4,521 research outputs found

    MAGSAT anomaly field inversion and interpretation for the US

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    Long wavelength anomalies in the total magnetic field measured by MAGSAT over the United States and adjacent areas are inverted to an equivalent layer crustal magnetization distribution. The model is based on an equal area dipole grid at the Earth's surface. Model resolution, defined as the closest dipole spacing giving a solution having physical significance, is about 220 km for MAGSAT data in the elevation range 300-500 km. The magnetization contours correlate well with large scale tectonic provinces. A higher resolution (200 km) model based on relatively noise free synthetic "pseudodata" is also presented. Magnetic anomaly component data measured by MAGSAT is compared with synthetic anomaly component fields arising from an equivalent source dipole array at the Earth's surface generated from total field anomaly data alone. An excellent inverse correlation between apparent magnetization and heat flow in the western U.S. is demonstrated. A regional heat flow map which is presented and compared with published maps, predicts high heat flow in Nebraska and the Dakotas, suggesting the presence of a "blind" geothermal area of regional extent

    A method of inversion of satellite magnetic anomaly data

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    A method of finding a first approximation to a crustal magnetization distribution from inversion of satellite magnetic anomaly data is described. Magnetization is expressed as a Fourier Series in a segment of spherical shell. Input to this procedure is an equivalent source representation of the observed anomaly field. Instability of the inversion occurs when high frequency noise is present in the input data, or when the series is carried to an excessively high wave number. Preliminary results are given for the United States and adjacent areas

    Equivalent source modeling of the main field using MAGSAT data

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    An iterative least squares estimation algorithm with the capability for including a priori statistical information was implemented to recover multiple magnetic dipole models of the Earth's main magnetic field. The dipoles are fixed to a specified radius at or below the core-mantle boundary and centered on equal area blocks. The algorithm can solve for dipole magnitudes only (fixed orientations), or allow full freedom of orientation and solve for vector components. External field parameters and observatory anomaly biases can also be estimated simultaneously. Time dependence is modeled using first time derivatives for dipole vector components. Single-epoch and time dependent dipole models are derived using MAGSAT and observatory annul means data. Equivalent spherical harmonic representation are computed in closed form from the dipole models and compared with truncated spherical harmonic models estimated in the standard way from the same data sets. In particular, a 21 deg spatial resolution model based on 93 dipoles was computed based on observatory annual means data and a selected MAGSAT data set and was compared with candidate IGRF 1975 models and their 1980 secular variation

    An equivalent layer magnetization model for the United States derived from MAGSAT data

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    Long wavelength anomalies in the total magnetic field measured field measured by MAGSAT over the United States and adjacent areas are inverted to an equivalent layer crustal magnetization distribution. The model is based on an equal area dipole grid at the Earth's surface. Model resolution having physical significance, is about 220 km for MAGSAT data in the elevation range 300-500 km. The magnetization contours correlate well with large-scale tectonic provinces

    A compilation of mineral occurrences and the relationship of occurrences to structural elements of the Kentucky and Tennessee region

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    A very prominent magnetic anomaly measured by MAGSAT over the eastern mid-continent of the United States was inferred to have a source region beneath Kentucky and Tennessee. Prominent aeromagnetic and gravity anomalies are also associated with the inferred source region. A crustal model constructed to fit these anomalies interpreted the complex as a large mafic plutonic intrusion of Precambrian age. The complex was named the Kentucky body. It was noticed that the Jessamine Dome, which is a locus of intense faulting and mineralization, occurs near the northern end of the Kentucky body, and that more generally there seemed to be a spatial relationship between mineral occurrence and the body. The relationship between mineral deposits in Kentucky and Tennessee and the Kentucky body was investigated. A compilation of mineral occurrences in the region, classified according to type and age, is presented

    Isomeric effects in the gas-phase reactions of dichloroethene, C2H2Cl2, with a series of cations

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    A study of the reactions of a series of gas-phase cations (NH4+_4^+, H3_3O+^+, SF3+_3^+, CF3+_3^+, CF+^+, SF5+^+, SF2+_2^+, SF+^+, CF2+_2^+, SF4+_4^+, O2+_2^+, Xe+^+, N2_2O+^+, CO2+_2^+, Kr+^+, CO+^+, N+^+, N2+_2^+, Ar+^+, F+^+ and Ne+^+) with the three structural isomers of dichloroethene, i.e. 1,1-C2_2H2_2Cl2_2, cis-1,2-C2_2H2_2Cl2_2 and trans-1,2-C2_2H2_2Cl2_2 is reported. The recombination energy of these ions spans the range 4.7-21.6 eV. Reaction rate coefficients and product branching ratios have been measured at 298 K in a selected ion flow tube. Collisional rate coefficients are calculated by modified average dipole orientation theory and compared with experimental data. Thermochemistry and mass balance have been used to predict the most feasible neutral products. Threshold photoelectron-photoion coincidence spectra have also been obtained for the three isomers of C2_2H2_2Cl2_2 with photon energies in the range 10-23 eV. The fragment ion branching ratios have been compared with those of the flow tube study to determine the importance of long-range charge transfer. A strong influence of the isomeric structure of dichloroethene on the products of ion-molecule reactions has been observed for H3_3O+^+, CF3+_3^+, and CF+^+. For 1,1-C2_2H2_2Cl2_2 the reaction with H3_3O+^+ proceeds at the collisional rate with the only ionic product being 1,1-C2_2H2_2Cl2_2H+^+. However, the same reaction yields two more ionic products in the case of cis-1,2- and trans-1,2-C2_2H2_2Cl2_2, but only proceeds with 14 % and 18 % efficiency, respectively. The CF3+_3^+ reaction proceeds with 56-80 % efficiency, the only ionic product for 1,1-C2_2H2_2Cl2_2 being C2_2H2_2Cl+^+ formed via Cl- abstraction, whereas the only ionic product for both 1,2-isomers is CHCl2+_2^+ corresponding to a breaking of the C=C double bond. Less profound isomeric effects, but still resulting in different products for 1,1- and 1,2-C2_2H2_2Cl2_2 isomers, have been found in the reactions of SF+^+, CO2+_2^+, CO+^+, N2+_2^+, and Ar+^+. Although these five ions have recombination energies above the ionization energy of any of the C2_2H2_2Cl2_2 isomers and hence the threshold for long-range charge transfer, the results suggest that the formation of a collision complex at short range between these ions and C2_2H2_2Cl2_2 is responsible for the observed effects

