PSR B1849+00 Probes the Tiny-Scale Molecular Gas?     

S. Stanimirović  J.M. Weisberg  J.M. Dickey  A. de la Fuente  K. Devine  A. Hedden  S.B. Anderson 《Astrophysics and Space Science》2004,292(1-4):167-172

In this paper we present and discuss the great difference in OH absorption spectra against PSR B1849+00 and SNR G33.6+0.1 along the same line-of-sight. This finding is important as it clearly demonstrates that statistics of absorbing molecular gas depends on the size of the background source.  相似文献   

Unusual comets (?) as observed from the Hubble Space Telescope     

Karen J. Meech  Harold A. Weaver 《Earth, Moon, and Planets》1996,72(1-3):119-131

In separate projects, the Hubble Space Telescope has been used to assess the nature of 3 unusual objects: Chiron, Pholus and P/Shoemaker-Levy 9. This paper will compare these objects and discuss how the unique capabilities of the HST may be used to address the issue of cometary activity in each. Chiron, which has exhibited obvious cometary characteristics for several years, might have a bound dust coma that is unresolvable from the ground. In an attempt to directly observe this bound coma, we have obtained a series of images of Chiron with the HST Planetary Camera. Inner coma structure out to 0.″2 has been detected. From these observations we infer a low bulk nucleus density for Chiron. Both HST and ground-based images of 5145 Pholus have been obtained to search for evidence of activity. The ground-based data give the most sensitive limits; however, it is shown that the WFPC-2 on HST can give limits 2–3 orders of magnitude more sensitive than conventional ground-based limits. Finally, as part of a collaborative effort, we have been obtaining HST observations of SL9 in order to determine the fragment sizes and to assess their nature (i.e., cometary vs. asteroid). Both ground-based observations from the UH 2.2m telescope on Mauna Kea and HST observations show that the near-nucleus dust is redder than the sun. While FOS spectra did not detect OH emission, the WFPC-2 HST data show that the inner coma remained very circular from July 1993 up until 2 weeks prior to impact, implying continued production of dust.  相似文献   

Towed array shape estimation using Kalman filters-theoreticalmodels     

Gray  D.A. Anderson  B.D.O. Bitmead  R.R. 《Oceanic Engineering, IEEE Journal of》1993,18(4):543-556

The dynamical behavior of a thin flexible array towed through the water is described by the Paidoussis equation. By discretizing this equation in space and time a finite-dimensional state-space representation is obtained where the states are the transverse displacements of the array from linearity in either the horizontal or vertical plane. The form of the transition matrix in the state-space representation describes the propagation of transverse displacements down the array. The outputs of depth sensors and compasses located along the array are shown to be related in a simple, linear manner to the states. From this state-space representation a Kalman filter which recursively estimates the transverse displacements and hence the array shape is derived. It is shown how the properties of the Kalman filter reflect the physics of the propagation of motion down the array. Solutions of the Riccati equation are used to predict the mean square error of the Kalman filter estimates of the transverse displacements  相似文献   

The deep seismicity of the Tyrrhenian Sea   总被引：4，自引：0，他引：4  

Helen Anderson  James Jackson 《Geophysical Journal International》1987,91(3):613-637

The study reappraises the deep seismicity of the Tyrrhenian Sea. Careful examination of the quality of reported hypocentres shows that the earthquakes define a zone dipping NW, about 200 km along strike, 50 km thick, and reaching a depth of about 500 km. The zone is slightly concave to the NW at a depth of 300 km, but, contrary to many previous reports, is not tightly concave, nor are there significant spatial gaps in the seismicity, which is effectively continuous with depth. Seismicity is, however, concentrated in the depth interval 250–300 km, where the dip of the seismic zone changes from 70° (above 250 km) to a more gentle dip of 45° at greater depths. Seven fault-plane solutions are available for the largest earthquakes in this depth interval, all of them consistent with a P -axis down the dip of the seismic zone, and all of them requiring movement on faults out of the plane of the subducting slab.
Two deep earthquakes near Naples lie well outside the main zone of activity; for one of which a fault-plane solution is available that has a P -axis not aligned with the dip of the seismic zone. The tightly concave slab-geometry favoured by other reports is supported mainly by the location of these events near Naples, which we think may represent deformation in a separate, probably shallower dipping, piece of subducted lithosphere.
The lack of shallow seismicity, and particularly of thrust faulting earthquakes, at the surface projection of the Benioff zone suggests that active subduction has ceased. Estimates of the convergence rate responsible for subduction in the last 10 Myr far exceed the present convergence rate of Africa and Eurasia, suggesting that the subduction was related instead to the stretching and thinning of the crust in the Tyrrhenian Sea.  相似文献   

Geophysical surveys on the East Pacific Rise — Galapagos Rise system     

Roger N. Anderson  Michael A. Hobart  R. P. Von  Herzen Daniel J. Fomari 《Geophysical Journal International》1978,54(1):141-166

