Showing papers in "Reviews of Geophysics in 1982"

Development of a turbulence closure model for geophysical fluid problems

Abstract: Applications of second-moment turbulent closure hypotheses to geophysical fluid problems have developed rapidly since 1973, when genuine predictive skill in coping with the effects of stratification was demonstrated. The purpose here is to synthesize and organize material that has appeared in a number of articles and add new useful material so that a complete (and improved) description of a turbulence model from conception to application is condensed in a single article. It is hoped that this will be a useful reference to users of the model for application to either atmospheric or oceanic boundary layers.

Generalized Nonlinear Inverse Problems Solved Using the Least Squares Criterion

Abstract: We attempt to give a general definition of the nonlinear least squares inverse problem. First, we examine the discrete problem (finite number of data and unknowns), setting the problem in its fully nonlinear form. Second, we examine the general case where some data and/or unknowns may be functions of a continuous variable and where the form of the theoretical relationship between data and unknowns may be general (in particular, nonlinear integrodierentia l equations). As particular cases of our nonlinear algorithm we find linear solutions well known in geophysics, like Jackson’s (1979) solution for discrete problems or Backus and Gilbert’s (1970) a solution for continuous problems.

Optical Properties of Snow

Abstract: Measurements of the dependence of snow albedo on wavelength, zenith angle, grain size, impurity content, and cloud cover can be interpreted in terms of single-scattering and multiple-scattering radiative transfer theory. Ice is very weakly absorptive in the visible (minimum absorption at λ = 0.46 µm) but has strong absorption bands in the near infrared (near IR). Snow albedo is therefore much lower in the near IR. The near-IR solar irradiance thus plays an important role in snowmelt and in the energy balance at a snow surface. The near-IR albedo is very sensitive to snow grain size and moderately sensitive to solar zenith angle. The visible albedo (for pure snow) is not sensitive to these parameters but is instead affected by snowpack thickness and parts-per-million amounts (or less) of impurities. Grain size normally increases as the snow ages, causing a reduction in albedo. If the grain size increases as a function of depth, the albedo may suffer more reduction in the visible or in the near IR, depending on the rate of grain size increase. The presence of liquid water has little effect per se on snow optical properties in the solar spectrum, in contrast to its enormous effect on microwave emissivity. Snow albedo is increased at all wavelengths as the solar zenith angle increases but is most sensitive around λ =1 µm. Many apparently conflicting measurements of the zenith angle dependence of albedo are difficult to interpret because of modeling error, instrument error, and inadequate documentation of grain size, surface roughness, and incident radiation spectrum. Cloud cover affects snow albedo both by converting direct radiation into diffuse radiation and also by altering the spectral distribution of the radiation. Cloud cover normally causes an increase in spectrally integrated snow albedo. Some measurements of spectral flux extinction in snow are difficult to reconcile with the spectral albedo measurements. The bidirectional reflectance distribution function which apportions the reflected solar radiation among the various reflection angles must be known in order to interpret individual satellite measurements. It has been measured at the snow surface and at the top of the atmosphere, but its dependence on wavelength, snow grain size, and surface roughness is still unknown. Thermal infrared emissivity of snow is close to 100% but is a few percent lower at large viewing angles than for overhead viewing. It is very insensitive to grain size, impurities, snow depth, liquid water content, or density. Solar reflectance and microwave emissivity are both sensitive to various of these snowpack parameters. However, none of these parameters can be uniquely determined by satellite measurements at a single wavelength; a multichannel method is thus necessary if they are to be determined by remote sensing.

Atmospheric gravity waves generated in the high‐latitude ionosphere: A review

Abstract: A review of theoretical and observational results describing atmospheric gravity wave (AGW)/traveling ionospheric disturbance (TID) phenomena at high latitudes is presented. Some recent experimental studies of AGW's using the Chatanika incoherent scatter radar and other geophysical sensors are reported. Specifically, the following features are described in detail: (1) cause/effect relations between aurorally generated AGW's and TID's detected at mid-latitudes, including probable ‘source signature’ identification, (2) AGW source phenomenology, particularly a semiquantitative assessment of the relative importance of Joule heating, Lorentz forces, intense particle precipitation, and other mechanisms in generating AGW's, and (3) detection of TID's in the auroral ionosphere. Several instances of F region electron density, temperature, and plasma periodicities accompanied by horizontal plasma velocities which were consistent with theoretical AGW/TID models are documented.

