Semi-analytic approach to understanding the distribution of neutral hydrogen in the Universe     

T. Roy Choudhury  T. Padmanabhan  R. Srianand 《Monthly notices of the Royal Astronomical Society》2001,322(3):561-575

Analytic derivations of the correlation function and the column density distribution for neutral hydrogen in the intergalactic medium (IGM) are presented, assuming that the non-linear baryonic mass density distribution in the IGM is lognormal. This ansatz was used earlier by Bi & Davidsen to perform one-dimensional simulations of lines of sight and analyse the properties of absorption systems. We have taken a completely analytic approach, which allows us to explore a wide region of the parameter space for our model. The analytic results have been compared with observations to constrain various cosmological and IGM parameters, whenever possible. Two kinds of correlation functions are defined: (i) along the line of sight (LOS); and (ii) across the transverse direction. We find that the effects on the LOS correlation owing to changes in cosmology and the slope of the equation of state of the IGM, γ , are of the same order, which means that we cannot constrain both the parameters simultaneously. However, it is possible to constrain γ and its evolution using the observed LOS correlation function at different epochs provided that one knows the background cosmology. We suggest that the constraints on the evolution of γ obtained using the LOS correlation can be used as an independent tool to probe the reionization history of the Universe. From the transverse correlation function, we obtain the excess probability, over random, of finding two neutral hydrogen overdense regions separated by an angle θ . We find that this excess probability is always less than 1 per cent for redshifts greater than 2. Our models also reproduce the observed column density distribution for neutral hydrogen, and the shape of the distribution depends on γ . Our calculations suggest that one can rule out γ >1.6 for z ≃2.31 using the column density distribution. However, one cannot rule out higher values of γ at higher redshifts.  相似文献   

Key features associated with the early and late South China summer monsoon onset     

Choudhury  Devanil  Nath  Debashis  Chen  Wen 《Theoretical and Applied Climatology》2022,147(1-2):99-108

Theoretical and Applied Climatology - Here we investigate the key features of early and late South China Sea summer monsoon (SCSSM) onset. To carry out the investigation, we select five extreme...  相似文献   

A very extended reionization epoch?     

A. Melchiorri  T. Roy Choudhury  P. Serra  A. Ferrara 《Monthly notices of the Royal Astronomical Society》2005,364(3):873-878

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Development of rational models for tunnel blast prediction based on a parametric study   总被引：2，自引：0，他引：2  

A. K. Chakraborty  A. K. Raina  M. Ramulu  P. B. Choudhury  A. Haldar  P. Sahoo  C. Bandopadhyay 《Geotechnical and Geological Engineering》2004,22(4):477-496

The empirical models available for prediction of the tunnel blast results like pull ratio, specific charge, specific drilling and overbreak have some inherent shortcoming in absence of any parametric study at the backdrop. Hence, the models use different constituting parameters and provide values which differ widely. After a thorough review of literature and field investigations in the drivages of mines and tunnels some parameters were identified. Those parameters were subjected to Multiple Linear Regression analyses to filter out the most influencing ones which represent the rockmass properties, the tunnel configurations and the blast designs. A parameter called Tunnel Blasting Index (TBI) was conceptualized and was expressed in terms of those most influencing parameters. All the blast results observed during the filed investigations could be well related to a single index TBI. Some adjustments on account of shape of the tunnel and joint orientations, which were not addressed in the available models, are suggested in the developed models.  相似文献   

Seismic uplift capacity of strip anchors in soil     

Deepankar Choudhury  Kanakapura. S. Subba Rao 《Geotechnical and Geological Engineering》2004,22(1):59-72

