Showing papers by "Vikram Sarabhai Space Centre published in 2008"
TL;DR: In this article, the authors highlight the recent developments in the field of squaraine chemistry, and highlight the relevance of these dyes as a class of functional organic materials useful for electronic and photonic applications.
Abstract: This feature article highlights the recent developments in the field of squaraine chemistry Attempts have been made to address the relevance of squaraine dyes as a class of functional organic materials useful for electronic and photonic applications Due to the synthetic access of a variety of squaraine dyes with structural variations and due to the strong absorption and emission properties which respond to the surrounding medium, these dyes have been receiving significant attention Therefore, squaraine dyes have been extensively investigated in recent years, from both fundamental and technological viewpoints
358 citations
TL;DR: The Integrated Campaign for Aerosols, gases and Radiation Budget (ICARB) was carried out under the Geosphere Biosphere Programme of the Indian Space Research Organization (ISRO-GBP) as discussed by the authors.
Abstract: During March–May 2006, an extensive, multi-institution, multi-instrument, and multi-platform integrated field experiment ‘Integrated Campaign for Aerosols, gases and Radiation Budget’ (ICARB) was carried out under the Geosphere Biosphere Programme of the Indian Space Research Organization (ISRO-GBP). The objective of this largest and most exhaustive field campaign, ever conducted in the Indian region, was to characterize the physico-chemical properties and radiative effects of atmospheric aerosols and trace gases over the Indian landmass and the adjoining oceanic regions of the Arabian Sea, northern Indian Ocean, and Bay of Bengal through intensive, simultaneous observations. A network of ground-based observatories (over the mainland and islands), a dedicated ship cruise over the oceanic regions using a fully equipped research vessel, the Sagar Kanya, and altitude profiling over selected regions using an instrumented aircraft and balloonsondes formed the three segments of this integrated experiment, which were carried out in tandem. This paper presents an overview of the ICARB field experiment, the database generated, and some of its interesting outcomes though these are preliminary in nature. The ICARB has revealed significant spatio-temporal heterogeneity in most of the aerosol characteristics both over land and ocean. Observed aerosol loading and optical depths were comparable to or in certain regions, a little lower than those reported in some of the earlier campaigns for these regions. The preliminary results indicate: • low (< 0.2) aerosol optical depths (AOD) over most part of the Arabian Sea, except two pockets; one off Mangalore and the other, less intense, in the central Arabian Sea at ∼ 18 ◦ N latitude;
189 citations
22 Jan 2008
TL;DR: In this article, the effect of intramolecular charge transfer on the geometries and the vibrational modes contributing to the linear electro-optic effect of the organic NLO material was analyzed.
Abstract: A comprehensive investigation on the intramolecular charge transfer (ICT) of an efficient π-conjugated potential push–pull NLO chromophore, 4-[ N , N -dimethylamino]-4′-nitro stilbene (DANS), from a strong electron-donor group (dimethylamino- N (CH 3 ) 2 ) to a strong electron-acceptor group (nitro-NO 2 ) through the π-conjugated bridge ( trans -stilbene) has been carried out from their vibrational spectra. The NIR FT-Raman and FT-IR spectra supported by the density functional theory (DFT) quantum chemical computations have been employed to analyze the effects of intramolecular charge transfer on the geometries and the vibrational modes contributing to the linear electro-optic effect of the organic NLO material. It has been observed that the changes in the endocyclic and exocyclic angles result from the charge-transfer interaction of the phenyl ring and the amino group in the electron-donor side of the NLO chromophore. The strongest vibrational modes contributing to the electro-optic effect have been identified and examined from the concurrent IR and Raman activation of ν (C C/C–C) mode, ring C C stretching modes, in-plane deformation modes, nitro modes and the umbrella mode of methyl groups. Furthermore, the splitting of the vinyl stretching modes and the electronic effects such as hyperconjugation and backdonation on the methyl hydrogen atoms causing the decrease of stretching frequencies and infrared intensities have also been analyzed in detail. The effect of frontier orbitals transition of electron density transfer and the influence of planarity between the phenyl rings of the stilbene moiety on the first hyperpolarizability have also been discussed.
185 citations
TL;DR: In this paper, a wide-ranging multi-platform data from a major field campaign conducted over Indian region was used to estimate the energy absorbed in ten layers of the atmosphere and found that during pre-monsoon season, most of Indian region is characterized by elevated aerosol layers.
Abstract: Wide-ranging multi-platform data from a major field campaign conducted over Indian region was used to estimate the energy absorbed in ten layers of the atmosphere. We found that during pre-monsoon season, most of Indian region is characterized by elevated aerosol layers.Three-fold increase in aerosol extinction coefficient was observed at higher atmospheric layers (>2 km) compared to that near the surface and a substantial fraction (as much as 50 to 70%) of aerosol optical depth was found contributed by aerosols above (reflecting) clouds. Consequent absorption and hence strong warming above clouds was found larger by several degrees (K) compared to that near the surface. The aerosol-induced elevated warming was mostly confined below 2 km over northern Indian Ocean while found up to 4 km over central India, thus exhibiting strong meridional gradients (similar to 4 K) at atmospheric levels above 2 km. Climate implications of the large elevated warming are discussed.