An overview of seasonal snow metamorphism

Abstract: The grains in seasonal snow undergo rapid and radical transformations in size, shape, and cohesion. These grain characteristics affect all of the basic properties of snow. Snow is characterized as either wet or dry depending on the presence of liquid water. Wet snow is markedly different at low and high liquid contents. Dry snow is characterized as either an equilibrium form or a kinetic growth form; that is, it is either well rounded or faceted. Of course, many snow grains display either transitional features between two of these categories or features which arise from other processes. Snow is classified depending on the dominant processes of its metamorphism.

Transport coefficients of low‐energy cosmic rays in interplanetary space

Abstract: The propagation of energetic particles along and across the interplanetary magnetic field is governed by the large-scale field geometry and by scattering in small-scale turbulent fields. Values of the scattering mean free path parallel to the field, λ∥(R), are reviewed in prompt solar bursts and nonimpulsive (corotating) events. Analysis of intensity and anisotropy profiles in combination is a powerful tool for elucidating λ∥(R). A consensus is found: at 1 AU, λ∥ = 0.08–0.3 AU over a wide range of rigidity, R = 5 × 10−4 to 5 GV. Efforts to explain the discrepancy between empirical values of λ∥ and scattering theory are discussed. Quantitative measures of λ∥ in rare scatter-free events, where λ∥ ≳ 1 AU, are discussed because they can provide important details of the scattering process and the magnetic power spectra. Cross-field diffusion due to random walk of field lines is revisited. Recent values deduced from magnetic power spectra in interplanetary space, magnetic diffusion at the sun, Jovian electron propagation, and cosmic ray events are evaluated. Again, a consensus is sought, and a reasonable mean is K⊥r/β = 1021 cm² s−1. Previous arguments against a significant K⊥r are reassessed, including the problem of the persistence of intensity fluctuations in cosmic ray events. Combining the consensus for K⊥r/β with that for λ∥ gives K⊥r/K∥ < 0.1 at 1 AU, and thus neglect of K⊥r in the modeling of solar cosmic ray events appears justified (although account needs to be taken of coronal propagation). The outlook for the future includes better empirical values of λ∥ down to Ep ∼ 10 keV and Ee ∼ 1 keV, comparison with scattering theories at these energies, and comparison between empirical and theoretical λ∥ in other regions such as the magnetosheath and upstream solar wind.

origin and emplacement of ophiolites

Abstract: Ever since their recognition, ophiolites have been a source of controversy, although all workers have agreed upon their importance. A number of conflicting schools of thought have become unified in the interpretation of ophiolite complexes as fragments of oceanic crust and mantle formed at spreading centers. An expanded version of the Penrose ophiolite definition is proposed, herein called the ophiolite association, taking into account rocks which bear upon the origin and significance of ophiolite complexes. An ideal ophiolite association includes the following units from bottom to top: (1) a crystalline basement and shallow water sedimentary sequence, (2) a tectonic unit of thrust slices of continental margin and rise and abyssal sediments and/or melange, (3) a metamorphic unit, as much as a few hundred meters thick, generally with higher-grade rocks over lower-grade ones, (4) ultramafic tectonite unit composed dominantly of multiply deformed peridotite, dunite, and minor chromitite, (5) a cumulate complex, ultramafic at the base grading to mafic or intermediate at the top, (6) a noncumulus unit of varitextured gabbro and minor trondhjemite (7) a sheeted dike complex, (8) an extrusive section of massive and pillowed flows and intercalated sediments, (9) an abyssal or bathyal sediment sequence which may include radiolarian chert, red pelagic limestone, metalliferous sediments, breccias, and/or pyroclastic deposits, and (10) postemplacement deposits of laterite, reef limestone, or shallow marine or subaerial sediments. This expanded ophiolite association allows greater precision in inferring original origin and emplacement. Some ophiolite complexes, particularly in the Tethys (herein designated Tethyan type), exhibit units 1, 2, 3, and 4, whereas others, particularly in the Cordilleran belt (herein termed Cordilleran type), lack units 1, 2, 3, and, in places, 4. The emplacement of the former probably occurred during collision of a passive margin with a subduction zone or incipient subduction zone. Emplacement of the Cordilleran type generally is not clear-cut. The nature of the spreading center which produced a given ophiolite is indicated more clearly by the presence or absence of units 5–9 than by chemistry or mineralogy. Some ophiolite complexes lack complete sections and may have formed along transform faults. Others contain volcanic pyroclastic deposits instead of pelagic sedimentary sequence and probably formed within or near an arc. Several ophiolite complexes may have originated by conversion of a transform fault into a subduction zone. Such conversion may result from normal plate evolution or from major plate reorganization. Major times of ophiolite emplacements appear to correspond with global plate reorganizations.

