Spatially intermittent fields in photospheric magnetoconvection     

P. J. Bushby  S. M. Houghton 《Monthly notices of the Royal Astronomical Society》2005,362(1):313-320

Motivated by recent high-resolution observations of the solar surface, we investigate the problem of non-linear magnetoconvection in a three-dimensional compressible layer. We present results from a set of numerical simulations which model the situation in which there is a weak imposed magnetic field. This weak-field regime is characterized by vigorous granular convection and spatially intermittent magnetic field structures. When the imposed field is very weak, magnetic flux tends to accumulate at the edges of the convective cells, where it forms compact, almost 'point-like' structures which are reminiscent of those observed in the quiet Sun. If the imposed field is slightly stronger, there is a tendency for magnetic flux to become concentrated into 'ribbon-like' structures which are comparable to those observed in solar plages. The dependence of these simulations upon the strength of the imposed magnetic field is analysed in detail, and the concept of the fractal dimension is used to make a further, more quantitative comparison between these simulations and photospheric observations.  相似文献   

Simultaneous operation of the Sea Beam multibeam echo-sounder andthe SeaMARC II bathymetric sidescan sonar system     

de Moustier  C. Lonsdale  P.F. Shor  A.N. 《Oceanic Engineering, IEEE Journal of》1990,15(2):84-94

An experiment aboard the Scripps Institution of Oceanography's RV Thomas Washington has demonstrated the seafloor mapping advantages to be derived from combining the high-resolution bathymetry of a multibeam echo-sounder with the sidescan acoustic imaging plus wide-swath bathymetry of a shallow-towed bathymetric sidescan sonar. To a void acoustic interference between the ship's 12-kHz Sea Beam multibeam echo-sounder and the 11-12-kHz SeaMARC II bathymetric sidescan sonar system during simultaneous operations, Sea Beam transmit cycles were scheduled around SeaMARC II timing events with a sound source synchronization unit originally developed for concurrent single-channel seismic, Sea Beam, and 3.5-kHz profile operations. The scheduling algorithm implemented for Sea Beam plus SeaMARC II operations is discussed, and the initial results showing their combined seafloor mapping capabilities are presented  相似文献   

A comparison of type II shock speeds     

R. J. Thompson  J. A. Kennewell  N. P. Prestage 《Solar physics》1996,166(2):371-378

Type II radio bursts are produced by material moving outwards in the solar atmosphere. Their drift in frequency allows the calculation of the radial speed with which the shock is moving- very basic information in assessing the likelihood that the shock will reach the Earth and its time of arrival. This paper compares the shock speeds derived from radio bursts observed by the Swept Frequency Interferometric Radiometer (SFIR) equipment at the US Air Force Radio Solar Telescope Network (RSTN) of observatories with those measured with the Culgoora radiospectrograph operated by IPS Radio and Space Services. The SFIR shock speeds are found to be 1.5–3.0 times larger than the Culgoora values which are consistent with earlier results. This difference appears to originate from the incorrect interpretation of events as a result of the smaller frequency range of the SFIR equipment.  相似文献   

Simulations of a giant hybrid air shower detector     

B. R. Dawson  Y. Dai H.  P. Sommers  S. Yoshida 《Astroparticle Physics》1996,5(3-4):239-247

We have used Monte Carlo simulations to investigate the capabilities of a giant air shower observatory designed to detect showers initiated by cosmic rays with energies exceeding 10¹⁹ eV. The observatory is to consist of an array of detectors that will characterise the air shower at ground level, and optical detectors to measure the fluorescence light emitted by the shower in the atmosphere. Using these detectors together in a ‘hybrid’ configuration, we find that precise geometrical reconstruction of the shower axis is possible, leading to excellent resolution in energy. The technique is also shown to provide very good reconstruction below 10¹⁹ eV, at energies where the ground array is not fully efficient.  相似文献   

Some Problems with Slow Rotation of CP Stars     

Ju. V. Glagolevsky 《Astrophysics》2005,48(2):194-207

Some difficulties in explaining the slow rotation of CP stars are discussed. The most likely hypotheses are (1) a loss of angular momentum involving a magnetic field during “pre-main sequence” evolution and (2) the slow rotation existed from the very start of the creation of these stars. The braking hypothesis is supported by only one property of CP stars— the lower the mass of the star is, the greater the difference between its average rotation velocity vsini and that of normal stars. On the other hand, there is another property— the lower the rotation speeds of CP stars are, the greater their fraction among normal stars. The latter property supports the hypothesis that the lower the initial rotation speed of a star is when it is created, the greater the probability will become chemically peculiar. This property is inherent in chemically peculiar stars both with and without a magnetic field. It is proposed that the cause of the slow rotation of CP stars must be sought in the very earliest phases of their formation, as should the cause of the separation into chemically peculiar magnetic, chemically peculiar nonmagnetic, and normal stars.__________Translated from Astrofizika, Vol. 48, No. 2, pp. 229–245 (May 2005).  相似文献   

A time series analysis of sound propagation in a strongly multipathshallow water environment with an adiabatic normal mode approach     