170 citations
TL;DR: In this paper, the authors report the results of a multi-instrumental (ground-based and space-borne) and multistation study on the development/inhibition of postsunset ESF during five moderate to intense geomagnetic storms occurred during the low and descending phase of the solar activity period, 2004-2006.
Abstract: [1] Development or inhibition of ESF during magnetically active periods has been an important space weather topic of interest during the recent past in view of its applications in the satellite based navigational systems. Particularly, the postsunset period exhibits significant variability for storm time development of ESF versus longitude. In this paper, we report the results of a multi-instrumental (ground based and space-borne) and multistation study on the development/inhibition of postsunset ESF during five moderate to intense geomagnetic storms occurred during the low and descending phase of the solar activity period, 2004–2006. It has been observed that, the prompt penetration of eastward electric fields into low latitudes and subsequent development of ESF occurred in all longitudinal sectors where the local time corresponds to postsunset hours during the entire main phase of the storm. In this paper, we show the development of plasma bubble irregularities over a wide longitudinal extent of 92° owing to the dusk time penetration of eastward electric fields into low latitudes. Either the sudden increase in AE-index and/or a marked decrease in Sym-H index may be used as proxies to determine the occurrence as well as the time of penetration of electric fields into equatorial and low latitudes. However, in such cases where the AE-index does not represent any sudden increase, the dSymH/dt seems to be the better index to determine the time of penetration. In this paper, is also presented an interesting case where the prompt penetration eastward electric fields dominated the existing strong westward electric fields and subsequently caused the onset of spread-F and scintillations at both VHF (244 MHz) as well as L-band (1.5 GHz) frequencies.
93 citations
TL;DR: In this paper, the authors investigated the characteristics of aerosols in the marine atmospheric boundary layer (MABL) over the Bay of Bengal (BoB), northern Indian Ocean (NIO), and Arabian Sea (AS) using extensive measurements of the mass concentrations and mass size distributions of composite aerosols and mass concentrations of black carbon (BC) aerosols on board an oceanographic research vessel during its 2-month-long cruise of March-May 2006 as a part of Integrated Campaign for Aerosols, Gases, and Radiation Budget (ICARB).
Abstract: [1] Characteristics of aerosols in the marine atmospheric boundary layer (MABL) over the Bay of Bengal (BoB), northern Indian Ocean (NIO), and Arabian Sea (AS) have been investigated using extensive measurements of the mass concentrations and mass size distributions of composite aerosols and mass concentrations of black carbon (BC) aerosols on board an oceanographic research vessel during its 2-month-long cruise of March–May 2006 as a part of Integrated Campaign for Aerosols, Gases, and Radiation Budget (ICARB). The data have been examined for the spatial distribution of aerosols over the oceans around India, for the north-south (latitudinal) and east-west (longitudinal) gradients, and to delineate the distinctiveness of these two oceanic regions with regard to aerosol properties. The results show that in general, aerosol concentrations (total and BC) are significantly higher over the BoB, compared to the AS. Over the BoB, there exists a strong latitudinal gradient with the aerosol concentrations (of total and BC) decreasing rapidly from north to south. On the other hand, over the AS the north-south gradient was very weak and showed an opposite trend; with the concentrations, particularly of BC, increasing from north to south. In the northern BoB, due north of 13°N, BC showed strong longitudinal gradient, with the concentrations increasing from west to east; whereas due south, the longitudinal gradients were weak or insignificant. Over the southern AS, BC tended to decrease westward from the western coast of India, reached a minimum at 60°E, and increased thereafter. No longitudinal gradients were noticed in northern AS. Despite these, the BC mass fraction (FBC) was nearly constant spatially over the entire BoB, implying an almost uniformly mixed nature of BC with the composite aerosols over the BoB. In contrast, over the AS, FBC showed an increase southward along the latitude and decrease westward along the longitude. In general, the BC mass fraction was nearly 3 times higher over the BoB compared to the AS implying thereby that the aerosols over the BoB are significantly more absorbing in nature than those over the AS.
75 citations
TL;DR: In this paper, the authors investigated the vertical coupling in the low-latitude atmosphere-ionosphere system driven by the 5-day Rossby W1 and 6-day Kelvin E1 waves.
70 citations
15 Jan 2008-Materials Science and Engineering A-structural Materials Properties Microstructure and Processing
TL;DR: In this article, an analytical and experimental study was made to evaluate the notched tensile and pin bearing strength of the woven glass fiber reinforced (GFR)-epoxy composite laminates.
Abstract: Analytical and experimental studies were made to evaluate the notched tensile and pin bearing strengths of the woven glass fiber reinforced (GFR)-epoxy composite laminates. The symmetric glass/epoxy woven mat cross-ply laminates containing 16 woven mats were prepared using hand lay-up technique with 45% of volume fraction. The laminates were cut into specimens as per ASTM standards. The experiments were done at three different feed rates, spindle speeds and hole diameters to examine the influence of drilling parameters on the notched tensile strength and bearing strengths of the laminates. The design of experiments and analysis of variance (ANOVA) techniques of Taguchi, were utilized to study the statistical influence of the drilling parameters on the extension of delamination. The fracture data of center-hole tensile specimens were correlated using a modification in one of the stress fracture criteria viz., the point stress criterion (PSC). The bearing strength was correlated with notched and un-notched tensile strengths of the woven fabric composite laminates.