Thermodynamics and lattice vibrations of minerals: 5. Applications to phase equilibria, isotopic fractionation, and high‐pressure thermodynamic properties

Abstract: In previous papers in this series, a model that uses available elastic, structural, and spectroscopic data on minerals has been used to predict the thermodynamic functions Cv (heat capacity), S (entropy), E (internal energy), and F (Helmholtz free energy). In this paper, four applications to problems of current geochemical and geophysical interest are presented: (1) interpretation of complex trends of calorimetric data; (2) calculation of phase equilibria; (3) calculation of oxygen-isotopic fractionation factors; and (4) estimation of the effect of pressure on thermodynamic functions. The model demonstrates that trends in high-temperature thermodynamic properties of silicates are determined by the position and relative numbers of high-frequency modes, generally antisymmetric (Si, Al)-O stretching modes. The position of these modes varies systematically with degree of polymerization of tetrahedra, and therefore high-temperature calorimetric behavior is relatively systematic as a function of crystal structure and mineral composition. Trends at low frequency are much more complex because the low-frequency optic modes that most strongly influence the low-temperature thermodynamic functions depend in a complex way on the size, coordination, and mass of cations and various polyhedra in the minerals. The heat capacity curves of kyanite, andalusite, and sillimanite and of quartz, coesite, and stishovite show crossovers that cannot be explained by Debye theory, which accounts only for acoustic mode behavior, but can be explained by the model spectra proposed because proper account is taken of the changing low- and high-frequency optic modes upon polymorphic transformations. The proposed model is sufficiently accurate that phase equilibrium problems can be addressed: the quartz-coesite-stishovite equilibrium curves, the kyanite-andalusite-sillimanite triple point, and the breakdown of albite to jadeite-plus-quartz are cited as specific examples. For each example, predicted slopes of equilibrium curves agree moderately well to excellently with slopes determined experimentally. The calculated slopes are sensitive to spectroscopic parameters, particularly to the distribution of optic modes in the far infrared; this sensitivity is discussed in detail for the albite breakdown reaction. The model can be used for prediction of isotopic fractionation factors if spectra of the isotopic forms of the mineral are known or postulated. A simple set of ‘rules’ for generating hypothetical spectra of 18O minerals from measured spectra of the 16O forms is given. Reduced partition functions are calculated for 13 minerals. At 298°K the model values of reduced partition function, 1000 ln α, of these minerals decrease in the order quartz > calcite ≳ albite > muscovite > clinoenstatite ≈ anorthite > diopside > pyrope > grossular > zircon > forsterite > andradite > rutile, in good agreement with experimental data. At 1000°K the first six minerals show small crossovers so that the order becomes calcite, muscovite ≈ albite, quartz, anorthite, and clinoenstatite; the differences in 1000 ln α at high temperature for these minerals are so small that the model probably cannot address the deviations from experimental trends. The model clearly defines the region in which the fractionation factors do not follow a 1/T² trend and should be useful for extrapolation of experimental data to low temperatures. Finally, a modified Gruneisen parameter model is proposed for shift of the lattice vibrational frequencies under compression, and thermodynamic properties to 1000 kbar, 1000°K, are given for nine minerals. At 1 Mbar, the predicted decrease in entropy at 298°K ranges from 54% (of the 1-bar value) for periclase to 25% for stishovite.

Stratospheric aerosols: Observation and theory

Abstract: Important chemical and physical roles of aerosols are discussed, and properties of stratospheric aerosols as revealed by experimental data are described. In situ measurements obtained by mechanical collection and scattered-light detection yield the overall size distribution of the aerosols, and analyses of preserved aerosol precursor gases by wet chemical, cryogenic and spectroscopic techniques indicate the photochemical sources of particle mass. Aerosol chemical reactions including those of gaseous precursors, those in aqueous solution, and those on particle surfaces are discussed, in addition to aerosol microphysical processes such as nucleation, condensation/evaporation, coagulation and sedimentation. Models of aerosols incorporating such chemical and physical processes are presented, and simulations are shown to agree with measurements. Estimates are presented for the potential aerosol changes due to emission of particles and gases by aerospace operations and industrial consumption of fossil fuels, and it is demonstrated that although the climatic effects of existing levels of stratospheric aerosol pollution are negligible, potential increases in those levels might pose a future threat.

