Tropospheric disturbance of 17–21 December 1974 and its effect on microwave propagation     

A. P. Mitra  Y. V. Somayajulu  S. P. Singal  S. C. Majumdar  T. R. Tyagi  B. M. Reddy  S. K. Agarwal  B. S. Gera  A. B. Ghosh  S. K. Sarkar 《Boundary-Layer Meteorology》1977,11(1):103-116

A number of troposphere-monitoring experiments were pressed into service during a large tropospheric disturbance which lasted for several days in December 1974. The experiments operated intensively included a monostatic Sodar system, a satellite radio beacon receiver at very low elevation angles, Meteorological Radiosondes, a 250-kW weather radar, a microwave LOS link and a microbarograph. This multi-pronged campaign yielded very useful results and the correspondences between various observations were striking. The Sodar and microwave link observations were visibly affected only when N reached a value of about –70 units; however, the satellite beacon observations showed anomalous behaviour even for lower N values. Rather spectacular coincidences occurred on one night: the large fading (up to 10 db) in the LOS link; highly disturbed conditions displayed in the entire Sodar range; the largest tropospheric effect on the satellite beacon system; and the most active superrefraction of the radar echoes. It was also noticed that these effects are obvious only during nights, while they remain masked or dampened in the sun-lit hours.  相似文献   

MHD instability due to non-homogeneity and non-uniformity of magnetic field in gravitating medium and its consequences in the central region of galaxies     

Z. Sarkar  R. Bondyopadhaya 《Astrophysics and Space Science》2007,312(3-4):215-225

The MHD wave instabilities due to non-uniform magnetic field and non-homogeneity of density have been studied. The reference (coordinate) system considered here is cylindrical type. The General Dispersion Relation (GDR) for the wave propagation in a gravitating but non-relativistic region has been derived. Similar to common knowledge, the said non-uniformities have been found to be responsible for the instability of the system. But interestingly many instability factors are produced due to presence of two types of non-uniformities simultaneously. This theory may add more clues for the event of instabilities, formation of hot plasma-bed in Galactic Central Region, and mass out-flow from there. Many conditions for instabilities could be obtained from GDR deduced here. However, a few conditions for critical wavelength of the MHD wave have been obtained in terms of system parameters (like gradient of magnetic field and rotation). This theory, in turn, may be helpful for the better understanding of the Explosion Theory of formation of outer structure of Galaxies like ours.  相似文献   

Cosmology and Astrophysics using the Post-Reionization HI     

Tapomoy Guha Sarkar  Anjan A. Sen 《Journal of Astrophysics and Astronomy》2016,37(4):33

We discuss the prospects of using the redshifted 21-cm emission from neutral hydrogen in the post-reionization epoch to study our Universe. The main aim of the article is to highlight the efforts of Indian scientists in this area with the SKA in mind. It turns out that the intensity mapping surveys from SKA can be instrumental in obtaining tighter constraints on the dark energy models. Cross-correlation of the HI intensity maps with the Ly α forest data can also be useful in measuring the BAO scale.  相似文献   

Analysis and quantification of contrasts in observed meteorological fields for foggy and non-foggy days   总被引：1，自引：0，他引：1  

Prashant Goswami  Sumana Sarkar 《Meteorology and Atmospheric Physics》2015,127(5):605-623

  相似文献   

The Kabanga Ni sulfide deposit, Tanzania: I. Geology, petrography, silicate rock geochemistry, and sulfur and oxygen isotopes   总被引：1，自引：0，他引：1  

Wolfgang D. Maier  Sarah-Jane Barnes  Arindam Sarkar  Ed Ripley  Chusi Li  Tim Livesey 《Mineralium Deposita》2010,45(5):419-441