67 citations
TL;DR: In this article, the authors used the radial velocity variances in the 82-98 km height region, which are mainly caused by gravity waves, measured by the meteor radar to determine the gravity wave momentum fluxes.
Abstract: [1] Wind measurements from continuous meteor radar observations during June 2004–May 2007 are used to estimate the gravity wave momentum fluxes in the Mesosphere Lower Thermosphere (MLT) region over Trivandrum (8.5°N, 76.9°E), a low-latitude station in India. The radial velocity variances in the 82–98 km height region, which are mainly caused by gravity waves, measured by the meteor radar are used to determine the gravity wave momentum fluxes. Using a novel method proposed by Hocking (2005), altitude profiles of momentum fluxes of short period (less than 2–3 h) gravity waves are estimated for three continuous years. Seasonal variation in the gravity wave momentum fluxes showed semi annual variation with equinoctial maximum and solstitial minimum. By using estimated gravity wave momentum fluxes, an attempt is made to quantify their contribution in driving the Mesospheric Semi Annual Oscillation (MSAO). The mean flow acceleration estimated from the divergence of gravity wave momentum fluxes is compared with the observed mean flow acceleration computed from monthly mean zonal winds during six MSAO cycles over three years. This comparison reveled that the gravity wave contribution toward the westerly phase of MSAO varies from ∼20–60% while that toward the easterly phase varies from ∼30–70%. Variations are observed from cycle-to-cycle in the gravity wave forcing toward both phases of MSAO. The significance of the present study lies in estimating the gravity wave momentum fluxes in the MLT region and quantifying their contribution toward the generation of MSAO over the low-latitude for the first time.
65 citations
TL;DR: In this paper, the occurrence of an additional layer, called F3 layer, in the equatorial ionosphere at American, Indian, and Australian longitudes during the super double geomagnetic storm of 7-11 November 2004 is presented using observations and modeling.
Abstract: [1] The occurrence of an additional layer, called F3 layer, in the equatorial ionosphere at American, Indian, and Australian longitudes during the super double geomagnetic storm of 7–11 November 2004 is presented using observations and modeling. The observations show the occurrence, reoccurrence, and quick ascent to the topside ionosphere of unusually strong F3 layer in Australian longitude during the first super storm (8 November) and in Indian longitude during the second super storm (10 November), all with large reductions in peak electron density (Nmax) and total electron content (GPS-TEC). The unusual F3 layers can arise mainly from unusually strong fluctuations in the daytime vertical E × B drift as indicated by the observations and modeling in American longitude. The strongest upward E × B drift (or eastward prompt penetration electric field, PPEF) ever recorded (at Jicamarca) produces unusually strong F3 layer in the afternoon hours (≈1400–1600 LT) of PPEF, with large reductions in Nmax and TEC; the layer also reappears in the following evening (≈1700–1800 LT) owing to an unusually large downward drift. At night, when the drift is unusually upward and strong, the F region splits into two layers.
63 citations
TL;DR: In this article, the authors examined the spatio-temporal and spectral variations during the period of ICARB (March to May 2006) and found that the AOD and the derived Angstrom parameters showed considerable variations across India during the above period.
Abstract: Spectral aerosol optical depth (AOD) measurements, carried out regularly from a network of observatories spread over the Indian mainland and adjoining islands in the Bay of Bengal and Arabian Sea, are used to examine the spatio-temporal and spectral variations during the period of ICARB (March to May 2006). The AODs and the derived Angstrom parameters showed considerable variations across India during the above period. While at the southern peninsular stations the AODs decreased towards May after a peak in April, in the north Indian regions they increased continuously from March to May. The Angstrom coefficients suggested enhanced coarse mode loading in the north Indian regions, compared to southern India. Nevertheless, as months progressed from March to May, the dominance of coarse mode aerosols increased in the columnar aerosol size spectrum over the entire Indian mainland, maintaining the regional distinctiveness. Compared to the above, the island stations showed considerably low AODs, so too the northeastern station Dibrugarh, indicating the prevalence of cleaner environment. Long-range transport of aerosols from tshe adjoining regions leads to remarkable changes in the magnitude of the AODs and their wavelength dependencies during March to May. HYSPLIT back-trajectory analysis shows that enhanced long-range transport of aerosols, particularly from the west Asia and northwest coastal India, contributed significantly to the enhancement of AOD and in the flattening of the spectra over entire regions; if it is the peninsular regions and the island Minicoy are more impacted in April, the north Indian regions including the Indo Gangetic Plain get affected the most during May, with the AODs soaring as high as 1.0 at 500 nm. Over the islands, the Angstrom exponent (α) remained significantly lower (∼1) over the Arabian Sea compared to Bay of Bengal (BoB) (∼1.4) as revealed by the data respectively from Minicoy and Port Blair. Occurrences of higher values of α, showing dominance of accumulation mode aerosols, over BoB are associated well with the advection, above the boundary layer, of fine particles from the east Asian region during March and April. The change in the airmass to marine in May results in a rapid decrease in α over the BoB.
TL;DR: In this paper, high-resolution airborne measurements of the altitude profiles of the mass concentrations (MB) of aerosol black carbon (BC) were made off Bhubaneswar (BBR, 85.82°E, 20.25°N), over northwest Bay of Bengal, in the altitude region upto 3 km.
