing solid and its pore fluid, which could serve to stabilize a fault against rapid spreading. Also, the solution for a pressurized cylindrical cavity leads to a time-dependent stress field near the cavity wall, and its relevance to time effects in the inception of hydraulic fractures from boreholes, or from drilled holes in laboratory specimens, is discussed. Various limiting cases are identified, and numerical values of the controlling porous media elastic parameters are given for several rocks.

Retrieval of atmospheric temperature and composition from remote measurements of thermal radiation

Abstract: This paper reviews the methods which may be used to estimate the state of the atmosphere, i.e., the distribution of temperature and composition, from measurements of emitted thermal radiation such as are made by remote sounding instruments on satellites. The principles of estimation theory are applied to a linear version of the problem, and it is shown that many of the apparently different methods to be found in the literature are particular cases of the same general method. As an aid to understanding, the optimum linear solution is described in terms of the geometry of n dimensions, with n = 3 for illustration. In generalizing the approach to the nonlinear problem there are two stages: (1) finding any member of the infinite family of possible solutions, which may be done by any convenient iterative method, and (2) finding the optimum solution by satisfying appropriate constraints.

The Elastic Properties of Composite Materials

Abstract: The determination of the elastic properties of composite materials (multiphase aggregates, polycrystals, and porous or cracked solids) from the elastic properties of the components may be approached in several ways. The problem may be treated statistically, via scattering theory, through variational principles, or by the assumption of specific geometries for the material under consideration. Each of these methods is reviewed in turn. The widely used Voigt-Reuss-Hill average can be a poor approximation for both two-phase composites and polycrystals, and its replacement by the two Hashin-Shtrikman bounds is recommended. For pore-containing or crack-containing media, specific geometry models must be considered if useful results are to be obtained. If aggregate theory is used to estimate the moduli of individual components of a composite whose bulk properties are known, the shear moduli of the component phases must be matched (within a factor of 2 or 3) for the method to be useful. Results for nonlinear composites (which allow calculation of the pressure variation of aggregate moduli) have been obtained for only a few special cases.

Lunar volcanism in space and time.

Abstract: The role of lunar volcanism in the history of the moon is documented using lunar-orbit and earth-based data along with characterizations derived from Apollo and Luna sample-return missions. Characteristics of mare and highland volcanic features are described, Apollo and Luna results are discussed, and the characteristics of other mare deposits and of other highland features of possible volcanic origin are summarized. Major conclusions are that: (1) there is little unequivocal morphologic evidence for highland volcanism, (2) lunar mare lavas appear to have originated from depths of 100 to 500 km, (3) impact melting does not appear to have been a factor in the generation of mare lavas, (4) mare volcanism was characterized by massive outpourings of very fluid volatile-poor lava analogous to terrestrial flood basalts, (5) mare volcanism took place from 3.83 to about 2.5 billion years ago, (6) the preferential occurrence of mare deposits in large impact basins appears to be generically unrelated to basin formation, and (7) a thicker farside crust may be responsible for the distinctive nearside-farside asymmetry of mare deposits.

Steady state flow of rocks

Abstract: Experimentally determined steady state flow properties and processes of important rock-forming materials are reviewed in reference to those of metals and ceramics and to physical conditions in the earth's crust and upper mantle. Dislocation motion controls the creep rate over a wide range of steady state conditions in the experiments, and the observation that the same processes have operated during natural deformations permits extrapolations of the mechanical data. Under these conditions, strain rate is related to stress raised to the power 2–9, depending on the material and conditions, and the resulting flow stresses and equivalent viscosities are compared at a representative geological strain rate of 10−14/s. The results are applied in brief discussions of diapirism, growth of folds, and flow in the upper mantle.