Knobles  D.P. Koch  R.A. 《Oceanic Engineering, IEEE Journal of》1996,21(1):1-13

Measured time series were generated by small omnidirectional explosive sources in a shallow water area. A bottom-mounted hydrophone recorded sound signals that propagated over a sloping bottom. The time series in the 250-500 Hz band were analyzed with a broad-band adiabatic normal mode approach. The measured waveforms contain numerous bottom interacting multipaths that are complicated by the subbottom structure that contains high-velocity layers near the water-sediment interface. Several of the details of the geoacoustic structure and the depth of the water column at the receiver are inferred from comparisons of the measured data to simulated time series. The sensitivity of broad-band matched-field ambiguity surfaces in the range-depth plane for a single receiver to selected waveguide parameters is examined. A consistent analysis is made where the simulated time series are compared to the measured time series along with the single-receiver matched-field localization solutions for ranges out to 5 km. In this range interval, it was found that the peak cross-correlation between the measured and simulated time series varied between 0.84 and 0.69. The difference between the GPS range and the range obtained from the matched-field solution varied from 0 to 63 m. The geoacoustic structure obtained in the analysis consists of an 8-m low-velocity sediment layer over an 8-m high-velocity layer followed by a higher velocity, infinite half-space  相似文献   

Semi-Classical Cosmological Models with Scalar, Dirac and Electromagnetic Fields     

Luis P. Chimento  Norberto A. Zuccalá 《Astrophysics and Space Science》1997,254(1):37-49

Exact solutions of the semi-classical Einstein equations for conformally invariant free quantum fields in an homogeneous and isotropic space-time, with cosmological constant and containing a classical scalar field, dust matter, an unquantised Dirac field and electromagnetic radiation are found. The initial behaviour of the semiclassical models is investigated. Some of the solutions found avoid the singularity and do not have particle horizons. This revised version was published online in July 2006 with corrections to the Cover Date.  相似文献   

Exposure history of the Torino meteorite     

R. Wieler  Th. Graf  P. Signer  S. Vogt  G. F. Herzog  C. Tuniz  D. Fink  L. K. Fifield  J. Klein  R. Middleton  A. J. T. Jull  P. Pellas  J. Masarik  G. Dreibus 《Meteoritics & planetary science》1996,31(2):265-272

Abstract— We determined He, Ne, Ar, ¹⁰Be, ²⁶Al, ³⁶Cl, and ¹⁴C concentrations, as well as cosmic-ray track densities and halogen concentrations in different specimens of the H6 chondrite Torino, in order to constrain its exposure history to cosmic radiation. The Torino meteoroid had a radius of ～20 cm and travelled in interplanetary space for 2.5–10 Ma. Earlier, Torino was part of a larger body. The smallest possible precursor had a radius of 55 cm and a journey through space longer than ～65 Ma. If the first-stage exposure took place in a body with a radius of >3 m or in the parent asteroid, then it lasted nearly 300 Ma. The example of Torino shows that it is easy to underestimate first-stage exposure ages when constructing two-stage histories.  相似文献   

Stellar models and Hyades: the Hipparcos test     

V. Castellani  S. Degl'Innocenti  P. G. Prada Moroni 《Monthly notices of the Royal Astronomical Society》2001,320(1):66-72

We compare theoretical stellar models for main sequence (MS) stars with the Hipparcos data base for the Hyades cluster to give a warning against the uncritical use of available theoretical scenarios and to show how formal MS fittings can be fortuitous if not fictitious. Moreover, we find that none of the current theoretical scenarios appears able to account for an observed mismatch between theoretical predictions and observations of the coolest Hyades MS stars. Finally, we show that current theoretical models probably give too faint He burning luminosities unlike the case of less massive He burning models, with degenerate progenitors, which have been suggested to suffer the opposite discrepancy.  相似文献   

Simulations of deep pencil-beam redshift surveys     

N. Yoshida  J. Colberg  S.D.M. White  A.E. Evrard  T.J. MacFarland †  H.M.P. Couchman  A. Jenkins  C.S. Frenk  F.R. Pearce  G. Efstathiou  J.A. Peacock  P.A. Thomas 《Monthly notices of the Royal Astronomical Society》2001,325(2):803-816

We create mock pencil-beam redshift surveys from very large cosmological N -body simulations of two cold dark matter (CDM) cosmogonies, an Einstein–de Sitter model ( τ CDM) and a flat model with Ω₀=0.3 and a cosmological constant (ΛCDM). We use these to assess the significance of the apparent periodicity discovered by Broadhurst et al. Simulation particles are tagged as 'galaxies' so as to reproduce observed present-day correlations. They are then identified along the past light-cones of hypothetical observers to create mock catalogues with the geometry and the distance distribution of the Broadhurst et al. data. We produce 1936 (2625) quasi-independent catalogues from our τ CDM (ΛCDM) simulation. A couple of large clumps in a catalogue can produce a high peak at low wavenumbers in the corresponding one-dimensional power spectrum, without any apparent large-scale periodicity in the original redshift histogram. Although the simulated redshift histograms frequently display regularly spaced clumps, the spacing of these clumps varies between catalogues and there is no 'preferred' period over our many realizations. We find only a 0.72 (0.49) per cent chance that the highest peak in the power spectrum of a τ CDM (ΛCDM) catalogue has a peak-to-noise ratio higher than that in the Broadhurst et al. data. None of the simulated catalogues with such high peaks shows coherently spaced clumps with a significance as high as that of the real data. We conclude that in CDM universes, the regularity on a scale of ∼130  h ⁻¹ Mpc observed by Broadhurst et al. has a priori probability well below 10⁻³.  相似文献