Abstract: During the Integrated Campaign for Aerosols, gases and Radiation Budget (ICARB) over India, high-resolution airborne measurements of the altitude profiles of the mass concentrations (MB) of aerosol black carbon (BC) were made off Bhubaneswar (BBR, 85.82°E, 20.25°N), over northwest Bay of Bengal, in the altitude region upto 3 km. Such high-resolution measurements of altitude profiles of aerosols are done for the first time over India. The profiles showed a near-steady vertical distribution of MB modulated with two small peaks, one at 800m and the other at ∼2000m. High resolution GPS (Global Positioning System) sonde (Vaisala) measurements around the same region onboard the research vessel Sagar Kanya (around the same time of the aircraft sortie) revealed two convectively well mixed layers, one from ground to ∼700m with an inversion at the top and the other extends from 1200m to ∼2000m with a second inversion at ∼2200m and a convectively stable region in the altitude range 700–1200m. The observed peaks in the MB profile are found to be associated with these temperature inversions. In addition, long-range transport from the Indo-Gangetic Plain (IGP) and deserts lying further to the west also influence the vertical profile of BC. Latitudinal variation of MB showed a remarkable land ocean contrast at the 500m altitude (within the well mixed region) with remarkably lower values over oceans, suggesting the impact of strong sources over the mainland. However, above the ABL (at 1500m), the latitudinal variations were quite weak, and this appears to be resulting from the impact of long-range transport. Comparison of the altitude profiles of MB over BoB off BBR with those obtained during the earlier occasion over the inland stations of Hyderabad and Kanpur showed similarities above ∼500m, with MB remaining around a steady value of ∼1 μg m−3. However, large differences are seen within the ABL. Even though the observed MB values are not unusually high, their near constancy in the vertical column will have important implications to radiative forcing.
TL;DR: The chemical composition of near surface aerosols at the tropical coastal location Trivandrum (8.55°N, 77°E) has been studied by analyzing the aerosol samples collected during a period of 16 months from October 2003 to January 2005 as mentioned in this paper.
Abstract: [1] Chemical composition of near-surface aerosols at the tropical coastal location Trivandrum (8.55°N, 77°E) has been studied by analyzing the aerosol samples collected during a period of 16 months from October 2003 to January 2005. The annual mean aerosol mass loading at this site averages to around 54 ± 19 μg m−3. The chemical analysis of samples revealed Cl−, SO42−, and NO3− to be the major anionic species and Na, NH4+, Fe, and Ca to be the major cationic species. The total mass loading and the mass concentration of various chemical species showed pronounced seasonal dependence. The active aerosol sources at this location have been investigated in detail to delineate their relative dominance in different seasons. Ions like Na, Cl, Mg, and K, which are mainly of oceanic origin, showed peak during monsoon, indicating sea spray to be the most important source mechanism during the period. On the other hand, mass loading of SO42−, NH4+, PO43−, Fe, and Al and that of trace elements exhibited their maximum in winter/summer, indicating the dominance of continental/crustal sources during the period. The sea salt and non-sea salt components of individual ions like SO42−, K, Ca, and Mg also revealed distinctly different seasonal patterns. In spite of being a coastal site, the non-sea salt components dominated at this location. Dominance of non-sea salt SO42− throughout the year indicates significant anthropogenic influence at this location. On the basis of this study, a first-cut model for the chemical composition of aerosols at this tropical coastal site has been evolved.
TL;DR: In this article, a thermodynamic shape memory polymers based on polyurethane with soft segment consisting of poly (tetramethylene oxide) (PTMO) and hard segment arising from urethane reaction of tolylene diisocyanate with OH terminal of PTMO and 1,4 butane diol (BD) were synthesized by a two-step process.
Abstract: Thermoresponsive shape memory polymers based on polyurethane with soft segment consisting of poly (tetramethylene oxide) (PTMO) and hard segment arising from urethane reaction of tolylene diisocyanate with OH terminal of PTMO and 1,4 butane diol (BD) were synthesized by a two- step process. The molar ratios of the reactants were varied to get polymers of different soft – hard segment contents. These were characterized by DSC, IR, DMTA, XRD, SEM analyses, and mechanical properties. The shape memory behavior was evaluated by cyclic tensile tests. PTMO served as the switching segments whose crystalline melting was responsible for the switching behavior. As the hard segment-content increased, transition temperature (Ttrans) diminished and so did the elongation and tensile strength of the polyurethane. Higher glassy/rubbery modulus ratio observed for higher hard segment-content polymer was conducive to better shape recovery properties. The influence of hard/soft segments on the thermal, mechanical, damping and shape memory properties of the copolymers are presented and correlated to their phase morphology, as investigated by FTIR and SEM. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2008
TL;DR: In this paper, the authors investigated the mechanical strength and modulus of carbon fiber-reinforced polybenzoxazine composites by changing the length of CFs and found that the increase in strength of composites with fiber length is attributed to the enhancement in effective contact area of fibers with the matrix.