Mare basalts: Crystal chemistry, mineralogy, and petrology

Abstract: The paper attempts a synthesis of the major-element chemistry, petrography, mineral chemistry, and crystal chemistry of the mare basalts returned by Apollo and Luna missions A classification of the mare basalts based on major-element chemistry is given, and textural sequences within each major-element group are identified The mineral chemistry and crystal chemistry of each mineral group are considered within the framework of the major-element groups and the textural sequences The various classes of models for the origin of the mare basalts and the nature of their source regions are discussed in the context of the major- and trace-element chemistries and experimental investigations

Magnetometers using RF-driven squids and their applications in rock magnetism and paleomagnetism

Abstract: A variety of magnetometer systems have been constructed utilizing superconducting quantum interference device (Squid) sensors. The magnetometers have high sensitivity and fast response times and eliminate the need for spinning or vibrating the sample to make measurements. The magnetometers have superconducting sense coils in which a dc persistent current is induced by the insertion of the sample. The current gives, via a flux transformer, a field at the Squid sensor, which is detected as the measure of the magnetization of the sample. The sense coils, the transformer, and the detector are housed within a superconducting shield. It was the advent of the Josephson junction devices which permitted the use of critical currents so small that flux-switching sensors such as those used in the magnetometers could be developed. The particular Squid used in these magnetometers is an RF-driven weak link sensor. The magnetometers have been used to observe magnetic transitions to identify magnetic phases. A high-field susceptometer with a superconducting magnet permits determination of saturation magnetization and magnetic characteristics. Standard paleomagnetic samples can be measured at a magnetic moment sensitivity of 10−8 G cm³. Small samples have been measured with a 3-mm access system whose noise level is 2 × 10−10 G cm³. Long cores can be measured by using a horizontal open access system. Continuous thermal demagnetization and intensity determinations are being developed.

The distribution of water vapor in the stratosphere

Abstract: This paper seeks to collect together and to assess the many measurements of stratospheric humidity which have been reported over the last 25 years, with a view to determining the average distribution of water vapor in the stratosphere and its variations in time and space. Variations with height, latitude, time, and season are considered. In addition, some consideration is given to the proper use of experimental values of humidity in discussions of the water budget and circulation of the stratosphere; it is emphasized that the considerable uncertainties which often exist in measured data often can preclude the drawing of quantitative conclusions.

Representation of magnetic fields in space

Abstract: Several methods by which a magnetic field in space can be represented are reviewed with particular attention to problems of the observed geomagnetic field. Time dependence is assumed to be negligible, and five main classes of representation are described by vector potential, scalar potential, orthogonal vectors, Euler potentials, and expanded magnetic field.

Sources and sinks for atmospheric N2O

Abstract: Observations of the temporal and spatial distribution of N_2O in solution are not yet sufficient to permit quantitative assessment of the role of the ocean in the budget of atmospheric N_2O. Consideration of the global nitrogen cycle suggests that the land should be the primary source of N_2O. The gas is removed in the atmosphere by photolysis and by reaction with O(¹D), and there may be additional sinks in the ocean.

Magnetospheric electric fields and their variation with geomagnetic activity

Abstract: Diverse and independent measurements of the variation of particle convection boundaries with geomagnetic activity are used to obtain relations between the magnitude of a large scale electric field and the Kp index. The relations are compared with measured fields and with models and are found to be consistent with diverse observations and probably more reliable than results previously obtained.

Magnetic fields of the magnetosheath

Abstract: The magnetic field of the magnetosheath is most naturally discussed in terms of its steady state and its fluctuating components Theory of the steady state field is quite well developed and its essential features have been confirmed by observations The interplanetary field is convected through the bow shock where its magnitude is increased and its direction changed by the minimal amount necessary to preserve the normal component across the shock Convection within the magnetosheath usually increases the magnitude still further near the subsolar point and further distortes the direction until the field is aligned approximately tangent to the magnetopause Fluctuations of the magnetosheath field are very complex, variable and not well understood Spectral peaks are common features which occur at different frequencies at various times Perturbation vectors of hydromagnetic waves tend to be aligned with the shock and magnetopause surfaces Magnetosheath waves may be generated upstream, within the magnetosheath, at the bow shock, or at the magnetopause, but the relative importance of these sources is not known

Crystal Chemistry of Silicate Minerals of Geophysical Interest

Abstract: Recent crystal chemical research on rock-forming silicate minerals has focused on the accurate refinement of room temperature and high-temperature crystal structures. The data are of sufficient accuracy to allow detailed examination of structural topology and intracrystalline equilibria and to provide some insight into exsolution phenomena, chemical bonding, and phase transitions. This paper summarizes the results of recent research on olivines, humites, garnets, aluminosilicates, pyroxenes, pyroxenoids, amphiboles, micas, silica minerals, feldspars, feldspathoids, and scapolites.