Summary. The East Pacific Rise at 12–15° S is topographically smooth with a crestal horst or linear volcanic peak marking the present axis of spreading. The Galapagos Rise at 14–17° S is topographically rough with a possible central graben marking the extinct spreading axis. The seafloor spreading magnetic anomalies on the East Pacific Rise are of low amplitude, but fracture-zone anomalies at 13–14° S have amplitudes of up to 1250 nT. Anomalies of this amplitude at the magnetic equator must be formed within the fracture zone by some combination of block reversal boundaries, anomalously-high magnetic intensities, and/or anomalously-large thicknesses of the magnetic layers within the fracture zone. Magnetization and major-element chemical analyses of basalts dredged from four locales along the fracture zone indicate that the large magnetic-anomaly amplitudes are caused by the high iron and titanium content of these ferrobasalts. The magnetic-anomaly profiles from the Galapagos Rise and its fracture-zone system are of normal amplitude and are extremely difficult to correlate internally or with the geomagnetic timescale.
Eighty-one heat-flow measurements indicate that the values measured are controlled by sediment thickness. Where the thickness of the sediment blanket is greater than 100 m, high heat flow is measured and possibly is representative of the total heat transfer at the seafloor. Where the sediment thickness is less than 100 m, seawater circulation in the oceanic crust is thought to remove most of the heat convectively; thus causing low conductive heat-flow values to be measured by the usual heat-flow apparatus. The heat loss by convective processes is probably a function also of topographic roughness and sediment permeability.  相似文献   

Polysynthetic feldspar twin-width distributions: Models for inversion twinning     

Norman H. Gray  James B. Anderson 《Mathematical Geology》1980,12(3):267-277

The lamella-width distribution of the transformation polysynthetic twins produced in volcanic anorthoclases during cooling reflects certain aspects of twin nucleation and growth kinetics. An analysis of simple hypothetical statistical models for the kinetics of the transformation process outlines the range and type of twin-width distributions possible. The general characteristics of these distributions, particularly at small twin widths, are sensitive to whether twin boundaries or twin centers nucleate. The rates of twin nucleation and growth during transformation also influence the general form, particularly the skewness, of the final twin-width distribution. The generally exponential nature of the upper tails to the distributions studied is apparently quite insensitive to kinetic details.  相似文献   

Lead isotopes as a tool for gossan assessment in base metal exploration     

Brian L. Gulson  Karen J. Mizon 《Journal of Geochemical Exploration》1979,11(3):299-320

Lead isotope measurements on gossans may be used as an exploration tool for Pb-Zn-Cu deposits of the “stratiform” type. The method is based on the homogeneous isotopic composition of stratiform orebodies and the close fit of their ratios to the so-called “growth curve”. These features also characterize oxidized outcrops. (gossans) of the primary sulphide ore. The retention of homogeneous Pb isotope ratios during oxidation was tested and proved for true gossans derived from known mineralization in which there was a variation in vertical depth from gossan to primary sulphide, different ages and different geological environments. The deposits included Pb-Zn-Cu (Woodlawn), Pb-Zn (Broken Hill), Zn-Pb (Dugald River), Zn-Cu-Pb (Currawang).The method has been extended to distinguish true gossans from false (pseudo) gossans where the base metals and associated trace elements have been scavenged from the surrounding rocks. It has also been applied to a company exploration program in which a number of ironstone cappings had been drilled and other geochemical information was available. Diamond drilling indicated that five prospects were barren, i.e. underlain by iron sulphides devoid of significant base metal mineralization. On the Pb isotope data, three of the prospects would be rejected; a fourth shows a particularly high potential and is worthy of further exploration, whilst the fifth initially showed high potential but a comparison of gossan and sulphide Pb isotope data after drilling suggests that the drill hole was sited on the fringes of economic mineralization.The Pb isotope method may be utilized further in exploration at the drilling stage. If sulphides are intersected, Pb isotopes may be used to distinguish barren sulphides from those with economic potential, even though they both may contain similar base metal concentrations.  相似文献   

Active tectonics of the Adriatic Region   总被引：14，自引：0，他引：14  

Helen Anderson  James Jackson 《Geophysical Journal International》1987,91(3):937-983

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High-resolution studies of radio sources in the Hubble Deep and Flanking Fields     

T. W. B. Muxlow  A. M. S. Richards  S. T. Garrington  P. N. Wilkinson  B. Anderson  E. A. Richards  D. J. Axon  E. B. Fomalont  K. I. Kellermann  R. B. Partridge  R. A. Windhorst 《Monthly notices of the Royal Astronomical Society》2005,358(4):1159-1194

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Upper mantle anisotropy: evidence from free oscillations     

Don L. Anderson  Adam M. Dziewonski 《Geophysical Journal International》1982,69(2):383-404

Summary Isotropic earth models are unable to provide uniform fits to the gross Earth normal mode data set or, in many cases, to regional Love-and Rayleigh-wave data. Anisotropic inversion provides a good fit to the data and indicates that the upper 200km of the mantle is anisotropic. The nature and magnitude of the required anisotropy, moreover, is similar to that found in body wave studies and in studies of ultramafic samples from the upper mantle. Pronounced upper mantle low-velocity zones are characteristic of models resulting from isotropic inversion of global or regional data sets. Anisotropic models have more nearly constant velocities in the upper mantle.
Normal mode partial (Frediét) derivatives are calculated for a transversely isotropic earth model with a radial axis of symmetry. For this type of anisotropy there are five elastic constant. The two shear-type moduli can be determined from the toroidal modes. Spheroidal and Rayleigh modes are sensitive to all five elastic constants but are mainly controlled by the two compressional-type moduli, one of the shear-type moduli and the remaining, mixed-mode, modulus. The lack of sensitivity of Rayleigh waves to compressional wave velocities is a characteristic only of the isotropic case. The partial derivatives of the horizontal and vertical components of the compressional velocity are nearly equal and opposite in the region of the mantle where the shear velocity sensitivity is the greatest. The net compressional wave partial derivative, at depth, is therefore very small for isotropic perturbations. Compressional wave anisotropy, however, has a significant effect on Rayleigh-wave dispersion. Once it has been established that transverse anisotropy is important it is necessary to invert for all five elastic constants. If the azimuthal effect has not been averaged out a more general anisotropy may have to be allowed for.  相似文献