摘要:

Shock‐wave data are now available for a variety of rocks, minerals, and oxides of geophysical interest in the pressure range appropriate for the lower mantle. These data are analyzed to obtain equation‐of‐state parameters with emphasis on the shock‐induced high‐pressure phases. Of twenty‐four materials for which Hugoniot data are analyzed, all but MgO, Al2O3, and MnO2undergo at least one shock‐induced phase change below 800 kb. Birch‐Murnaghan parameters for the raw Hugoniots, metastable Hugoniots, adiabats, and 25°C isotherms are obtained for the high‐pressure phases. On correcting the raw Hugoniot data for MgO and Al2O3for strength effects, we find that the calculated adiabatic equations of state are in good agreement with recent ultrasonic data. The zero‐pressure densities of high‐pressure phases are obtained by constraining the adiabats calculated from the Hugoniot data such that the zero‐pressure densities and the zero‐pressure slopes of the adiabats satisfy Anderson's seismic equation of state.Probable crystallographic structures of the high‐pressure phases are inferred from the classical laws of crystal chemistry and, in some cases, from static high‐pressure recovery experiments on analog compounds. Shock data for SiO2(stishovite) indicate that transformation to the fluorite‐type structure (observed in TiO2) does not take place under shock, at least to ∼2000 kb. Fe2O3probably transforms to either the perovskite or B rare earth structure with a zero‐pressure density of 5.96 g/cm³. MgAl2O4(spinel) may transform to the CaFe2O4structure with a zero‐pressure density of 4.19 g/cm³. Feldspars transform to the hollandite structure (density, of ∼3.85 g/cm³). Olivine‐rich rocks containing greater than 10% FeO either disproportionate to the ilmenite and rock salt structure or transform to a new post‐spinel polymorph having the Sr2PbO4structure. Pyroxenes containing greater than 10% FeO probably transform to the ilmenite structure. High‐pressure forms of sillimanite and andalusite have calculated densities of 4.00 and 3.95 g/cm³, respectively. This probably represents disproportionation reaction products, Al2O3+ SiO2(stishovite), which would give a density of 4.09 g/cm³. The Birch‐Murnaghan second‐order parameter ξ is nearly zero for MgO and Al2O3. 0.73 for stishovite, and ∼1 for the high‐pressure phases of the olivines and pyroxenes. The values ofK′ =dK/dPare calculated along the Hugoniots and adiabats and are found to decrease at a rate of −0.5 to −1.6 cm³/g when the

ISSN:8755-1209    

DOI:10.1029/RG007i004p00667

年代:1969

数据来源: WILEY

摘要:

The Néel theory of magnetization relaxation has been systematically and quantitatively tested for six samples containing synthetic and natural single domain grains. The results of a wide range of experiments involving time, temperature, and direct and alternating magnetic fields were compared with theoretical results predicted by using the Néel theory and the experimental grain size‐coercivity distribution of each sample. Good agreement was found for many experiments involving small field remanences, where grain interaction effects are negligible. Most of the discordant results could be explained by using either the Preisach theory or an extended form of the Néel theory, which takes account of interactions. The two methods are shown to be equivalent, but the Néel theory is more useful in analyzing thermal processes that are intractable by the Preisach method. The similarity of the behavior of synthetic and natural samples suggests that monodomain material is important in many rocks, but experiments on a wide variety of rock types are needed for verific

ISSN:8755-1209    

DOI:10.1029/RG007i004p00709

年代:1969

数据来源: WILEY

摘要:

The impact of dust particles on space vehicles near the earth gives important information about interplanetary dust. Satellite impact measurements have been made with two types of detectors: acoustic and penetration detectors. These types of detectors correspond to momentum and energy restrictions, respectively, on the impact rate of dust. The early acoustic data showed a greatly enhanced rate near the earth, which suggested that there was a high concentration near the earth (‘dust belt’). Quite to the contrary, from theory no sensible enhancement in concentration can be derived but, instead, a moderate enhancement in impact rate, depending on the geocentric velocity of the particles. The dust belt would therefore exist only as a consequence of the measurement technique.Here we review the theory that allows us to calculate the following, for a given geocentric velocity of dust particles and a distribution of velocities (all isotropic) : (1) the concentration; (2) the flux; and (3) the impact rate (momentum‐ and energy‐limited) at various distances from the earth and for various detector velocities so as to bracket all possible experimental situations. With the use of these results, it is then possible to analyze and compare results from different experiments. Of the experimental data reviewed, both U. S. and Russian, only certain are selected for a more detailed analysis. The theory accounts well for the observed penetration data in space vehicles flown near the earth and the moon; they lead to a zodiacal dust model similar to van de Hulst's model, having a particle population predominantly in the micron and decamicron size range (rather than submicron), with a flat (rather than steep) size distribution, and with geocentric velocities of 5 to 15 km/sec.A theoretical analysis is also presented for a nonisotropic velocity distribution. This is of practical importance for very small particles, which are partly supported by solar radiation pressure and therefore develop a streaming velocity with respect to the earth; in addition, the earth itself introduces an asymmetry, since its orbital velocity varies throughout the year. Numerical results are given for predicted asymmetries in concentration, influx of dust into the atmosphere, and in the satellite impact rates. Impact rates from some satellites exhibited such asymmetries, and these cases support the hypothesis that the heliocentric dust orbits have small inclination and small eccentricity. These results are inconclusive because the data are scant; a systematic analysis of all available impact data with respect to asymmetries is

ISSN:8755-1209    

DOI:10.1029/RG007i004p00759

年代:1969

数据来源: WILEY

摘要:

Satellite observations of energetic particles and magnetic fields during the last years have contributed greatly toward understanding our environment. Studies of nonadiabatic processes that occur in the radiation belts have led to a qualitative understanding of several source and loss mechanisms responsible for the observed particle population. A plasma instability leading to spontaneous emission of VLF and ELF wave has been found that limits the maximum flux of protons and electrons that can be trapped in the geomagnetic field. It has been shown that diffusion of particles into the magnetosphere is accompanied by a gain in particle energy; this process probably accounts for most of the lower‐energy particles found in the Van Allen belts. A comprehensive description of the geomagnetic field has been completed. Corrections for the current systems at the magnetospheric boundary and in the magnetotail have been applied in a comprehensive model. Secular changes in the main field have been interpreted in terms of core motion in the earth. Initial measurements have been performed of the galactic cosmic‐ray gradient in the solar system between Venus and Mars. Theories for explaining this gradient in terms of observable properties of the solar wind are being perfected. The relative abundance and energy spectra of hydrogen and helium isotopes in cosmic radiation have been explained in terms of interstellar matter traversed by the cosmic‐ray part

ISSN:8755-1209    

DOI:10.1029/RG007i004p00799

年代:1969

数据来源: WILEY