Vertical crustal movements from leveling data and their relation to geologic structure in the eastern United States

Abstract: Through analysis of leveling data, rates of relative vertical motion have been determined for points of a grid of profiles over much of the eastern United States. The rates are commonly too large to be attributed to leveling errors and therefore must reflect true earth movements. With some exceptions, modern movements appear to be related to earlier Phanerozoic trends, but the rates of modern movements are much larger than average rates over the past 130 m.y. Thus movements are either episodic or oscillatory about a long-term trend. If oscillatory, the periods must be less than about 106 yr and may be much smaller. The spatial patterns of the rates of relative motion, or tilts, can be correlated with certain geological features. The Atlantic and Gulf coastal plains are tilting downward away from the continental interior. Along a profile from Savannah, Georgia, to Philadelphia, Pennsylvania, the Atlantic Coastal Plain is also tilting downward to the north, with a perturbation at the Cape Fear arch. The Appalachian Highlands are rising relative to the Atlantic Coast at rates of up to 6 mm/yr. The pattern of maxima and minima in relative velocities along the profiles crossing the Appalachian Highlands province suggests elongated zones of relative uplift and subsidence paralleling either the Appalachian drainage divide or the trend of Appalachian structure. The maxima correlate strongly with topographic highs. There is a suggestion of correlation of modern seismicity with extremes of relative velocities in the Appalachian Highlands province. The Interior Plains are tilting downward to the east at a rate of 2 × 10−8 rad/yr, the implication being that glacial rebound is not an important factor south of the Great Lakes at the present time. The wavelength between successive zones of relative uplift in the Appalachian Highlands is about 300 km, suggesting that at least the entire thickness of the lithosphere is involved, and probably the asthenosphere as well. Possible explanations for these phenomena include the effects of hydrologic variations, phase transitions in the lower crust and/or the upper mantle, asthenospheric currents driven by ocean and ice loads or plate motions, and stresses derived from nonequilibrium crustal loads or crustal unloading by erosion.

The theory of whistler propagation

Abstract: Naturally occurring whistlers provide a powerful ground-based technique for probing the magnetosphere. The major features of the theory of whistler propagation in ducts in the plasmasphere are well understood. The details of how the whistler propagates upward from its source and excites the duct and how it escapes from the duct and reaches the ground are not so clear. Recent direction-finding experiments are designed to find the point at which the signal emerges from the lower ionosphere; from this the motion in longitude of the duct in which the whistler has propagated is inferred. It is thus important to understand the propagation of the signal between the duct and the lower ionosphere in order to interpret direction-finding experiments. Another problem of current interest is the amplification of whistlers. In this paper, existing theoretical understanding of the propagation of whistlers is reviewed. The region between the lower ionosphere and the ducts and the mechanisms of excitation of and leakage from the ducts are discussed. An attempt is made to identify those problems which require solution if the whistler technique is to be used to its full effect.