    Shifts in hexapod diversification and what Haldane could have said

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    Data on species richness and taxon age are assembled for the extant hexapod orders (insects and their six-legged relatives). Coupled with estimates of phylogenetic relatedness, and simple statistical null models, these data are used to locate where, on the hexapod tree, significant changes in the rate of cladogenesis (speciation-minus-extinction rate) have occurred. Significant differences are found between many successive pairs of sister taxa near the base of the hexapod tree, all of which are attributable to a shift in diversification rate after the origin of the Neoptera (insects with wing flexion) and before the origin of the Holometabola (insects with complete metamorphosis). No other shifts are identifiable amongst supraordinal taxa. Whilst the Coleoptera have probably diversified faster than either of their putative sister lineages, they do not stand out relative to other closely related clades. These results suggest that any Creator had a fondness for a much more inclusive clade than the Coleoptera, definitely as large as the Eumetabola (Holometabola plus bugs and their relatives), and possibly as large as the entire Neoptera. Simultaneous, hence probable causative events are discussed, of which the origin of wing flexion has been the focus of much attention

    Comparison of Magnetic Anomalies of Lithospheric Origin Measured by Satellite and Airborne Magnetometers over Western Canada

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    Crustal magnetic anomaly data from the OGO 2, 4 and 6 (Pogo) satellites are compared with upward-continued aeromagnetic data between 50 deg -85 deg N latitude and 220 deg - 260 deg E longitude. Agreement is good both in anomaly location and in amplitude, giving confidence that it is possible to proceed with the derivation and interpretation of satellite anomaly maps in all parts of the globe. The data contain a magnetic high over the Alpha ridge suggesting continental composition and a magnetic low over the southern Canada basin and northern Canadian Arctic islands (Sverdrup basin). The low in the Sverdrup basin corresponds to a region of high heat flow, suggesting a shallow Curie isotherm. A ridge of high field, with two distinct peaks in amplitude, is found over the northern portion of the platform deposits and a relative high is located in the central portion of the Churchill province. No features are present to indicate a magnetic boundary between Slave and Bear provinces, but a trend change is evident between Slave and Churchill provinces. South of 60 deg latitude a broad magnetic low is located over very thick (40-50 km) crust, interpreted to be a region of low magnetization

    A Selected Ion Flow Tube Study of the Reactions of Several Cations with the Group 6B Hexafluorides SF6, SeF6, and TeF6

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    The first investigation of the ion chemistry of SeF6_6 and TeF6_6 is presented. Using a selected ion flow tube, the thermal rate coefficients and ion product distributions have been determined at 300 K for the reactions of fourteen atomic and molecular cations, namely H3_3O+^+, CF3+_3^+, CF+^+, CF2+_2^+, H2_2O+^+, N2_2O+^+, O+^+, CO2+_2^+, CO+^+, N+^+, N2+_2^+, Ar+^+, F+^+ and Ne+^+ (in order of increasing recombination energy), with SeF6_6 and TeF6_6. The results are compared with those from the reactions of these ions with SF6_6, for which the reactions with CF+^+, CF2+_2^+, N2_2O+^+ and F+^+ are reported for the first time. Several distinct processes are observed amongst the large number of reactions studied, including dissociative charge transfer, and F−^-, F, F2−_2^- and F2_2 abstraction from the neutral reactant molecule to the reagent ion. The dissociative charge transfer channels are discussed in relation to vacuum ultraviolet photoelectron and threshold photoelectron-photoion coincidence spectra of XF6_6 (X = S, Se, and Te). For reagent ions whose recombination energies lie between the first dissociative ionisation limit, XF6_6 →\rightarrow XF5+_5^+ + F + e−^-, and the onset of ionisation of the XF6_6 molecule, the results suggest that if dissociative charge transfer occurs, it proceeds via an intimate encounter. For those reagent ions whose recombination energies are greater than the onset of ionisation, long-range electron transfer may occur depending on whether certain physical factors apply, for example non-zero Franck-Condon overlap. From the reaction kinetics, limits for the heats of formation of SeF4_4, SeF5_5, TeF4_4 and TeF5_5 at 298 K have been obtained; Δf\Delta_fHo^o(SeF4_4) < -369 kJ mol−1^{-1}, Δf\Delta_fHo^o(SeF5_5) < -621 kJ mol−1^{-1}, Δf\Delta_fHo^o(TeF4_4) > -570 kJ mol−1^{-1}, and Δf\Delta_fHo^o(TeF5_5) < -822 kJ mol−1^{-1}
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