The Kabanga Ni sulfide deposit represents one of the most significant Ni sulfide discoveries of the last two decades, with current indicated mineral resources of 23.23 Mt at 2.64% Ni and inferred mineral resources of 28.5 Mt at 2.7% Ni (Nov. 2008). The sulfides are hosted by a suite of ∼1.4 Ga ultramafic–mafic, sill-like, and chonolithic intrusions that form part of the approximately 500 km long Kabanga–Musongati–Kapalagulu igneous belt in Tanzania and Burundi. The igneous bodies are up to about 1 km thick and 4 km long. They crystallized from several compositionally distinct magma pulses emplaced into sulfide-bearing pelitic schists. The first magma was a siliceous high-magnesium basalt (approximately 13.3% MgO) that formed a network of fine-grained acicular-textured gabbronoritic and orthopyroxenitic sills (Mg# opx 78–88, An plag 45–88). The magma was highly enriched in incompatible trace elements (LILE, LREE) and had pronounced negative Nb and Ta anomalies and heavy O isotopic signatures (δ¹⁸O +6 to +8). These compositional features are consistent with about 20% contamination of primitive picrite with the sulfidic pelitic schists. Subsequent magma pulses were more magnesian (approximately 14–15% MgO) and less contaminated (e.g., δ¹⁸O +5.1 to +6.6). They injected into the earlier sills, resulting in the formation of medium-grained harzburgites, olivine orthopyroxenites and orthopyroxenites (Fo 83–89, Mg# _opx 86–89), and magmatic breccias consisting of gabbronorite–orthopyroxenite fragments within an olivine-rich matrix. All intrusions in the Kabanga area contain abundant sulfides (pyrrhotite, pentlandite, and minor chalcopyrite and pyrite). In the lower portions and the immediate footwall of two of the intrusions, namely Kabanga North and Kabanga Main, there occur numerous layers, lenses, and veins of massive Ni sulfides reaching a thickness of several meters. The largest amount of high grade, massive sulfide occurs in the smallest intrusion (Kabanga North). The sulfides have heavy S isotopic signatures (δ³⁴S wr = +10 to +24) that broadly overlap with those of the country rock sulfides, consistent with significant assimilation of external sulfur from the Karagwe–Ankolean sedimentary sequence. However, based partly on the relatively homogenous distribution of disseminated sulfides in many of the intrusive rocks, we propose that the Kabanga magmas reached sulfide saturation prior to final emplacement, in staging chambers or feeder conduits, followed by entrainment of the sulfides during continued magma ascent. Oxygen isotope data indicate that the mode of sulfide assimilation changed with time. The heavy δ¹⁸O ratios of the early magmas are consistent with ingestion of the sedimentary country rocks in bulk. The relatively light δ¹⁸O ratios of the later magmas indicate less bulk assimilation of the country rocks, but in addition the magmas selectively assimilated additional S, possibly through devolatization of the country rocks or through cannibalization of magmatic sulfides deposited in the conduits by preceding magma surges. The intrusions were tilted at ca. 1.37 Ga, during the Kibaran orogeny and associated synkinematic granite plutonism. This caused solid-state mobilization of ductile sulfides into shear zones, notably along the base of the intrusions where sulfide-hornfels breccias and lenses and layers of massive sulfides may reach a thickness of >10 m and can extend for several 10 s to >100 m away from the intrusions. These horizons represent an important exploration target for additional nickel sulfide deposits.  相似文献   

An experimental investigation to characterise soil macroporosity under different land use and land covers of northeast India   总被引：2，自引：0，他引：2  

Sangeeta Shougrakpam  Rupak Sarkar  Subashisa Dutta 《Journal of Earth System Science》2010,119(5):655-674

Saturated macropore flow is the dominant hydrological process in tropical and subtropical hilly watersheds of northeast India. The process of infiltration into saturated macroporous soils is primarily controlled by size, network, density, connectivity, saturation of surrounding soil matrix, and depthwise distribution of macropores. To understand the effects of local land use, land cover and management practices on soil macroporosity, colour dye infiltration experiments were conducted with ten soil columns (25 × 25 × 50 cm) collected from different watersheds of the region under similar soil and agro-climatic zones. The sampling sites included two undisturbed forested hillslopes, two conventionally cultivated paddy fields, two forest lands abandoned after Jhum cultivation, and two paddy fields, one pineapple plot and one banana plot presently under active cultivation stage of the Jhum cycle. Digital image analyses of the obtained dye patterns showed that the infiltration patterns differed significantly for different sites with varying land use, land cover, and cultivation practices. Undisturbed forest soils showed high degree of soil macroporosity throughout the soil profile, paddy fields revealed sealing of macropores at the topsoil due to hard pan formation, and Jhum cultivated plots showed disconnected subsoil macropores. The important parameters related to soil macropores such as maximum and average size of macropores, number of active macropores, and depthwise distribution of macropores were estimated to characterise the soil macroporosity for the sites. These experimentally derived quantitative data of soil macroporosity can have wide range of applications in the region such as water quality monitoring and groundwater pollution assessment due to preferential leaching of solutes and pesticides, study of soil structural properties and infiltration behaviour of soils, investigation of flash floods in rivers, and hydrological modelling of the watersheds.  相似文献   