Abstract: The mechanical strength and modulus of chopped carbon fiber (CF)-reinforced polybenzoxazine composites were investigated by changing the length of CFs. Tensile, compressive, and flexural properties were investigated. The void content was found to be higher for the short fiber composites. With increase in fiber length, tensile strength increased and optimized at around 17 mm fiber length whereas compressive strength exhibited a continuous diminution. The flexural strength too increased with fiber length and optimized at around 17 mm fiber length. The increase in strength of composites with fiber length is attributed to the enhancement in effective contact area of fibers with the matrix. The experimental results showed that there was about 350% increase in flexural strength and 470% increase in tensile strength of the composites with respect to the neat polybenzoxazine, while, compressive properties were adversely affected. The composites exhibited an optimum increase of about 800% in flexural modulus and 200% in tensile modulus. Enhancing the fiber length, leads to fiber entanglement in the composites, resulted in increased plastic deformation at higher strain. Multiple branch matrix shear, debonded fibers and voids were the failures visualized in the microscopic analyses. Defibrillation has been exhibited by all composites irrespective of fiber length. Fiber debonding and breaking were associated with short fibers whereas clustering and defibrillation were the major failure modes in long fiber composites. Increasing fiber loading improved the tensile and flexural properties until 50–60 wt% of fiber whereas the compressive property consistently decreased on fiber loading. Copyright © 2008 John Wiley & Sons, Ltd.
TL;DR: In this paper, the authors used a ground-based multi-wavelength solar radiometer (MWR) from October 2001 to February 2006 to estimate spectral aerosol optical depth (AOD) at ten discrete channels in the visible and near IR regions.
Abstract: Spectral aerosol optical depth (AOD) at ten discrete channels in the visible and near IR regions were estimated over Dibrugarh, located in the northeastern part of India, using a ground-based multi-wavelength solar radiometer (MWR) from October 2001 to February 2006. The observations reveal seasonal variations with low values of AODs in retreating monsoon and high values in the pre-monsoon season. Generally the AODs are high at shorter wavelengths and low at longer wavelengths. AOD spectra are relatively steep in winter compared to that in the monsoon period. The average value of AOD lies between 0.44±0.07 and 0.56±0.07 at 500 nm during the pre-monsoon season and between 0.19±0.02 and 0.22±0.02 during re-treating monsoon at the same wavelength. Comparison of MWR observation on Dibrugarh with satellite (MODIS) observation indicates a good correspondence between ground-based and satellite derived AODs. The synoptic wind pattern obtained from National Centre for Medium Range Weather Forecasting (NCMRWF), India and back trajectory analysis using the NOAA Hybrid Single-Particle Lagrangian Integrated Trajectory (HYSPLIT4) Model indicates that maximum contribution to aerosol extinction could be due to transport of pollutants from the industrialized and urban regions of India and large amounts of desert and mineral aerosols from the west Asian and Indian desert. Equal contributions from Bay-of-Bengal (BoB), in addition to that from the Indian landmass and west Asian desert leads to a further increase of AOD over the region of interest in the pre-monsoon seasons.
TL;DR: In this paper, the theoretical spectra of Jovian X-ray aurora were derived from the Chandra Xray telescope using Monte Carlo simulations of the energy and charge relaxation of precipitating ions.
Abstract: Spectra of Jovian X-ray auroras observed from the North and South poles with the Chandra X-ray telescope are analyzed and compared with predicted spectra of the charge-exchange mechanism. To determine the theoretical spectra of Jovian X-ray auroras, we model numerically the collisionally induced evolution of energy and charge distributions of Oq+ and Sq+ ions, precipitating into the Jovian atmosphere. Monte Carlo simulations of the energy and charge relaxation of the precipitating ions are carried out with updated cross-sections of the ion stripping, electron capture, and gas-ionization collisions. X-ray and Extreme Ultraviolet (EUV) spectra of cascading radiation induced by individual energetic sulfur and oxygen ions are calculated, and relative intensities of X-ray emission lines are determined. Synthetic spectra of X-ray and EUV photons are computed at different initial kinetic energies and compositions of ion-precipitating fluxes. Theoretical spectra with adjustable initial energies and relative fraction of sulfur and oxygen ions are shown to be in good agreement with the spectra of X rays detected from the South and North polar regions. The abundances and initial energies of the precipitating ions are inferred by comparing synthetic and observed X-ray spectra. Comparisons are performed independently for the North and South pole emissions. Abundances ofmore » the precipitating sulfur ions are found to be four to five times smaller than those of oxygen ions, and averaged ion energies are determined to lie between 1 and 2 MeV/amu. Slightly different ion flux compositions are found to describe the observed spectra of X-ray emission from the North and South poles.« less
TL;DR: In this article, columnar optical depth and near-surface mass concentration of aerosols over Visakhapatnam, an urban location along the east coast of India during the summer monsoon period (May-August 2005), were measured simultaneously along with chemical sampling for water-soluble ionic species (NH4+, Na+, K+, Mg2+, Ca2+, Cl−, NO3−, SO42−, and HCO3−).