The evolution of solar ultraviolet luminosity

Abstract: Astronomical observations of stars analogous to the sun are used to construct a tentative account of the evolution of solar UV luminosity. Evidence exists that the young sun was a much more powerful source of energetic particles and radiation than it is today, and while on the main sequence, solar activity has declined as an inverse power law of age as a consequence of angular momentum loss to the solar wind. Observations of pre-main sequence stars indicate that before the sun reached the main sequence, it may have emitted as much as ten thousand times the amount of ultraviolet radiation that it does today. The impact of the results on knowledge of photochemistry and escape of constituents of primordial planetary atmospheres is discussed.

Magnetic properties of marine limestones

Abstract: Numerous paleomagnetic investigations have been carried out on limestones, but their rock magnetic properties have often been neglected. In this review, geological and rock magnetic factors which determine whether marine limestones are suitable for paleomagnetic study are summarized, and laboratory techniques for the identification of the magnetic mineralogy are evaluated. The conditions under which pelagic limestones form are especially favorable for the acquisition of a stable primary natural remanence (NRM), probably by a postdepositional alignment process. The effectiveness and timing of acquisition of this remanence are influenced by the thoroughness of bioturbation. Other remanence components can be acquired during diagenesis, in the soft sediment right after deposition or much later even in the indurated limestone. The magnetic mineralogy of a limestone is difficult to describe optically because of the small grain size and low concentration; magnetic techniques are more convenient. Direct analysis of extracted magnetic minerals is handicapped by the difficulty of obtaining a representative extract. A particularly useful technique involves combined observations of isothermal remanent magnetization (IRM) acquisition in strong fields up to at least 4 T and the subsequent destruction of this IRM during stepwise or continuous thermal demagnetization. The magnetic minerals most commonly identified by these techniques are magnetite, goethite, hematite, and maghemite. These magnetic minerals occur in different combinations in the marine limestones of central Europe and the Mediterranean realm. Pyrite is common in certain limestones, where it can be the precursor of an unwelcome late goethite. Pyrrhotite may be responsible for unstable magnetization in the Swiss Helvetic limestones but otherwise is rare. Thermal demagnetization of some limestones causes changes of susceptibility and coercivity at quite low temperatures (300°–400°C) due to the breakdown of a metastable mineral such as maghemite, goethite, or pyrrhotite. At high temperatures (above 500°C), pronounced changes of magnetic mineralogy occur in all limestones because of the growth of new magnetite. This necessitates great care in interpretation of thermal demagnetization of high blocking temperature components and often results in observational difficulties due to viscous remanent (VRM) behavior of the newly formed magnetite. VRM is a widespread component of NRM in many limestones, but it can usually be removed effectively by standard magnetic cleaning techniques. The susceptibility serves as a simple control of mineralogical change during heating and can serve as an indicator of lithologic variation of magnetic mineral concentration and type in magnetostratigraphic profiles. The susceptibility is anisotropic and shows a classical ‘depositional’ pattern of the distribution of principal axes. However, the probable postdepositional mechanism of NRM acquisition (especially in bioturbated sediments) implies that the anisotropy pattern results from postdepositional compaction of the sediment.

The lunar regolith: Chemistry, mineralogy, and petrology

Abstract: The data base on the lunar regolith is surveyed to form a synthesis of the lunar regolith chemistry, mineralogy, and petrology. The data were derived from samples collected by the Apollo missions 11-17 and the Luna 16, 20, and 24 probes. The missions were sent to sample formations and areas which typified the common observed features of the lunar surface. Drive tubes were used to extract samples from beneath the surface in order to study the relationship between the regolith and the bedrock, as well as to identify the processes that formed the regolith, which is regarded as the prime source of raw materials for early lunar industrial activities. Regolith origins are now understood to be destructive processes of comminution and constructional processes of agglutinate formation. Mixing occurs on the local scale, although lateral transport is inefficient on the moon. The usual contents of the fraction of regolith less than 10 microns in diameter are Al2O3, CaO, Na2O, K2O, light REE, and Th.

