Statistical analysis of dynamic fibrils observed from NST/BBSO observations     

Thambaje Gopalan Priya  Jiang-Tao Su  Jie Chen  Yuan-Yong Deng  Debi Prasad Choudhury 《天文和天体物理学研究(英文版)》2018,(2)

We present the results obtained from the analysis of dynamic fibrils in NOAA active region(AR) 12132, using high resolution Hα observations from the New Solar Telescope operating at Big Bear Solar Observatory. The dynamic fibrils are seen to be moving up and down, and most of these dynamic fibrils are periodic and have a jet-like appearance. We found from our observations that the fibrils follow almost perfect parabolic paths in many cases. A statistical analysis on the properties of the parabolic paths showing an analysis on deceleration, maximum velocity, duration and kinetic energy of these fibrils is presented here. We found the average maximum velocity to be around 15 kms~(-1) and mean deceleration to be around 100 ms~(-2). The observed deceleration appears to be a fraction of gravity of the Sun and is not compatible with the path of ballistic motion due to gravity of the Sun. We found a positive correlation between deceleration and maximum velocity. This correlation is consistent with simulations done earlier on magnetoacoustic shock waves propagating upward.  相似文献   

Analysis of the Variability of a Two-Dimensional Finite Strain Estimate     

Kingshuk Roy Choudhury 《Mathematical Geosciences》2009,41(5):535-553

Finite strain estimation is a widely used technique for the study of rock deformation in structural geology. One particular algorithm proposed by Shimamoto and Ikeda uses the ‘average shape matrix’ of deformed markers. This paper provides a detailed error analysis for resulting strain estimates in two dimensions. When the number of markers exceeds 100, estimators of components of the strain tensor are shown to have an approximately Gaussian distribution with variances that increase with their mean. Equal variance estimators are obtained by applying a log transform for the elongation and an arcsin transformation for the orientation estimates. Confidence interval formulae for strain tensor components are proposed. Lithology specific constants arising in these formulae are estimated from undeformed samples. The results are validated by application to simulated data as well as observational data from thin sections of sandstone sampled from SE Ireland.  相似文献   

Glimpsing through the high-redshift neutral hydrogen fog     

S. Gallerani  A. Ferrara  X. Fan  T. Roy Choudhury 《Monthly notices of the Royal Astronomical Society》2008,386(1):359-369

We analyse the transmitted flux in a sample of 17 QSOs spectra at 5.74 ≤ z _em≤ 6.42 to obtain tighter constraints on the volume-averaged neutral hydrogen fraction, x _{H  i} , at z ≈ 6. We study separately the narrow transmission windows (peaks) and the wide dark portions (gaps) in the observed absorption spectra. By comparing the statistics of these spectral features with a semi-analytical model of the Lyα forest, we conclude that x _{H  i} evolves smoothly from 10^−4.4 at   z = 5.3  to 10^−4.2 at   z = 5.6  , with a robust upper limit x _{H  i} < 0.36 at   z = 6.3  . The frequency and physical sizes of the peaks imply an origin in cosmic underdense regions and/or in H  ii regions around faint quasars or galaxies. In one case (the intervening H  ii region of the faint quasar RD J1148+5253 at   z = 5.70  along the line of sight of SDSS J1148+5251 at   z = 6.42  ) the increase of the peak spectral density is explained by the first-ever detected transverse proximity effect in the H  i Lyα forest; this indicates that at least some peaks result from a locally enhanced radiation field. We then obtain a strong lower limit on the foreground QSO lifetime of t _Q > 11 Myr. The observed widths of the peaks are found to be systematically larger than the simulated ones. Reasons for such discrepancy might reside either in the photoionization equilibrium assumption or in radiative transfer effects.  相似文献   

Cosmic microwave background polarization constraints on radiative feedback     

C. Burigana  L. A. Popa  R. Salvaterra  R. Schneider  T. Roy Choudhury  A. Ferrara 《Monthly notices of the Royal Astronomical Society》2008,385(1):404-410

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Equivalent-Linear Seismic Ground Response Analysis of Some Typical Sites in Mumbai   总被引：4，自引：0，他引：4  