Abstract: [1] Columnar optical depth and near-surface mass concentration of aerosols over Visakhapatnam, an urban location along the east coast of India during the summer monsoon period (May–August 2005), were measured simultaneously along with chemical sampling for water-soluble ionic species (NH4+, Na+, K+, Mg2+, Ca2+, Cl−, NO3−, SO42−, and HCO3−). The mean aerosol optical depth (AOD) (0.5 μm) and Angstrom parameters (α, β) during this period were obtained as 0.72 ± 0.39 and 0.88 ± 0.39, 0.48 ± 0.36, respectively. The total surface aerosol mass concentration varied from 95 to 128 (μg/m3), out of which coarse mode dominated by 45%. While Cl−, Na+, K+, and Mg2+ (sea salts) contributed nearly 56%, SO42− and NO3− (anthropogenic constituents) contributed 33% in surface aerosol constituents. During this period, high spectral variability in AOD, negative curvature of second-order Angstrom coefficient (α1), abundance of columnar submicron aerosols, role of air mass trajectories as tracers of long-range transport, cation deficiency, and sea-salt dependence on wind speed are some of the observations over Visakhapatnam. The synergy of the results from these complementary measurements can be reflected while computing the aerosol radiative forcing.
TL;DR: In this paper, the authors describe the mechanical and thermal characterisation of cross-linked GAP and GAP-hydroxyl terminated polybutadiene (HTPB) networks.
Abstract: This paper describes the mechanical and thermal characterisation of cross-linked glycidyl azide polymer (GAP) and GAP–hydroxyl terminated polybutadiene (HTPB) networks. Cross-linked GAP and GAP–HTPB networks were prepared by reacting GAP diol and GAP–HTPB diol mixture with different diisocyanates. The physical and mechanical characteristics were found to be influenced by the type of isocyanate curing agents, [NCO]/[OH] equivalent ratios and concentration of GAP. For all the three types of curing agents, GAP–HTPB blends of 50 : 50 to 30 : 70 ratios show higher mechanical strength over the virgin networks of GAP or HTPB. Thermal decomposition of cross-linked GAP–HTPB networks was evaluated by thermogravimetric analysis (TGA). The kinetic parameters for the decomposition of GAP–HTPB blends were found to be dependant on the concentration of GAP and HTPB in the blend. The cross-linked GAP–HTPB blends were subjected to dynamic mechanical analysis (DMA). The glass transition characteristics of the blends were evaluated by DMA and it was found that blends prepared with GAP content up to 30% showed single transition in the loss tangent trace indicating no phase separation in the cured network.
TL;DR: In this paper, the effect of microballoon concentration on tensile, compressive, and flexural strengths of polybenzoxazine foams was studied, and the specific tensile and compressive properties were optimized at about 68% by volume of micro-balloon, while flexural strength decreased marginally on increasing the micro balloon content.
Abstract: Syntactic foams of polybenzoxazine, containing moderately high volume percentage of glass microballoons, were prepared. The specific gravity decreased with increase in microballoon content. The disproportionate decrease in specific gravity was ascribed to entrapment of air voids during compaction. The high content of microballoon increased the possibility for air voids that tended to get accumulated. The effect of microballoon concentration on tensile, compressive, and flexural strengths of the foams was studied. Tensile and compressive properties were optimized at about 68% by volume of microballoon while flexural strength decreased marginally on increasing the microballoon content. Althought the specific tensile and compressive strength showed a maximum followed by a decrease, the specific flexural strength systematically increased with microballoon content. The increased packing density of syntactic foam of a given constituent composition increased the compressive strength. The property variation was corroborated by morphological features, as evidenced in scanning electron micrographs. The syntactic foams showed “multiple resin-neck formation” and “disc-shaped microballoon regions.” The crushing of microballoons during molding was inevitable when compaction was effected to achieve a density beyond the theoretical one. Low-density syntactic foams tend to fail at lower loads because of fracturing of microballoons. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008
TL;DR: In this article, the authors deduced single scattering albedos (SSA) from extensive and collocated measurements of spectrally resolved aerosol scattering (σ sca ) and absorption (σ abs ) coefficients, carried out for the first time over the entire Bay of Bengal (BoB).
Abstract: [1] Aerosol single scattering albedos (SSA) were deduced from extensive and collocated measurements of spectrally resolved aerosol scattering (σ sca ) and absorption (σ abs ) coefficients, carried out for the first time over the entire Bay of Bengal (BoB). Notwithstanding the high values of σ sca , and σ abs , comparatively higher values of SSA were noticed over the head BoB (north of 16°N) and coastal regions while lower values of SSA persisted over the Central BoB indicating the dominance of absorbing aerosols far away from continental source regions. At mid visible wavelength (550 nm) SSA ranged from 0.84 to 0.96 over different parts of the BoB showing large heterogeneity. However, more than 80% of the values lay between 0.9 and 0.95. Spectral variation of SSA was distinctly different over the northern and southern BoB.
TL;DR: In this paper, the long term variations of low-latitude mesospheric semi-annual oscillation and quasi-biennial oscillation in the zonal wind are studied using observations available from 1977 to 2006.
Abstract: [1] Long term variations of low-latitude mesospheric semi-annual oscillation (MSAO) and quasi-biennial oscillation (MQBO) in the zonal wind are studied using observations available from 1977 to 2006. The MQBO showed large inter-annual variability in the phase difference between 77.5 km and 72.5 km indicating such variability in eddy viscosity. Association of these oscillations with stratospheric quasi-biennial oscillation (SQBO) is investigated. In general, the MQBO at 77.5 km is out-of-phase with SQBO and this relation fails during strong and long SQBO. The MSAO at 72.5 km and 77.5 km generally shows strong westward phase during strong eastward phase of SQBO. The most striking feature observed in the present study is the strong modulation of the MSAO by MQBO revealing a non-linear type of interaction with presence of sidebands of the MSAO.
