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991.
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R. Ramesh 《Journal of Astrophysics and Astronomy》2000,21(3-4):237-240
We present observations of the ‘quiet’ Sun close to the recent solar minimum (Cycle 22), with the Gauribidanur
radioheliograph. Our main conclusion is that coronal streamers also influence the observed radio brightness temperature. 相似文献
993.
Offset-dependent characteristics of seismic scattering are useful for characterizing fractured reservoirs. We use two models that have different background medium properties and different azimuthal AVO responses to study elastic wave propagation and scattering in gas-saturated, heterogeneously fractured reservoirs. Heterogeneous fracture density distributions are built through stochastic modelling. Synthetic seismograms are generated by 3-D finite difference modelling, and waveforms along crack-normal and strike directions are considered in this paper. The multiple signal classification (MUSIC) frequency estimator is used in waveform estimation to provide frequency-domain attributes related to seismic wave scattering by fracture heterogeneity. Our results indicate that the strength of the scattering field is a function of the background medium. The strength also increases with increasing fracture scatterer density and with decreasing correlation length of spatial variations of fracture density. The scattering field is weak at the top of the fractured reservoir. The first-order results are dominated by velocity anisotropy of the mean fracture density field. However, the base of the fractured reservoir corresponds to a strong scattering field on which fracture heterogeneity has a larger effect and is characterized by the loss of coherence. 相似文献
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The digitized lava-flow margins of well-defined extended eruptions occurring at Vesuvio in 1760, 1794, 1861, 1906, 1929 and 1944 are found to follow fractal behaviours inside a scaling region enclosed between 50 and 400 m. Although the invariance region is well respected, the fractal dimension D varies from one lava flow to another: the more irregular the lava-flow margin, the larger the value of D. The ascertained dependence of D on the duration of premonitory activity, preceding the emission of lavas, might provide some insight into the inner volcanic processes before the eruption and into the dynamical processes operating during flow emplacement. 相似文献
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Large degrees of circular polarization at near-infrared wavelengths have been reported in the OMC1 star-forming region. This discovery, in combination with compelling evidence for the existence of non-spherical aligned grains in star formation regions, has prompted us to investigate scattering from spheroidal particles as a possible mechanism for the production of large circular polarization in reflection nebulae. We use a dipole calculation to model the small particle limit and a T -matrix code to treat arbitrarily sized particles. We find that size distributions of perfectly aligned spheroids, with only modest 2:1 axis ratios, are capable of producing circular polarization of up to 50 per cent when scattering unpolarized incident light. This is the case even for dielectric materials, such as 'astronomical silicate', as long as sufficient large particles are included in the size distribution. We consider the effects of particle alignment and find that spinning oblate spheroids should be much more efficient circular polarizers than equivalent prolate spheroids. 相似文献
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The quantitative spectroscopy of stellar objects in complex environments is mainly limited by the ability of separating the object from the background. Standard slit spectroscopy, restricting the field of view to one dimension, is obviously not the proper technique in general. The emerging Integral Field (3D) technique with spatially resolved spectra of a two‐dimensional field of view provides a great potential for applying advanced subtraction methods. In this paper an image reconstruction algorithm to separate point sources and a smooth background is applied to 3D data. Several performance tests demonstrate the photometric quality of the method. The algorithm is applied to real 3D observations of a sample Planetary Nebula in M31, whose spectrum is contaminated by the bright and complex galaxy background. The ability of separating sources is also studied in a crowded field in M33. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim) 相似文献
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