A measurement of the cosmic microwave background temperature 1280 MHz     

A. Raghunathan  Ravi Subrahmanyan 《Journal of Astrophysics and Astronomy》2000,21(1-2):1-17

The absolute temperature of the cosmic microwave background (CMB) has been measured at a frequency of 1280 MHz. The observation was made with a modified version of the L-band receiver used in the Giant Metre wavelength Radio Telescope (GMRT): the feed horn was replaced by a corrugated plate and the receiver was placed on the ground, directed at zenith, and shielded from ground radiation by an aluminium screen with corrugated edges. Novel techniques have been adopted for

?reducing and cancelling unwanted contributions to the system temperature of the receiver and

?calibrating the contributions from the feed assembly and receiver.

The thermodynamic temperature of the CMB is estimated to be 3.45 ± 0.78 K.  相似文献   

An excursion set model for the distribution of dark matter and dark matter haloes     

Ravi K. Sheth 《Monthly notices of the Royal Astronomical Society》1998,300(4):1057-1070

A model of the gravitationally evolved dark matter distribution, in the Eulerian space, is developed. It is a simple extension of the excursion set model that is commonly used to estimate the mass function of collapsed dark matter haloes. In addition to describing the evolution of the Eulerian space distribution of the haloes, the model allows one to describe the evolution of the dark matter itself. It can also be used to describe density profiles, on scales larger than the virial radius of these haloes, and to quantify the way in which matter flows in and out of Eulerian cells. When the initial Lagrangian space distribution is white noise Gaussian, the model suggests that the Inverse Gaussian distribution should provide a reasonably good approximation to the evolved Eulerian density field, in agreement with numerical simulations. Application of this model to clustering from more general Gaussian initial conditions is discussed at the end.  相似文献   

The behavior of total lightning activity in severe Florida thunderstorms     

Earle Williams  Bob Boldi  Anne Matlin  Mark Weber  Steve Hodanish  Dave Sharp  Steve Goodman  Ravi Raghavan  Dennis Buechler 《Atmospheric Research》1999,51(3-4)

The development of a new observational system called LISDAD (Lightning Imaging Sensor Demonstration and Display) has enabled a study of severe weather in central Florida. The total flash rates for storms verified to be severe are found to exceed 60 fpm, with some values reaching 500 fpm. Similar to earlier results for thunderstorm microbursts, the peak flash rate precedes the severe weather at the ground by 5–20 min. A distinguishing feature of severe storms is the presence of lightning ‘jumps' — abrupt increases in flash rate in advance of the maximum rate for the storm. The systematic total lightning precursor to severe weather of all kinds — wind, hail, tornadoes — is interpreted in terms of the updraft that sows the seeds aloft for severe weather at the surface and simultaneously stimulates the ice microphysics that drives the intracloud lightning activity.  相似文献   

Iterative methods for non-classically damped dynamic systems     

Firdaus E. Udwadia  Ravi Kumar 《地震工程与结构动力学》1994,23(2):137-152

Non-classically damped structural systems do not easily lend themselves to the modal superposition method because these systems yield coupled second-order differential equations. In this paper, a variety of new computationally efficient iterative methods for determining the response of such systems are developed. The iterative approaches presented here differ from those presented earlier in that they are computationally superior and/or are applicable to the determination of the responses of broader classes of structural systems. Numerical examples, which are designed to evaluate the efficacy of these schemes, show the vastly improved rates of convergence when compared to earlier iterative schemes.  相似文献   

The non-linear probability distribution function in models with local primordial non-Gaussianity     

Tsz Yan Lam  Ravi K. Sheth 《Monthly notices of the Royal Astronomical Society》2009,395(3):1743-1748

We use the spherical evolution approximation to investigate non-linear evolution from the non-Gaussian initial conditions characteristic of the local f _nl model. We provide an analytic formula for the non-linearly evolved probability distribution function (PDF) of the dark matter which shows that the underdense tail of the non-linear PDF in the f _nl model should differ significantly from that for Gaussian initial conditions. Measurements of the underdense tail in numerical simulations may be affected by discreteness effects, and we use a Poisson counting model to describe this effect. Once this has been accounted, our model is in good quantitative agreement with the simulations. In principle, our calculation is an important first step in programs which seek to reconstruct the shape of the initial PDF from observations of large-scale structures in the Lyα forest and the galaxy distribution at later times.  相似文献   