TL;DR: In this paper, the altitude of major convective outflow in the troposphere is identified and is considered to represent the convective tropopause, which is found to match well with the local minimum of potential temperature lapse rate obtained from simultaneous radiosonde observations.
Abstract: [1] Using the MST radar observations at Gadanki (13.5°N, 79.2°E), a tropical station, the altitude of major convective outflow in the troposphere is identified and is considered to represent the convective tropopause. This is found to match well with the altitude of local minimum of potential temperature lapse rate obtained from simultaneous radiosonde observations. The convective tropopause altitudes are also compared with the cloud top altitudes obtained using satellite brightness temperature (BT) data and are found to match in the case of deep convection. The thickness of the tropical tropopause layer follows very closely the convective tropopause altitude and has little dependence on the cold point tropopause altitude. The thickness of the tropopause layer is found to shrink when convection reaches high altitudes. This occurs mainly during the monsoon months of July, September, and October.
TL;DR: In this article, the authors investigated the effect of boundary layer blockage on the internal flow choking in a dual-thrust motor and found that the blockage is greater in magnitude for the choked case than for the unchoked case.
Abstract: Theoretical studies have been carried out to examine internal flow choking in the inert simulators of a dual-thrust motor. Using a two-dimensional k-omega turbulence model, detailed parametric studies have been carried out to examine aerodynamic choking and the existence of a fluid throat at the transition region during the startup transient of dual-thrust motors. This code solves standard k-omega turbulence equations with shear flow corrections using a coupled second-order-implicit unsteady formulation. In the numerical study, a fully implicit finite volume scheme of the compressible, Reynolds-averaged, Navier-Stokes equations is employed. It was observed that, at the subsonic inflow conditions, there is a possibility of the occurrence of internal flow choking in dual-thrust motors due to the formation of a fluid throat at the beginning of the transition region induced by area blockage caused by boundary-layer-displacement thickness. It has been observed that a 55% increase in the upstream port area of the dual-thrust motor contributes to a 25% reduction in blockage factor at the transition region, which could negate the internal How choking and supplement with an early choking of the dual-thrust motor nozzle. If the height of the upstream port relative to the motor length is too small, the developing boundary layers from either side of the port can interact, leading to a choked,flow. On the other hand, if the developing boundary layers are far enough apart, then choking does not occur. The blockage factor is greater in magnitude for the choked case than for the unchoked case. More tangible explanations are presented in this paper for the boundary-layer blockage and the internal flow choking in dual-thrust motors, which hitherto has been unexplored.
TL;DR: In this paper, a vibrational spectral analysis of the novel nonlinear optical (NLO) material, Glycine Sodium Nitrate (GSN) is carried out using NIR FT-Raman and FT-IR spectroscopy, supported by Density Functional Theoretical (DFT) computations to derive equilibrium geometry, vibrational wave numbers and first hyperpolarizability.
TL;DR: In this paper, the optical and physical properties of columnar and near-surface aerosols were carried out from Manora Peak, Nainital (a sparsely inhabited, high altitude location, ∼2 km above mean sea level, in the Himalayas), during the Integrated Campaign for Aerosols, gases and Radiation Budget (ICARB) under the Geosphere Biosphere Programme of the Indian Space Research Organization (ISRO-GBP).
Abstract: Collocated measurements of the optical and physical properties of columnar and near-surface aerosols were carried out from Manora Peak, Nainital (a sparsely inhabited, high altitude location, ∼2 km above mean sea level, in the Himalayas), during the Integrated Campaign for Aerosols, gases and Radiation Budget (ICARB) under the Geosphere Biosphere Programme of the Indian Space Research Organization (ISRO-GBP). Under this, observational data of spectral aerosol optical depths (AOD), mass concentration of aerosol black carbon (MB), mass concentration (MT) and number concentration (Nt) of composite (total) aerosols near the surface and meteorological parameters were collected during the period February 15 to April 30, 2006. Though very low (<0.1 at 500 nm) AODs were observed during clear days, as much as a four-fold increase was seen on hazy days. The Angstrom exponent (α), deduced from the spectral AODs, revealed high values during clear days, while on hazy days α was low; with an overall mean value of 0.69 ± 0.06 for the campaign period. BC mass concentration varied between 0.36 and 2.87 μg m−3 and contributed in the range 0.7 to 1.8% to the total aerosol mass. Total aerosol number concentration and BC mass concentration showed diurnal variation with a midnight and early morning minimum and a late afternoon maximum; a pattern quite opposite to that seen in low altitude stations. These are attributed to the dynamics of the atmospheric boundary layer.
TL;DR: In this article, extensive and collocated measurements of several aerosol parameters were made over the eastern Arabian Sea, during the inter-monsoon and summer monsoon seasons of 2003 as a part of the Arabian Sea Monsoon Experiment (ARMEX).
TL;DR: In this paper, the properties of polybenzoxazine filled with chopped silica fibers and their syntactic foams of varying composition and densities were investigated, including tensile, compressive, and flexural properties with change in composition.