Global air pollution and climatic change

Abstract: This report reviews the findings on pollutant emissions, removal processes, and pollutant concentrations, as well as some aspects of climatic change. It is shown that emissions from agricultural burning in the tropics exceed significantly U.S. annual emission rates. The direct anthropogenic global particle production amounts to about 7% of that naturally produced. More than 70% of the man-made particles are in the form of gaseous precursors. The anthropogenic contribution to the total global particle production is about 15%. Natural emissions of particulates and CO, CO2, CH4, H2S, NO2, and NH3 exceed man-made emissions by orders of magnitude. Only SO2 is produced predominantly by human activities. Emission estimates, especially those of CO, CO2, CH4, NH3, and N2O, differ greatly among the different investigators. The major removal processes of particulate and gaseous pollutants in the troposphere and stratosphere are reviewed. Trends and current levels of particulate and gaseous pollutants for urban and background stations are discussed. The tropospheric and stratospheric global monitoring programs are outlined. The discussion of changes in the upper atmosphere centers around stratospheric aerosols CO2, CO, CH4, H2O, and O3. The relative effects of nuclear tests, SST flights, and propellants and refrigerants are discussed. Recent hypotheses of ozone destruction by chlorofluoromethanes and bromines are presented. Next evidence of climatic change is given. Natural and man-made external causes of climatic change, including fluctuations in solar emission, orbital changes, changes in CO2, dust, and land use, and internal causes of climatic change, which include Antarctic ice surges, decreases in ocean salinity, and almost intransitivity, are discussed. All these factors are interrelated via complicated feedback mechanisms. Hypotheses concerning the physical causes, the time scales, and the initial stages of ice ages are reviewed. It appears that major ice ages seem to occur every 100,000 yr and that after an interglacial interlude we are on the brink of a period of colder climate. It has been estimated that the mean temperature of the planetary atmosphere in its surface layers has decreased by about 0.3°C since the 1940's despite an 11% increase of CO2 above the nineteenth century preindustrial level of 290 ppm. It appears that the natural climatic cooling trend is roughly 3 times more powerful than the present influence of CO2. However, in the near future, far-reaching adverse climatic and ecological consequences can be expected because the CO2 increase is too rapid for the regulatory mechanisms of the oceans. The impact of an increasing aerosol loading cannot be assessed reliably yet. The net effect will probably be small or one of warming. Presently, the heat release of the order of 15–20 TW from global energy production is still relatively small. But with the continuation of the present energy growth rate, within one generation, waste heat production may reach 100–300 TW, an amount found sufficient in natural processes to cause climatic changes. Some problems related to land use changes are also discussed, especially overgrazing, which may lead to desertification, and tropical deforestation, which may alter the atmospheric circulation.

Rare gas fractionation patterns in terrestrial samples and the Earth-atmosphere evolution model

Abstract: Rare gas abundances in various terrestrial materials can be classified into three distinct fractionation patterns: type 1 shows a progressive enrichment in the heavier gases (shales, natural gases, holocrystalline submarine basalts, seawater and groundwater), type 2 shows a large enrichment of Ne and a slight apparent enrichment of Xe (glassy margins of submarine basalts, a subaerial basalt, an olivine xenolith from Hawaii), and type 3 shows large enrichments of Ne, Kr, and Xe relative to Ar (thucolite and a shale sample). The type 1 pattern is formed by the low-temperature adsorption of rare gases usually via the rare gases dissolved in seawater. The type 2 pattern is interpreted as a combination of high-temperature diffusion, solubility, and bulk melting in the mantle. The type 3 pattern probably represents a hybrid pattern. These patterns are consistent with an earth-atmosphere evolution model which assumes (1) that the present rare gas abundances in the atmosphere closely approximate the rare gas inventory of the whole earth except for xenon and (2) that the average rare gas abundance pattern in the solid earth is roughly similar to that in the atmosphere except for Xe.

The current status of speculations on the composition of the core of the Earth

Abstract: The earth's core, at a pressure of between 1.3 and 3.7 Mbar and a temperature between 3000° and 5000°C, consists primarily of Fe-Ni in the inner core and molten Fe alloyed with from 8 to 20% of a light element, most likely S or Si, giving an average atomic number for the elements in the outer core of about 23. If the core accreted before the mantle, then the apparent disequilibrium of some elements with respect to core and mantle is explained, but it is difficult to explain the presence of appreciable quantities of a light element that are required in the core. Alternatively, the core formed by melting, coalescence, and subsequent collapse of metallic particles in a protoearth that was relatively homogeneous initially. Such collapse would raise the temperature of the earth as much as 2300°K. Silicon must be present in the core if (Fe + Mg)/Si> 1 in the mantle and if that ratio for the bulk earth lies within the limits set by the sun and chondrites. If Si is present, the apparent chemical disequilibrium between metallic Si in the core and Fe2+ and Fe3+ in the mantle must be explained. Arguments for S in the core based on the abundance of S in rocks of crustal and mantle origin appear to have little foundation; however, there are strong geochemical and cosmochemical arguments justifying the presence of S in the core if the earth accreted at relatively low temperatures. The intriguing speculation that the bulk of the earth's potassium is present in the outer core, thus providing energy for core and mantle, remains to be tested by more partitioning experiments. A number of explanations exist to explain the apparent disequilibrium of some elements between core and mantle; however, none is satisfactory. The core formed soon after the earth accreted; in fact, the age generally taken as the age of the earth probably represents the time of core formation.