The role of metastable species in the thermosphere

Abstract: Metastable excited states of various ions and neutrals in the thermosphere provide reservoirs for the temporary storage of a large portion of solar EUV energy, and permit the conversion of this photon energy by kinetic or vibrational heating, ion formation metastable species formation and nonlocal energy deposition. The present paper reviews current understandings of the chemistry of O(1D), O(1S), O(+)(2D), O(+)(2P), N(2D), N(2P), N(+)(1S), N(+)(1D), NO(+)(a), O2(+)(a), N2(A) and the vibrationally excited states of O2, N2, O2(+) and N2(+). The role of these species in the overall thermospheric energy budget and ionization balance is also quantified. It is noted that of the species considered, O(1D), O(1S), O(+)(1S), O(+)(2D), N(2D), N2(A) and the vibrationally excited states of the oxygen and nitrogen molecules are most significant, with a major fraction of the kinetic heating of the thermosphere taking place through O(1D).

Coupling function between solar wind parameters and geomagnetic indices

Abstract: Results of past studies on the correlation of a function of solar wind parameters with a geomagnetic index are reviewed and examined to identify the most relevant coupling function among those hitherto proposed. For the AL index, three functions, BsV, BsV², and Akasofu's e, are compared, where Bs is the southward component of the interplanetary magnetic field and V is the solar wind velocity, and the importance of BsV² is then confirmed by a new presentation of the prediction of each function and the AL index. For the Dst index, validities of the above three functions are also compared, but a new function, BsVP1/3, where P is the dynamic pressure of the solar wind, is proposed as the one that surpasses all of the other three functions in predicting Dst. Implications of the two coupling functions are discussed, and possible directions for future work on this problem are suggested.

Properties of blowing snow

Abstract: The size and shape of windblown snow particles determine not only the mass transported by turbulent fluxes but also the rate of phase change from ice to water vapor that occurs in this multiphase flow. These properties and particle densities dictate particle fall velocity and therefore the vertical distribution of mass and surface area, which strongly influence the gradients and fluxes of sensible heat and water vapor within the transport layer. Initial movement at the snow surface depends more on availability and impact forces of loose particles than on aerodynamic drag. Cohesion between surface particles and particle restitution coefficient are important properties that determine threshold wind speeds for snow transport. Threshold speeds for blowing snow vary over such a large range in nature that formulations predicting transport rate as a function of wind speed should include threshold speed as a parameter. The expression derived by Iversen et al. (1975) is compared with low-level snow transport in the atmospheric boundary layer. Self-similarity of wind profiles in blowing snow is a property of the flow that has been exploited for scale modeling of snow deposition around obstacles, both outdoors and in wind tunnels. Good quantitative results are obtained by careful attention to similitude requirements.

Secular effects of oceanic tidal dissipation on the Moon's orbit and the Earth's rotation

Abstract: The earth and moon are considered as a two-body system in gravitational isolation from the sun and other planets. The lunar orbit is taken as circular, and the solid earth is assumed to be a rigid sphere (with no tidal deformation) so that there are no precessional torques other than those arising from the tidally deformed ocean. Numerical solutions to Laplace's tidal equations, with dissipation by linear bottom friction, are used to obtain ocean-wide distributions of tidal amplitude for two idealized continentalities: a single circular continent (spherical cap) centered at the north pole, and a single spherical cap centered on the equator. Calculations are made for two different values of the frictional resistance coefficient, thus giving rise to four sets of solutions. The computed tidal amplitudes are used to calculate the oceanic tidal torque, which in turn is used to integrate the orbital equations backward in time for solution of the two-body problem. The Coriolis parameter and the tidal frequencies change with time, thus requiring that the tidal equations be solved several times during the course of each orbital integration. In this manner, the earth's rotational and the moon's orbital histories are determined on a geologic time scale for each of the four models. The calculated position of the lunar orbit at 4.5 billion years ago is found to range from 38 to 53 earth radii in the four models and corresponds to a sidereal month of 330 to 550 hours. The sidereal day would have been 12 to 18 hours, with a relative inclination of 3° to 22° between the terrestrial and lunar poles. These results are in sharp contrast to those from previous studies of the earth-moon system, most of which indicated a Roche limit approach of the two bodies roughly 1 to 2 billion years ago and presented therefore a time scale difficulty in theories of lunar origin. This contrast arises mainly from the fact that previous modelers avoided solution of Laplace's tidal equations by prescribing a constant frictional phase lag angle between the angular position of the moon and the major axis of the second-degree harmonic of the tidally deformed surface of the earth. The amount of phase lag was established from the present astronomically deduced rate of tidal dissipation, but this precluded dynamic variations in tidal torque over geologic time, which are critical for determination of the orbital time scale. The present rate of oceanic tidal dissipation is evidently anomalously high because of the near resonance of the oceanic response in both frequency and shape to the tidal forcing. For the earth and moon to have evolved to their present separation from a distance of less than about 35 earth radii solely on the basis of oceanic tidal dissipation would have required the M2 response to have been near resonance for a significant portion of geologic history, a rather implausible scenario. The calculations reported here show that, on the contrary, frictional coupling between the earth and moon was much weaker than at present throughout most of the orbital history, and consequently the moon's closest approach could not have been as small as 35 earth radii. Although idealized continentalities were used in these calculations, the general conclusion of a weaker frictional coupling in the past is believed to be relatively insensitive to the continental configuration. This is because at faster paleorotation, the semidiurnal tidal frequencies would be resonant only with smaller-scale normal modes of the ocean rather than with global scale modes as at present. The time scale difficulty heretofore present in models of orbital evolution is thereby eliminated. It should be noted that the orbital history developed here supports the theory of binary planetary accretion for lunar origin as opposed to fission or tidal capture.