Non-Gaussian distribution and clustering of hot and cold pixels in the five-year WMAP sky     

Graziano Rossi  Ravi K. Sheth  Changbom Park  Carlos Hernández-Monteagudo 《Monthly notices of the Royal Astronomical Society》2009,399(1):304-316

We present measurements of the clustering of hot and cold patches in the microwave background sky as measured from the Wilkinson Microwave Anisotropy Probe 5-year data. These measurements are compared with theoretical predictions which assume that the cosmological signal obeys Gaussian statistics. We find significant differences from the simplest Gaussian-based prediction. However, the measurements are sensitive to the fact that the noise is spatially inhomogeneous (e.g. because different parts of the sky were observed for different lengths of time). We show how to account for this spatial inhomogeneity when making predictions. Differences from the Gaussian-based expectation remain even after this more careful accounting of the noise. In particular, we note that hot and cold pixels cluster differently within the same temperature thresholds at few-degree scales. While these findings may indicate primordial non-Gaussianity, we discuss other plausible explanations for these discrepancies. In addition, we find some deviations from Gaussianity at sub-degree scales, especially in the W band, whose origin may be associated with extragalactic dust emission.  相似文献   

Perturbation theory and excursion set estimates of the probability distribution function of dark matter, and a method for reconstructing the initial distribution function     

Tsz Yan Lam  Ravi K. Sheth 《Monthly notices of the Royal Astronomical Society》2008,386(1):407-415

  相似文献   

Textural studies on red sediments of Bhimunipatnam,Andhra Pradesh,east coast of India     

K. N. Murali Krishna  K. S. N. Reddy  Ch. Ravi Sekhar  P. Ganapathi Rao  K. Bangaku Naidu 《Journal of the Geological Society of India》2017,89(1):111-111

The study presents the textural characteristics of late Quaternary red sediments of Bhimunipatnam to understand the process of formation of these sediments. The red sediments are classified into (a) yellow sediments (b) reddish brown sediments (c) brick red sediments and (d) light yellow sediments sequence in the vertical litho section. The yellow sediments, rests on the khondalite basement, comprises of medium grained, moderate to poorly sorted and positively skewed. The rounded pebble beds with trough cross bedding indicate high energy turbulent conditions of deposition. The fining upward sequences indicate sediments were deposited under decreasing energy conditions under fluvial regime.The iron bearing minerals like garnets and pyriboles have undergone chemical weathering under high oxidizing environment resulting in addition of silt and clay to the reddish brown and brick red sediments and concretions were formed by carbonate precipitation. These processes caused changes in the mean grain size and sorting nature of these sediments which are originally aeolian in origin. The light yellow sediments were medium to fine grained, well sorted and similar to modern dune sands in terms of textural parameters. These sediments were deposited under low oxidation environmental conditions and acquired yellow colour due to Fe hydroxides.  相似文献   

Geospatial Technologies for National Geomorphology and Lineament Mapping Project – A Case Study of Goa State     

Tapas R. Martha  Deepshikha Ghosh  K. Vinod Kumar  A. Lesslie  M. V. Ravi Kumar 《Journal of the Indian Society of Remote Sensing》2013,41(4):905-920

A three level classification system, based on the genesis of landforms, was used to map the geomorphology of the Goa state. The first level corresponds to the process that was responsible for landform generation, the second level or the intermediate level was assigned based on the morphography, and the third level corresponds to the individual landforms units identified based on the morphostructure. The mapping was carried out using IRS-P6 LISS-III (23.5 m) satellite image as the primary data source. Ancillary data such as geological map, topographic map, digital elevation model (DEM), field data collected by global positioning system (GPS) and web portals for image visualisation, were also used for the mapping purpose. A new software designed for mapping landforms based on the genesis, was used in this study to create a seamless geomorphology and lineament database of the Goa state in a GIS environment. A total of 58 landforms within six types of genetic classes were mapped in this area. Similarly, structural and geomorphic lineaments were also delineated using the satellite data. The database created has multi-purpose usability such as environmental studies, mining activity assessment, coastal zone management and wasteland development, since the classification system used is focused on processes, not theme specific.  相似文献   