V. S. Phanikanth  Deepankar Choudhury  G. Rami Reddy 《Geotechnical and Geological Engineering》2011,29(6):1109-1126

Any earthquake event is associated with a rupture mechanism at the source, propagation of seismic waves through underlying rock and finally these waves travel through the soil layers to the particular site of interest. The bedrock motion is significantly modified at the ground surface due to the presence of local soil layers above the bedrock beneath the site of interest. The estimation of the amplifications in ground response due to the local soil sites is a complex problem to the designers and the problem is more important for mega cities like Mumbai in India, where huge population may get affected due to devastations of earthquake. In the present study, the effect of local soil sites in modifying ground response is studied by performing one dimensional equivalent-linear ground response analysis for some of the typical Mumbai soil sites. Field borelog data of some typical sites in Mumbai city viz. Mangalwadi site, Walkeswar site, BJ Marg near Pandhari Chawl site are considered in this study. The ground responses are observed for range of input motions and the results are presented in terms of surface acceleration time history, ratio of shear stress to vertical effective stress versus time, acceleration response spectrum, Fourier amplitude ratio versus frequency etc. The typical amplifications of ground accelerations considering four strong ground motions with wide variation of low to high MHA, frequency contents and durations are obtained. Results show that MHA, bracketed duration, frequency content have significant effects on the amplification of seismic accelerations for typical 2001 Bhuj motion. The peak ground acceleration amplification factors are found to be about 2.50 for Mangalwadi site, 2.60 for Walkeswar site and 3.45 for BJ Marg site using 2001 Bhuj input motion. The response spectrum along various soil layers are obtained which will be useful for designers for earthquake resistant design of geotechnical structures in Mumbai for similar sites in the absence of site specific data.  相似文献   

Seismic yield accelerations of MSW landfills by pseudo-dynamic approach   总被引：2，自引：2，他引：0  

Deepankar Choudhury  Purnanand Savoikar 《Natural Hazards》2011,56(1):275-297

Estimation of seismic yield acceleration for the municipal solid waste landfills is the basic step for design to mitigate the hazard of earthquake on landfills. For most of the landfills constructed in the seismic zones, influence of the vertical component of seismic acceleration is often neglected assuming that it is not in phase with the horizontal seismic acceleration and hence does not affect the design parameters largely. Conventional pseudo-static analysis using limit equilibrium method does not consider the variation of seismic inertial forces with time and also effect of vertically propagating shear and primary waves, which can have serious effect on the design values. To overcome the limitations of existing methodology, in the present study, pseudo-dynamic approach, which takes into account the effects of time, primary and secondary waves, frequency of excitation, is applied for the first time for dynamic analysis of landfills for three different typical practical geometries of landfills. Chosen sets of landfills resemble some of the existing landfill geometries. Design equations and charts are developed for factor of safety against base sliding and seismic yield acceleration for these three typical landfill geometries. The results are compared with the similar results obtained by using conventional pseudo-static analysis, and the influence of vertical seismic acceleration on factor of safety against base sliding and yield acceleration is reported.  相似文献   

A review of strong motion studies in Gujarat State of western India     

Pallabee Choudhury  Sumer Chopra  Ketan Singha Roy  B. K. Rastogi 《Natural Hazards》2014,71(3):1241-1257

The present work reviews the strong motion studies done in Gujarat State of western India. Prior to the 2001 Bhuj earthquake, no strong motion instrument was in operation in Gujarat. After the earthquake, number of research institutes/universities from India and abroad deployed strong motion instruments to study aftershock activity, source dynamics, path and site effects. The strong motion recordings have enhanced the general understanding of the physics of earthquakes in the region. An attempt has been made to develop attenuation relationship for the Gujarat region from the actual ground motions recorded by the strong motion networks. The Government of Gujarat with the help from Asian Development Bank, World Bank (WB), Ministry of Science and Technology and Ministry of Earth Sciences, Government of India, has established a permanent dense network of strong motion accelerograph (SMA) all over Gujarat. In addition, the Institute of Seismological Research has been established in Gandhinagar, Gujarat, with the help of WB for carrying out seismological research. Recently, many important studies have been carried out using actual acceleration data obtained from a dense network of 54 SMA, as well as synthetic data generated using region-specific ground motion parameters. The recorded data are used to obtain region-specific ground motion parameters and ground motion prediction equation. A deterministic hazard analysis for the entire state of Gujarat has been carried out using site-specific ground motion parameters. The estimated peak ground acceleration and modified Mercalli intensity values have been used to estimate the vulnerability of the different types of buildings in 31 cities of Gujarat. As Gujarat has three distinct regions having varied geological conditions, the recorded strong motion data gave an opportunity to study the effect of geological and local-site conditions on the response spectra. This study for an intra-plate region like Gujarat is a pioneer work. Still, lots of research work need to be carried out as more and more data are available, such as development of more robust ground motion prediction equations and a 3D-velocity structure of Gujarat. Generation of shake maps in real time and a credible early earthquake warning system is need of the hour for disaster mitigation and management.  相似文献   

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   

GPS measurement of land subsidence in Gandhinagar,Gujarat (Western India), due to groundwater depletion     

Pallabee Choudhury  Kalpna Gahalaut  Rakesh Dumka  V. K. Gahalaut  Abhay Kumar Singh  Sanjay Kumar 《Environmental Earth Sciences》2018,77(22):770

A continuous GPS measurement site, ISRR, at Gandhinagar (Western India), documented ~?5 mm/year of surface subsidence rate during 2009–2016. Preliminary modelling using an analytical solution indicates that the observed surface subsidence rate at the ISRR site is consistent with the ground water depletion reported from Gandhinagar. An assessment of data from GPS sites at Lucknow and Varanasi in the Indo-Gangetic plains in Northern India does not indicate any significant subsidence at these sites which is also consistent with the in situ observations of insignificant depletion of ground water in the region.  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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