Aeronomic effects of the South Atlantic Anomaly

Abstract: Experimental and theoretical values for precipitated electron and proton fluxes in the area of the South Atlantic geomagnetic anomaly are reviewed. Observations of airglow which have been reported in the literature suggest that there is no enhancement in the Brazilian region, though there may be some in the southeastern part of the Atlantic area. There is evidence from various types of observations that the electron density in the ionosphere is greater over the South Atlantic than at comparable places elsewhere, especially during magnetic disturbances, and similarly the ion density shows enhanced values. Electron and neutral temperatures may also be higher than normal in the anomalous region. Observations are required in all these fields, coordinated with particle flux measurements from rockets and satellites in the area, to confirm the reality of the effects and their relationship to particle precipitation.

Numerical models of the ocean circulation

Abstract: Numerical models of the large-scale circulation of the oceans have developed into a useful tool for the interpretation of oceanographic data and the planning of new observational programs. Idealized numerical models with simplified geometry and physics have extended the analytic theory of the wind-driven ocean circulation into the range in which inertial effects determine the solution. Recent numerical work has shown how stratification and baroclinic instability further modify a wind-driven ocean circulation. Other results obtained by simplified numerical models include important predictions about the spectral properties of geostrophic turbulence in the ocean. Another class of numerical models has been developed which attempts to model the geometry and physics of the ocean circulation in a more detailed way, allowing a quantitative comparison with observations. Interesting results have been obtained for the Indian Ocean which simulate the seasonal variations of the Somali Current. Other Soviet and U.S. model studies using the observed density field as input show that pressure torques acting on bottom topography can be as large as the torques exerted by the wind acting at the surface. As yet, detailed simulations of the ocean circulation in a major ocean basin which include the effect of mesoscale eddies have not been undertaken.

An evaluation of recent quantitative magnetospheric magnetic field models.

Abstract: Magnetospheric field models involving dipole tilt effects are discussed, with particular reference to defined magnetopause models and boundary surface models. The models are compared with observations and with each other whenever possible. It is shown that models containing only contributions from magnetopause and tail current systems are capable of reproducing the observed quiet time field just in a qualitative way. The best quantitative agreement between models and observations take place when currents distributed in the inner magnetosphere are added to the magnetopause and tail current systems. One region in which all the models fall short is the region around the polar cusp. Obtaining physically reasonable gradients should have high priority in the development of future models.

The sensitivity and dynamic range of inertial seismographs

Abstract: The literature on earth noise over the spectrum of periods from 0.1 s to 12 h is reviewed and presented as accelerations which, when expressed as spectral-noise density, allow all seismographs to be considered on a common basis of sensitivity. Their ultimate sensitivity is limited by thermal agitation of their suspended masses and is calculable by considering the energy flow to and from their lossy elements in accordance with principles elucidated by Nyquist. Practical instruments, including those with feedback loops, are considered as examples. Seismometers and accelerometers are treated here as resonant systems having one degree of freedom. The electrical outputs of their transducers are classified as ‘velocity’ or ‘displacement.’ The dynamic range required to record these outputs depends on the range of accelerations imposed on the instrument and upon the combined filtering effects of the resonant system and transducer. Gravitational variation, tilt, and tidal accelerations are considered as they affect the instrumental problem.

Parameterization of the planetary boundary layer in atmospheric general circulation models

Abstract: The interaction between the atmosphere and the underlying surface occurs through the planetary boundary layer. The basic information required for describing the large-scale processes in terms of the boundary layer processes relates to the turbulent fluxes of momentum, heat, and moisture, the height of the boundary layer, and surface parameters such as soil moisture content and topography. The numerical models of the atmospheric general circulation incorporate the (statistical) effects of the boundary layer processes in terms of the large-scale parameters computed by the model through parameterization techniques. This paper reviews various parameterization techniques used in current general circulation models and suggests the need for a sensitivity test to find the best one.