Geodynamics of NW India: Subduction, lithospheric flexure, ridges and seismicity     

M. Ravi Kumar  D. C. Mishra  B. Singh  D. Ch. Venkat Raju  M. Singh 《Journal of the Geological Society of India》2013,81(1):61-78

Spectral analysis of digital data of the Bouguer anomaly map of NW India suggests maximum depth of causative sources as 134 km that represents the regional field and coincides with the upwarped lithosphere — asthenosphere boundary as inferred from seismic tomography. This upwarping of the Indian plate in this section is related to the lithospheric flexure due to its down thrusting along the Himalayan front. The other causative layers are located at depths of 33, 17, and 6 km indicating depth to the sources along the Moho, lower crust and the basement under Ganga foredeep, the former two also appear to be upwarped as crustal bulge with respect to their depths in adjoining sections. The gravity and the geoid anomaly maps of the NW India provide two specific trends, NW-SE and NE-SW oriented highs due to the lithospheric flexure along the NW Himalayan fold belt in the north and the Western fold belt (Kirthar -Sulaiman ranges, Pakistan) and the Aravalli Delhi Fold Belt (ADFB) in the west, respectively. The lithospheric flexures also manifest them self as crustal bulge and shallow basement ridges such as Delhi — Lahore — Sagodha ridge and Jaisalmer — Ganganagar ridge. There are other NE-SW oriented gravity and geoid highs that may be related to thermal events such as plumes that affected this region. The ADFB and its margin faults extend through Ganga basin and intersect the NW Himalayan front in the Nahan salient and the Dehradun reentrant that are more seismogenic. Similarly, the extension of NE-SW oriented gravity highs associated with Jaisalmer — Ganganagar flexure and ridge towards the Himalayan front meets the gravity highs of the Kangra reentrant that is also seismogenic and experienced a 7.8 magnitude earthquake in 1905. Even parts of the lithospheric flexure and related basement ridge of Delhi — Lahore — Sargodha show more seismic activity in its western part and around Delhi as compared to other parts. The geoid highs over the Jaisalmer — Ganganagar ridge passes through Kachchh rift and connects it to plate boundaries towards the SW (Murray ridge) and NW (Kirthar range) that makes the Kachchh as a part of a diffused plate boundary, which, is one of the most seismogenic regions with large scale mafic intrusive that is supported from 3-D seismic tomography. The modeling of regional gravity field along a profile, Ganganagar — Chandigarh extended beyond the Main Central Thrust (MCT) constrained from the various seismic studies across different parts of the Himalaya suggests crustal thickening from 35-36 km under plains up to ～56 km under the MCT for a density of 3.1 g/cm³ and 3.25 g/cm³ of the lower most crust and the upper mantle, respectively. An upwarping of ～3 km in the Moho, crust and basement south of the Himalayan frontal thrusts is noticed due to the lithospheric flexure. High density for the lower most crust indicates partial eclogitization that releases copious fluid that may cause reduction of density in the upper mantle due to sepentinization (3.25 g/cm³). It has also been reported from some other sections of Himalaya. Modeling of the residual gravity and magnetic fields along the same profile suggest gravity highs and lows of NW India to be caused by basement ridges and depressions, respectively. Basement also shows high susceptibility indicating their association with mafic rocks. High density and high magnetization rocks in the basement north of Chandigarh may represent part of the ADFB extending to the Himalayan front primarily in the Nahan salient. The Nahan salient shows a basement uplift of ～ 2 km that appears to have diverted courses of major rivers on either sides of it. The shallow crustal model has also delineated major Himalayan thrusts that merge subsurface into the Main Himalayan Thrust (MHT), which, is a decollment plane.  相似文献