Magneto-atmospheric waves     

Toufik E. Abdelatif 《Solar physics》1990,129(2):201-219

The nature and properties of magneto-atmospheric (MA) waves are discussed here. A simple model atmosphere, isothermal and permeated by a uniform vertical magnetic field, was used to find that there are two type of modes with avoided crossing of the eigencurves in a K - diagram. The properties of these waves are found to be identical to the uncoupled fast and slow MA waves away from the crossings (specially for small horizontal wave number K 1). The energy density for the fast modes is found to vanish with height and is responsible for the 3-min umbral oscillations, while the slow mode energy density is harmonic.  相似文献   

Aeolian processes on Venus     

Greeley  Ronald  Arvidson  Raymond E. 《Earth, Moon, and Planets》1990,(1):127-157

In this pre-Magellan review of aeolian processes on Venus we show that the average rate of resurfacing is less than 2 to 4 km/Ga, based on the impact crater size frequency distribution derived from Venera observations, reasonable values of the impact flux, and the assumption of steady state conditions between crater production and obliteration. Viscous relaxation of crater topography, burial by volcanic deposits, tectonic disruption, chemical and mechanical weathering and erosion, and accumulation of windblown sediments probably all contribute to resurfacing. Based on the rate of disappearance of radar-bright haloes around impact craters, the rate of removal of blocky surfaces has been estimated to be about 10^–2 km/Ga. Pioneer-Venus altimetry data show that the average relative permittivity (at 17 cm radar wavelength) of the surface is too high for exposure of soils 10 cm deep, except for ~5% of the planet located primarily in tessarae terrains. The tectonically disrupted tessarae terrains may be sites of soil generation caused by tectonic disruption of bedrock and the presence of relatively steep slopes, or they may be terrains that serve as traps for windblown material. The overall impression is that Venus is a geologically active planet, but one dominated by volcanism and tectonism. On the other hand, theoretical considerations and experimental data on weathering and transport of surface materials suggest rather different conditions. Thermochemical arguments have been advanced that show: (1) CO₂ and SO₂ incorporate into weathering products at high elevation, (2) transport of weathered material by the wind to lower-elevation plains, and (3) re-equilibration of weathered material, releasing both CO₂ and SO₂. In addition, kinetic data suggest a rate of anhydrite formation of 1 km/Ga, a value comparable to the soil erosion rate on Mars, a planet with an active aeolian environment. Experiments and theoretical studies of aeolian processes show that measured surface winds are capable of moving sand and silt on Venus. Assuming that there is a ready sand supply, the flux could be as high as 2.5 × 10^–5 g/cm/s, a value comparable to desert terrains on Earth. In an active aeolian abrasion environment, sand grains could have lifetimes <10³ years. In addition, comminuted debris may be cold-welded to surfaces at the same time as abrasion is occurring. Magellan altimetry and SAR observations should allow assessment of which model for venusian surface modification (active vs. inactive surficial processes) is correct, given the global coverage, high spatial resolution, the calibrated nature of the data, and the potential during extended missions of acquiring multiple SAR views of the surface.Geology and Tectonics of Venus, special issue edited by Alexander T. Basilevsky (USSR Acad. of Sci. Moscow), James W. Head (Brown University, Providence), Gordon H. Pettengill (MIT, Cambridge, Massachusetts) and R. S. Saunders (J.P.L., Pasadena).  相似文献   

Food production — Problems and prospects     

Anifowoshe TO 《GeoJournal》1990,20(3):243-247

This paper reviews the different problems associated with food production in relation to population growth. A number of solutions are put forward to improve the situation.  相似文献   

Kriging with imprecise (fuzzy) variograms. I: Theory   总被引：2，自引：0，他引：2  

A. Bardossy  I. Bogardi  W. E. Kelly 《Mathematical Geology》1990,22(1):63-79

Imprecise variogram parameters are modeled with fuzzy set theory. The fit of a variogram model to experimental variograms is often subjective. The accuracy of the fit is modeled with imprecise variogram parameters. Measurement data often are insufficient to create good experimental variograms. In this case, prior knowledge and experience can contribute to determination of the variogram model parameters. A methodology for kriging with imprecise variogram parameters is developed. Both kriged values and estimation variances are calculated as fuzzy numbers and characterized by their membership functions. Besides estimation variance, the membership functions are used to create another uncertainty measure. This measure depends on both homogeneity and configuration of the data.  相似文献   

Investigation of the electrostatic charging of dust particles in a Paul-trap     

S. Pintér  J. Švestka  E. Grün 《Astrophysics and Space Science》1990,171(1-2):217-218

Dust particles (glass, tungsten, and nickel) with sizes ranging from 0.25 to 3m were levitated in a Paul-trap and charged by ions or electrons. For ions the particle potential is limited at field strength of about 1×10⁹ V m^–1 by a temperature-dependent discharge mechanism. The particles interaction with 2 to 20 keV electrons always leads to positive surface potentials which can be explained in terms of a decreased absorption of electrons by small particles. Micrometer sized agglomerates were used for the investigation of the electrostatic fragmentation. Fragmentation takes place in a twofold manner: small surface flufl can be removed or the parent particle can be disrupted into smaller agglomerates.Paper presented at the 11th European Regional Astronomical Meetings of the IAU on New Windows to the Universe, held 3–8 July, 1989, Tenerife, Canary Islands, Spain.  相似文献   