Mare basalt petrogenesis—A review of experimental studies

Abstract: Experimental results relevant to the fundamental question of the origin of mare basalts are examined with particular reference to guidelines for an appropriate evaluation of experiments. The petrogenesis of mare basalts remains a controversial subject as no petrogenetic scenario has yet been able to satisfy all the geochemical and geophysical constraints. Several generalizations hold true if one accepts that high-pressure equilibria provide some useful but limited information on mare source regions in the lunar interior. Petrogenesis of lowand high-Ti suites is identified. If assimilative processes are involved in the petrogenesis of the high-Ti suite, the high-pressure experiments on the resultant hybrid liquids have little bearing on their origins.

Interaction of solar wind with Mars as seen by charged particle traps on Mars 2, 3, and 5 satellites

Abstract: The near-Martian plasma measurements carried out by means of charged particle traps aboard the Mars 2, 3, and 5 satellites are reviewed. The planetary bow shock was observed; in a number of satellite passes near the planet the boundary of the obstacle that causes the bow shock was crossed. Arguments supporting the existence of an intrinsic magnetic field of Mars and of the possible existence of a quasi-isotropic plasma zone in the Martian magnetic tail are given. The behavior of the plasma and magnetic field at the obstacle boundary gives evidence favoring a magnetic nature for the obstacle (i.e., favoring the existence of a Martian magnetosphere).

Atmospheric Chemical Kinetics Data Survey

Abstract: Chemical kinetic data for more than 250 gas phase reactions of possible importance in the atmosphere have been reviewed. The results of available evaluations of the rate data are tabulated with selected recent experimental values. A preferred value for each reaction rate constant is indicated. A brief discussion of some of the experimental techniques that have been utilized is also given. This discussion emphasizes the capabilities and limitations of the individual techniques. The tables should be of use to those interested in modeling the complex chemical processes of the stratosphere, troposphere, or the urban atmosphere.

On the problem of simulating the Earth's induction effects in modeling polar magnetic substorms

Abstract: A major problem encountered in trying to model the current system associated with a polar magnetic substorm from ground-based magnetic observations is the difficulty of adequately evaluating the earth's induction effects Two methods for simulating these effects are reviewed here Method 1 simply reduces the earth to a perfect conductor and leads to very simple field equations Method 2 considers the earth as a ‘horizontally’ layered body of finite conductivity but requires a large amount of computational time The performances of both methods are compared when the substorm current system can be approximated by an infinitely long electrojet flowing over a flat earth In this case it appears that for most substorm modeling problems it is sufficient to treat the earth as a perfect conductor The depth of this perfect conductor below the earth's surface should be selected in function of the source frequency content

Saltation and Martian sandstorms

Abstract: A brief topical review of current knowledge available for predicting the characteristics of sandstorms is presented. Particular emphasis is placed on extrapolations to Mars where saltation of loose surface material appears to be a necessary condition for initiation of observed dust storms. Considerable data are now available to predict threshold winds under earth's environmental conditions, but techniques for predicting the quantity of material transported and the height and velocity to which it travels are still crude. Extrapolation of current knowledge to Mars is probably sufficient only for estimating purposes and landing craft design. The upcoming Viking landing mission on Mars will, it is hoped, shed additional light on the problem.

A comparison of one-dimensional theoretical models of stratospheric minor constituents

Abstract: The density profiles of stratospheric minor constituents are compiled from several one-dimensional theoretical models that have appeared in the literature. The models are compared with each other and also with observations if data are available. The similarity and disparity of the models are clarified and discussed. If a large difference exists among the models, attempts are made to interpret it as much as possible in terms of the physical and chemical data employed in each model. The inadequacy of one-dimensional models in calculating the realistic ozone density distributions and the problems related to it are briefly discussed.

Investigation of Pc 3 frequency geomagnetic pulsations in conjugate areas around L = 4: A review of some USSR and U.S. results

Abstract: Two extensive series of measurements of magnetospheric ULF waves in the Pc 3 frequency range (22–100 mHz) have been carried out in pairs of conjugate areas (Kerguelen-Sogra and Siple-Lac Rebours) which have approximately the same geomagnetic latitudes (L ∼ 35–40) but are separated by more than 8 hours in local time The results of these measurements are compared in this review, and some differences in the results are discussed in terms of possibly differing ionospheric and ground conditions in the conjugate areas Additional, possibly cooperative, studies to investigate further the magnetospheric wave phenomena in this frequency range are outlined