Uplift capacity of horizontal strip anchors in soil embedded under an inclined ground surface has been obtained under seismic conditions. Limit equilibrium approach with logspiral failure surface together with pseudo-static seismic forces has been adopted. The results have been presented in the form of seismic uplift capacity factors as functions of ground inclination, embedment ratio, angle of internal friction of the soil and seismic acceleration coefficients. The uplift capacity factors have been worked out separately for cohesion, surcharge and density components. Effect of the vertical seismic acceleration coefficient has been found to always reduce the uplift capacity whereas the effect of horizontal seismic acceleration coefficient has been found to reduce the uplift capacity in most of the cases. The obtained results of seismic uplift capacity factors are found to be the minimum when compared with the results available in literature on the basis of planar failure surface. This revised version was published online in July 2006 with corrections to the Cover Date.  相似文献   

A connectionist approach to SODAR pattern classification     

Choudhury  S. Mitra  S. 《Geoscience and Remote Sensing Letters, IEEE》2004,1(2):42-46

SODAR (or acoustic radar) systems are a useful tool to efficiently probe the lower planetary boundary layer (LPBL). The observations obtained by these systems can prove to be extremely useful if classified and interpreted correctly. The manual identification of different types of SODAR-recorded lower atmospheric microstructures is a laborious task and can be performed only by an expert having wide experience with the system and the variety of observations recorded by it. In this letter, we have developed a connectionist system to classify or identify SODAR patterns. The results demonstrate that the multilayer perceptron-based model is capable of successfully identifying the different SODAR patterns.  相似文献   

Magneto-viscoelastic disturbances in a ‘hereditary layer’     

Ajoy Roy Choudhury 《Pure and Applied Geophysics》1973,104(1):453-459

Summary The dynamical response due to the interaction of viscoelastic field and magnetic field in a layer ofVito Volterra's hereditary model is analysed by making use of Laplace-transform.  相似文献   

Behaviour ofP phase spectrum in relation to magnitude calculation     

M. A. Choudhury 《Pure and Applied Geophysics》1973,103(1):397-404

Summary P phase spectra within 0.1 and 1 Hz of 15 earthquakes of magnitude lying between 5.2 and 6.4 and having normal depths are examined. The spectra show a maximum around 0.5 Hz for all events. This is the principal maximum for smaller shocks. Error in magnitude calculation usingP phase amplitude of frequencies higher than about 0.5 Hz arise fromQ-type losses while that in lower frequencies from differences in source function.Institut de Physique du Globe, Université Paris VI. Contribution no. 48The main results of this paper were presented by the author at the General Assembly meeting of the European Seismological Commission, Luxembourg, September 1970 under the title variation de la magnitude en fonction de la période (1–10s) des ondesP.  相似文献   

On axisymmetric vibration in a transversely isotropic finite cylindrical shell acted upon by a magnetic field     

Ajoy Roy Choudhury 《Pure and Applied Geophysics》1972,97(1):70-75

Summary The present note seeks to investigate the axisymmetric vibration in a transversely isotropic finite cylindrical shell in presence of axial magnetic field by making use of the electromagnetic equations of Maxwell and the equations of elasticity. The frequency with its minimum value is obtained.  相似文献   

A comparative analysis of satellite-observed visible reflectance and 37 GHz polarization difference to assess land surface change over the Sahel zone, 1982–1986     

B. J. Choudhury 《Climatic change》1990,17(2-3):193-208

Applicability of visible reflectance to monitor land surface change particularly for arid and semi-arid regions has been studied fairly extensively. Visible reflectance over the Sahel zone from January 1982 to December 1986 has been derived from observations by the advanced very high resolution radiometer (AVHRR) on board the NOAA-7 and NOAA-9 satellites and compared against concurrent observations of 37 GHz polarization difference by the scanning multichannel microwave radiometer (SMMR) on board the Nimbus-7 satellite. Analysis of radiative transfer models suggested a high correlation to exist between these two sets of observations, and the observed spatial and temporal patterns of these data sets did show an overall high correlation. Land surface change resulting from the 1984 drought affected both of these data sets, but not in identical fashion. The present study suggests that potentially new information about physical and biophysical characteristics of the surface could be obtained by synergistic use of these two data sets.  相似文献