First record of palynofossils from the Fulra Limestone Formation (Middle Eocene) of Kachchh basin,Gujarat and their palaeoenvironmental implications     

Samir Sarkar  J. P. Mandal 《Journal of the Geological Society of India》2014,83(6):641-646

The present paper deals with the first record of a palynofloral assemblage recovered from the Fulra Limestone Formation exposed at the confluence of Fulra and Panandhro Nalas in the west of Babia hill, Kachchh basin, Gujarat. The recovered palynoflora consists of dinoflagellate cysts, fungal spores and ascostromata, pteridophyte spores, gymnosperm and angiosperm pollen. Some of the important constituents of the palynofloral assemblage are: Lygodiumsporites, Polypodiaceasporites, Polypodiisporites, Margocolporites, Tricolporopilites, Tricolporopollis, Graminidites, Aplanosporites, Phragmothyrites, Spiniferites, Operculodinium and Achomosphaera. Palynological data suggest that the Fulra Limestone Formation was mostly laid down in a shallow marine environment under a warm and humid tropical climate. Abundance of terrestrial palynofossils in some of the samples clearly points towards the relative proximity of the shore. A late middle Eocene age has been assigned for the Fulra Limestone Formation on the basis of recorded palynofossils.  相似文献   

Holographic dark energy model with linearly varying deceleration parameter and generalised Chaplygin gas dark energy model in Bianchi type-I universe     

Sanjay Sarkar 《Astrophysics and Space Science》2014,349(2):985-993

The paper deals with a spatially homogeneous and anisotropic Bianchi type-I universe filled with two minimally interacting fluids; matter and holographic dark energy components. The nature of the holographic dark energy for Bianchi type-I space time is discussed. An exact solution to Einstein’s field equations in Bianchi type-I line element is obtained using the assumption of linearly varying deceleration parameter. Under the suitable condition, it is observed that the anisotropy parameter of the universe approaches to zero for large cosmic time and the coincidence parameter increases with increasing time. We established a correspondence between the holographic dark energy models with the generalised Chaplygin gas dark energy model. We also reconstructed the potential and dynamics of the scalar field which describes the Chaplygin cosmology. Solution of the field equations shows that a big rip type future singularity will occur for this model. It has been observed that the solutions are compatible with the results of recent observations.  相似文献   

Fisher Matrix-based Predictions for Measuring the <Emphasis Type="Italic">z</Emphasis> = 3.35 Binned 21-cm Power Spectrum using the Ooty Wide Field Array (OWFA)     

Anjan Kumar Sarkar  Somnath Bharadwaj  Sk. Saiyad Ali 《Journal of Astrophysics and Astronomy》2017,38(1):14

We use the Fisher matrix formalism to predict the prospects of measuring the redshifted 21-cm power spectrum in different k-bins using observations with the upcoming Ooty Wide Field Array (OWFA) which will operate at 326.5 MHz. This corresponds to neutral hydrogen (HI) at z = 3.35, and a measurement of the 21-cm power spectrum provides a unique method to probe the large-scale structures at this redshift. Our analysis indicates that a 5σ detection of the binned power spectrum is possible in the k range 0.05 ≤ k ≤ 0.3 Mpc^?1 with 1000 hours of observation. We find that the signal- to-noise ratio (SNR) peaks in the k range 0.1?0.2 Mpc^?1 where a 10σ detection is possible with 2000 hours of observations. Our analysis also indicates that it is not very advantageous to observe beyond 1000 h in a single field-of-view as the SNR increases rather slowly beyond this in many of the small k-bins. The entire analysis reported here assumes that the foregrounds have been completely removed.  相似文献   

Derivations of coupled equations appropriate to the lower regions of the ionosphere     

S. S. De  B. K. Sarkar  Bithika Ghosh  Manasi Mal  B. Ghosh  S. Bandyopadhyay  S. K. Adhikari  A. C. Sen 《Astrophysics and Space Science》1991,184(2):281-285

Coupled equations are derived for different ionospheric parameters through susceptibility dyadic appropriate to the medium. The nature of dispersion within the medium is studied.  相似文献