Abstract: Polybenzoxazine filled with chopped silica fibers and their syntactic foams of varying composition and densities were processed. The composition and density variations were achieved by regulating the relative concentrations of silica fiber and glass microballoons (MB). The variation of tensile, compressive, and flexural properties with change in composition was investigated. For the silica-fiber filled materials, the property attained a maximum at about 40% volume content of fiber, and thereafter, the properties showed a diminishing trend. The incorporation of microspheres significantly lowered the strength of silica filled materials. However, the decrease in the specific flexural strength was less pronounced and the strength was unaffected beyond a certain microsphere content. During processing and mechanical testing, a large quantity of fibers was fractured, which reduced the strength of silica filled systems. The diminution in material strength on embedding microspheres is attributed to the presence of stress concentrating loci as evidenced from SEM analysis. The various factors leading to the property variation with composition are discussed with microscopic analyses, like clustering of fibers, crack propagation, fiber pull out, and debonding of fibers from resin phase. Dynamic mechanical analysis revealed an improved damping property for the filled materials in contrast to the unfilled polymer. The Tg (deduced from tan δ maximum from DMTA) decreased in silica fiber containing materials and on incorporating the MB, the values reverted back to that of the neat polymer. Both silica and MB conferred better thermal and thermooxidative stabilities to the polybenzoxazine. However, the degradation mechanism is nonoxidative in nature. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2008
TL;DR: In this paper, the second-order kinetics of hydroxyl-terminated glycidyl azide polymer (GAP) and different isocyanate curatives (TDI, IPDI, and MDCI) at various temperatures viz. 30, 40, 50, and 60°C were investigated.
Abstract: Reactions between hydroxyl-terminated glycidyl azide polymer (GAP) and different isocyanate curatives such as toluene diisocyanate (TDI), isophorone diisocyanate (IPDI), and methylene diicyclohexyl isocyanate (MDCI) at various temperatures viz. 30, 40, 50, and 60°C were followed by Fourier transform infra red spectroscopy. The reactions were found to follow second-order kinetics. With TDI and IPDI at 30°C, a two-stage reaction was observed. For GAP-TDI system, the second stage was slower than the first while for GAP-IPDI system, the second stage was faster than the first indicating dominance of autocatalytic effect. The stage separation occurred due to the difference in reactivity of the isocyanate groups and was found to narrow down with increase in temperature. The viscosity build up due to the curing reaction was followed for GAP-TDI system for comparison. The stage separation was evident in the viscosity build up also. Rheokinetic analysis done based on data generated showed a linear correlation between viscosity build up and fractional conversion. The kinetic and activation parameters evaluated from the data showed the relative difference in reactivity of the three diisocyanates with GAP. Both the approaches suggested that the reactivity of the isocyanates employed for the present study could be arranged as TDI > IPDI ≫ MDCI. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008
TL;DR: In this article, the authors present simultaneous observations of temporal and spatial variability of total electron content (TEC) and GPS amplitude scintillations on L 1 frequency (1.575 GHz) during the time of equatorial spread F (ESF) while the MST radar (53 MHz) located at Gadanki (13.5° N, 79.2° E, Dip latitude 6.3° N), a low latitude station, made simultaneous observations.
Abstract: . In this paper, we present simultaneous observations of temporal and spatial variability of total electron content (TEC) and GPS amplitude scintillations on L1 frequency (1.575 GHz) during the time of equatorial spread F (ESF) while the MST radar (53 MHz) located at Gadanki (13.5° N, 79.2° E, Dip latitude 6.3° N), a low latitude station, made simultaneous observations. In particular, the latitudinal and longitudinal extent of TEC and L-band scintillations was studied in the Indian region for different types of ESF structures observed using the MST radar during the low solar activity period of 2004 and 2005. Simultaneous radar and GPS observations during severe ESF events in the pre-midnight hour reveal that significant GPS L band scintillations, depletions in TEC, and the double derivative of the TEC index (DROTI), which is a measure of fluctuations in TEC, obtained at low latitudes coincide with the appearance of radar echoes at Gadanki. As expected, when the irregularities reach higher altitudes as seen in the radar map during pre-midnight periods, strong scintillations on an L-band signal are observed at higher latitudes. Conversely, when radar echoes are confined to only lower altitudes, weak scintillations are found and their latitudinal extent is small. During magnetically quiet periods, we have recorded plume type radar echoes during a post-midnight period that is devoid of L-band scintillations. Using spectral slopes and cross-correlation index of the VHF scintillation observations, we suggest that these irregularities could be "dead" or "fossil" bubbles which are just drifting in from west. This scenario is consistent with the observations where suppression of pre-reversal enhancement (PRE) in the eastward electric field is indicated by ionosonde observations of the height of equatorial F layer and also occurrence of low spectral width in the radar observations relative to pre-midnight period. However, absence of L-band scintillations during post-midnight event, when radar observed plume like structures and scintillations were recorded on VHF band, raises questions about the process of evolution of the irregularities. A possible explanation is that whereas small scale (~3 m) irregularities are generated through secondary waves that grow on the walls of km scale size irregularities, in this case evolution of the Rayleigh-Taylor instability itself did not extend to irregularities of scale sizes of a few hundred meters that produce scintillation on a L-band signal.