Nonlinear force-free magnetic fields

Abstract: The nonlinear properties of force-free magnetic fields are reviewed with particular reference to the mechanisms for the sudden release of stored energy in flares during the quasi-steady evolution of solar fields. It is shown that in the solar atmosphere, force-free fields with a nonconstant scalar function in the field equations are more likely to occur than those with a constant scalar function, and the nonlinear properties of these fields may give rise to many interesting physical effects. Consideration is then given to two possible mechanisms of field evolution: a model in which a force-free field in a medium of infinite electrical conductivity evolves in response to slowly changing boundary conditions brought about by photospheric motions in the solar active region, and a model in which a field in a medium of small finite electrical conductivity evolves in response to the slow Ohmic dissipation of the electric current.

The ionosphere as a source for magnetospheric ions

Abstract: Ion composition measurements within the past several years have shown O(+), He(+), and other ions of terrestrial origin to compose a substantial fraction of the magnetospheric ion population. This review examines (1) observations of topside ionospheric composition, (2) mechanisms for energization and injection of ionospheric ions into the magnetosphere, and (3) observations of ions of ionospheric origin in various regions of the magnetosphere, including the plasmasphere, ring current, magnetotail plasma sheet and lobes, and boundary layer and magnetosheath.

Electromagnetic induction in the moon

Abstract: The moon constitutes a nonhydromagnetic, but electrically conducting, target for the solar wind whose response reaches a peak as frequency increases and diminishes with further increase in frequency, suggesting the presence of the magnetic quadrupole moment. Magnetometer measurements of induction using Explorer and Apollo instruments are studied from both the harmonic and transient standpoint, and the resulting determination of internal bulk electrical conductivity is discussed. The closeness of the estimated internal temperature to the Ringwood-Essene solidus at 150-250 km depths suggests a layer of enhanced conductivity in lieu of high temperature. A reduced core radius estimate with a one-sigma upper limit of 360 km is reported. The discussion of lunar electrodynamics presented is restricted to the problem of induction, with only passing reference to flow fields and regional electric fields.

Electrical resistivity of minerals and rocks at high temperature and pressure

Abstract: After a general discussion of semiconductor and dielectric physics, experimental data on the dependence of electrical resistivity of different minerals on their cation composition within a wide temperature interval (up to 1100°C) are described The different types of dependence of resistivity on pressure are noted for minerals with a predominantly ionic type of electrical conductivity and for those whose conductivity depends on changes in valence of iron cations Experimental data are presented that show the changes of electrical resistivity of minerals due to polymorphism, dehydration, and carbonate decomposition Next the results of studies of the resistivity of water-saturated sedimentary rocks at high pressures and temperatures are described Data are given on (1) the effect of porosity, pore configuration, mineral composition, concentration of electrolyte, content of clay, type of cement, and type of cementation on the relationship between resistivity and pressure, ρ = f(P) and (2) the resistivity of water-saturated magmatic rocks at high temperatures (up to 260°C) and pressures (up to 15 kbar) The dependence of resistivity of different groups of rocks on their mineral and chemical composition and on their structure under conditions of high temperature and pressure is analyzed Finally, basic trends are indicated of interest for geophysics and for the use of experimental data on electrical properties of minerals