Statistical study of optical galactic warps     

M. L. Sanchez-Saavedra  E. Battaner  E. Florido 《Astrophysics and Space Science》1990,171(1-2):239-241

A list of northern spiral NGC galaxies, with inclinations higher than 74°, and exhibiting warps have been obtained from the Palomar Sky Survey (POSS). Approximate position angles and warping angles are given for each galaxy, in order to have a first reference for statistical studies of the warping phenomenon. When we take into account that warps are not always observed (depending on their relative position with respect to the observer) the resulting frequency of warps was so high that a tentative conclusion is that all disks are warped.Paper presented at the 11th European Regional Astronomical Meetings of the IAU on New Windows to the Universe, held 3–8 July, 1989, Tenerife, Canary Islands, Spain.  相似文献   

Astron observations of the Rapid Burster MXB 1730-335 and constraints on burster parameters from spectra of trailing bursts     

A. D. Kaminker  G. G. Pavlov  Yu. A. Shibanov  V. G. Kurt  E. Yu. Shafer  A. S. Smirnov  V. M. Shamolin  I. F. Kopaeva  E. K. Sheffer 《Astrophysics and Space Science》1990,173(2):171-189

We report on eight X-ray bursts detected by ASTRON from the Rapid Burster (RB) on 13 and 28 April and 16 August, 1983. Six of them (trailing bursts), with durations of 1.5–2 min, rise times of 5–10 s and intervals of 1–1.5 hours, exhibit spectral softening during the burst decay and may be related to the type I bursts. Two of the bursts (triangle bursts) observed on 28 April at interval of 28 min with much longer rise times (30–50 s) and longer durations (3 min), do not show distinct spectral softening. Persistent flux from RB on 16 August was estimated asF _p(2.0–2.4)×10^–9 erg cm^–2 s^–1. Spectral evolution of two trailing bursts was investigated by fitting their spectra in consecutive time intervals with the blackbody (BB), isothermal scattering photosphere (SP) and thermal bremsstrahlung (TB) models. Around the burst maxima the SP model fits the data best whereas in the burst tails the TB model is generally better. The BB model is worse than at least one of the two others. Interpretation of the burst spectra in terms of the BB radiation leads to improbably small neutron star mass and radius (M<0.86M ,R _NS<5 km) if the peak luminosity does not exceed the Eddington limit. Interpretation of the spectra around the burst maxima (3–15 s from the burst onset) in terms of an isothermal SP yields reasonable constraints onM,R _NS, and distanceD. For instance, for the hydrogen photosphere we obtainedM=(1.0–2.1)M R _NS=(7.1–16.4) km ifD=11 kpc. If one postulatesM=1.4M , thenD=(8.5–13) kpc for hydrogen photosphere; if, besides,D=11 kpc, thenR _NS=(8.1–13.3) km. It follows also from the SP-interpretation that the photosphere radius may increase up to 20–30 km in maxima of the trailing bursts when the luminosity becomes close to the Eddington luminosity.  相似文献   

Drift in toroidal configurations     

E. A. Evangelidis 《Astrophysics and Space Science》1990,174(2):195-210

It is found that charged particles of positive energiesE, when constrained on axisymmetric isoflux surfaces , execute sinusoidal motions with typical frequencies =(2E/m)^1/2). In general, it was found that under equilibrium condition p=J ^B/cthe particles develop a non-ambipolar drift velocityv _d =(cµ/eb)[1+q ² +2(q/)²]p.  相似文献   

Transmission and reflection of Rayleigh waves at a thin low velocity vertical layer: A laboratory and theoretical study     

P. F. Biagi  T. S. Gegechkory  E. N. Its  P. Manjgaladze  V. Sgrigna  D. Zilpimiani 《Pure and Applied Geophysics》1990,133(2):317-327

A three-dimensional ultrasonic modelling was used to study the Rayleigh waves reflected from a thin low velocity vertical layer and those transmitted through the same zone. The reflection and transmission coefficients of the Rayleigh waves have been calculated by an approximate method based on the Green function technique. Nonrigid contact boundary conditions were used for the low velocity layer modelling. A rather good correspondence between the theoretical and experimental absolute values of the reflection and the transmission coefficients was obtained. Some discrepancies between experimental and theoretical results, in particular with regard to phase shifts, may be explained by a certain inadequacy of the theoretical model to the low velocity layer investigated by the ultrasonic modelling.  相似文献   

A combined study of macroseismic data and focal mechanisms applied to the West-Bohemian earthquake,Czechoslovakia     

J. Zahradník  M. Antonini  G. Grünthal  J. Janský  D. Procházková  E. Schmedes  A. Špičak  J. Zedník 《Pure and Applied Geophysics》1990,133(1):53-71

The main shock of the West-Bohemian earthquake swarm, Czechoslovakia, (magnitudem=4.5, depthh=10 km) exhibits an irregular areal distribution of macroseismic intensities 6° to 7° MSK-64. Four lobes of the 6° isoseismal are found and the maximum observed intensity is located at a distance of 8 km from the instrumentally determined epicentre. This distribution can be explained by the energy flux of the directS wave generated by a circular source, the hypocentral location and focal mechanism of which are taken from independent instrumental studies. The theoretical intensity, which is assumed to be logarithmically proportional to the integrated squared ground-motion velocity (i.e.,I=const+log v ² (t)dt), fits the observed intensity with an overall root-mean-square error less than 0.5°. It is important that the present intensity data can also be equally well explained by the isotropic source. The fit was attained by means of a horizontally layered model though large fault zones and an extended sedimentary basin suggest a significant lateral heterogeneity of the epicentral region. The results encourage a broader application of the simple modelling technique used.  相似文献