Mechanical properties of snow

Abstract: : The investigation of the mechanical properties of seasonal snow cover aims mostly at applications in avalanche release and avalanche control but also at no less important problems such as vehicle mobility in snow, snow removal, or construction on snow. Primary needs are (1) constitutive equations, that is, relations between the stress tensor and the motion, and (2) fracture criteria which limit the region of validity of constitutive equations., Both can be tackled from the aspect of continiuum theories and structure theories. With modern continiuum theories the characteristic nonlinear behavior of snow can be taken into account and also the strong dependence on stress and strain history. When thermodynamics is introduced, more insight into the deformation and fracture processes can be gained. High initial deformation rates cause low dissipation, elastic behavior, and brittle fracture, whereas when dissipative mechanisms can develop, ductile fracture occurs. The advantage of structural theories lies in the immediate physical insight into deformation mechanisms, but the disadvantage is that only simple states of stresses acting macroscopically on a snow sample can be considered. Different approaches have been elaborated: for low-density snow the concept of chains (a series of stress-bearing grains) or the neck growth model (consideration of stress concentrations in bonds between grains) and for high-density snow the pore collapse model (snow idealized as a material containing air voids). Structural constitutive equations were applied to the calculation of stress waves in snow. Recorded acoustic emissions, indicating intergranular bond fractures, can also be used for the construction of constitutive equations.

Global nature of field‐aligned currents and their relation to auroral phenomena

Abstract: Low- and high-altitude spacecraft carrying scientific magnetometers have probed the magnetic fields of the terrestrial magnetosphere. These measurements reveal that field-aligned currents, possibly powered by dynamo action, flow over large regions of the earth's neighborhood. The purpose of this paper is to report on highlights of the U.S. contributions to the IMS effort in the area of understanding the global nature of the field-aligned currents and their relation to auroral and ionospheric phenomena. It has been established now that field-aligned currents flow on the same field lines connecting distant regions to low ionospheric altitudes. Similarly, currents flow on L shells extending over several hours of local time. However, it is becoming increasingly apparent that the sources of the currents in the various latitudes and longitudes are not the same. For this reason we discuss separately the following current-laden regions: (1) the noon sector, (2) the dawn and dusk sectors, (3) the midnight sector, and (4) the polar cap. Special attention is paid to polar cap ionospheric irregularities and the relation of auroral kilometric radiation to field-aligned currents and optical auroral emissions.

Low‐level VLF and LF radio emissions observed at Earth and Jupiter

Abstract: Major observational features of magnetospheric VLF-LF radio noise that have been uncovered by the IMP, Hawkeye and ISEE satellites, are examined with emphasis on the capabilities of the noise to diagnose local and remote plasma parameters. The relationship of the radiation to not less than 1-mV/m electrostatic upper hybrid emissions is assessed, and indirect observational evidence suggests that upper noise is associated with the generation of the VLF-LF radiation. Theoretical luminosities of mechanisms including synchrotron radiation, linear mode conversion of upper hybrid waves and nonlinear scattering of upper hybrid waves off plasma density irregularities are estimated, and the wave-wave scattering hypothesis is considered to be a viable mechanism if the presence of low-frequency waves as a scattering agent can be established.

A review of recent advances in tornado vortex dynamics

Abstract: Advances in understanding the dynamics of tornado vortices since the Lubbock symposium of 1976 are reviewed. The focus is on results obtained from experimental work with laboratory models and from numerical simulations of both real and laboratory events. Progress is shown to have been made in two areas: an increased appreciation for the possible complexities of tornado vortex structure and an improved understanding of the vorticity dynamics of columnar vortices. With respect to the former the evolution of the central core flow from a single-celled to a two-celled flow is described, and a mechanism capable of producing the multiple-vortex phenomenon is discussed. In the latter area the arrangements of vortex lines within the surface inflow layer and in the cores of one- and two-celled vortices are presented. It is seen that both the laboratory work and the numerical simulations indicate a key role in the dynamics of the vortex core played by the vortex breakdown phenomenon. Wherever possible, the theoretical findings are related to recent high-quality observations of actual tornadoes.

The cometary atmosphere and its interaction with the solar wind

Abstract: The present state of knowledge of cometary atmospheres and their interaction with the solar winds is assessed. Current models of the magnetospheres and ionospheres of comets are based on either an atmospheric chemistry approach, minimizing the effects of atmospheric dynamics and thermodynamics, while the other method takes the exact opposite viewpoint and roles and significance are reversed. A major difficulty in completing the models is a lack of data on the chemical composition of the cometary nucleus, although UV observations have revealed the H, C, O, and S elemental abundance ratios. It is suggested that further studies of collisional ionization by energetic electrons be performed to characterize processes in the cometary atmosphere. Hydronium may be the dominat molecule in the inner coma, if the assumption that the ionosphere is a magnetic field-free cavity separated from the solar wind by an unyielding tangential discontinuity surface is accurate. Particular observations and theoretical attention are recommended for the solar wind interaction in the intermediate range, i.e., 2.5-5 AU, when the cometary atmosphere develops.

The sign convention for quadrature Parkinson arrows in geomagnetic induction studies

Abstract: Time series analysis, which is basic to modem geophysical data processing, involves a choice between working with a time dependence of e +i'ot or e -i'ot. In published work the choice made is sometimes not explicitly stated, leaving ambiguity in the interpretation of complex quantities with quadrature parts. Parkinson arrows are used in geomagnetic induction studies to summarize anomalous vertical magnetic fluctuations at different observing stations and to indicate regions of high electrical conductivity. Such arrows are now regularly computed as real and quadrature pairs. The general convention is often adopted of 'reversing' a calculated real arrow so that it will point toward a conductivity increase, but for quadrature arrows the practice between various published papers has generally not been so consistent. The present paper demonstrates that consistent practice for reversing or not reversing quadrature Parkinson arrows is possible when the initial convention for time dependence is taken into account. A reversal practice is determined for interpretation in terms of a simple channeling model. A related matter is the definition of phase. Phase values are also generally ambiguous unless the time dependence used (e -iot or e +iot) is stated.

Optimization of geodetic networks

Abstract: It is the aim of this paper to give a concise review of the current state of research, the trends, and the outstanding problems in the optimization of geodetic networks. The datum problem, which has to be solved in the computational stage, is referred to as the zero-order design problem. In the real design stage, three classes of problems have arisen: the configuration problem as the first-order design problem, the weight problem as the second-order design problem, and the optimal improvement of an existing network as the third-order design problem. Furthermore, all subproblems must be analyzed within the three general objectives of geodetic networks: precision, reliability, and cost.

Three‐dimensional seismic inhomogeneities in the lithosphere and asthenosphere: Evidence for decoupling in the lithosphere and flow in the asthenosphere

Abstract: Newly discovered three-dimensional seismic velocity anomalies in the earth's lithosphere and asthenosphere are interpreted in terms of decoupling in the lithosphere and flow in the asthenosphere. A rough estimate of flow in the asthenosphere under California was made using Stokes' law on the basis of the size of anomalies and density contrast estimated from the velocity contrast. The result depends on the cause of velocity anomalies. If they are due to any effect which obeys Birch's law, the flow is as rapid as that of Morgan's (1972) plume driving the plate motion. On the other hand, if they are due to partial melts in thin cracks or at grain boundaries, the flow is negligible in accordance with Elsasser's (1971) model of plate tectonics driven quietly by the ridge-push and slab-pull in which the asthenosphere under the base of the plate is only passively dragged. The result of Cockerham and Ellsworth (1979, 1980) that the low-velocity zone in the mantle under central California appeared to have preserved the shape of the downgoing slab for up to 10 m. y. supports Elsasser's rather than Morgan's model. The shapes of low-velocity bodies found by Iyer and his colleagues under several geothermal areas in the western United States may be interpreted in terms of decoupling between the upper brittle lithosphere and the ductile substratum. The misalignment of the axis of the low-velocity column from the center of the granite outcrop in the Roosevelt Hot Springs area, Utah, and the inclined low-velocity column under the Geysers-Clear Lake region, California, indicate a decoupled lithosphere in these areas, whereas the upright column found under Yellowstone National Park suggests an intact lithosphere there.

ULF/lower‐ELF electromagnetic field measurements in the polar caps

Abstract: A review is given of the known properties of naturally occurring ULF/lower-ELF (0.022–100 Hz) electromagnetic pulsations in the polar caps. Because polar cap measurements by Western scientists in this particular frequency range have been surprisingly limited (even when allowance is made for the logistical difficulties), emphasis is given in the review (1) to a description of the gaps in our knowledge of the properties of the pulsations and (2) to the uniqueness of the geomagnetic field configuration in the polar caps and its implications. Given the gaps in knowledge and the unique field configuration, the opportunities for innovative research on ULF/lower-ELF electromagnetic pulsations in the polar caps appear to be particularly promising.