Showing papers by "Vikram Sarabhai Space Centre published in 2002"

Aerosol radiative forcing due to enhanced black carbon at an urban site in India

Abstract: [1] During a comprehensive aerosol field campaign, simultaneous measurements were made of aerosol spectral optical depths, black carbon mass concentration (Mb), total (Mt) and size segregated aerosol mass concentrations over an urban continental location, Bangalore (13° N, 77°E, 960 m msl), in India. Large amounts of BC were observed; both in absolute terms and fraction of total mass (∼11%) and sub-micron mass (∼23%) implying a significantly low single scatter albedo. The aerosol visible optical depth (τp) was in the range 0.24 to 0.45. Estimated surface forcing is as high as −23 W m−2 and top of the atmosphere (TOA) forcing is +5 W m−2 during relatively cleaner periods (τp ∼ 0.24). The net atmospheric absorption translates to an atmospheric heating of ∼0.8 K day−1 for cleaner periods and ∼1.5 K day−1 for less cleaner periods (τp ∼ 0.45). Our observations raise several issues, which may have impacts to regional climate and monsoon.

A pulsating auroral X-ray hot spot on Jupiter

Abstract: Jupiter's X-ray aurora has been thought to be excited by energetic sulphur and oxygen ions precipitating from the inner magnetosphere into the planet's polar regions1,2,3. Here we report high-spatial-resolution observations that demonstrate that most of Jupiter's northern auroral X-rays come from a ‘hot spot’ located significantly poleward of the latitudes connected to the inner magnetosphere. The hot spot seems to be fixed in magnetic latitude and longitude and occurs in a region where anomalous infrared4,5,6,7 and ultraviolet8 emissions have also been observed. We infer from the data that the particles that excite the aurora originate in the outer magnetosphere. The hot spot X-rays pulsate with an approximately 45-min period, a period similar to that reported for high-latitude radio and energetic electron bursts observed by near-Jupiter spacecraft9,10. These results invalidate the idea that jovian auroral X-ray emissions are mainly excited by steady precipitation of energetic heavy ions from the inner magnetosphere. Instead, the X-rays seem to result from currently unexplained processes in the outer magnetosphere that produce highly localized and highly variable emissions over an extremely wide range of wavelengths.

Aerosol black carbon over a tropical coastal station in India

Abstract: [1] Extensive ground based measurements of aerosol Black Carbon (BC) and size segregated aerosol mass concentration (total) were carried out (during August 2000 to October 2001) for the first time at a tropical coastal station, Trivandrum, India. BC depicted diurnal (with a nocturnal peak and mid-after noon low) and seasonal variations (highest during local winter and lowest during monsoon). The diurnal variations were closely associated with the boundary-layer dynamics. Daily mean BC concentration as high as ∼6 μg m−3 was observed on several occasions. Mass apportionment showed a BC share ranging from ∼3% to as high as ∼12% to the total aerosol mass, depending on the season. The presence of such large amount of BC over land might influence the regional aerosol forcing.

Effect of iodine doping on the bandgap of plasma polymerized aniline thin films

Abstract: Pure and iodine-doped polyaniline thin films are prepared by ac plasma polymerization technique. Doping of iodine is carried out in situ as well as by employing iodine chamber methods. The structural analyses of pure and iodine-doped polyaniline thin films are carried out by FTIR spectroscopic studies. Optical bandgaps of these films are evaluated from UV-VIS absorption studies. Direct and indirect transition energy gaps are determined from Tauc plots. The structural changes of polyaniline upon doping and the reduction of optical bandgap are explained on the basis of the results obtained from FTIR spectroscopic and UV-VIS absorption studies.

Chemical, microphysical, and radiative effects of Indian Ocean aerosols

Abstract: Received 19 April 2002; revised 2 August 2002; accepted 6 August 2002; published 14 December 2002. [1] Extensive and long-term multistation measurements of aerosol properties and radiative fluxes were carried out in the haze plume off the South Asian continent. These experiments are carried out at Kaashidhoo Climate Observatory (KCO) (4.95� N, 73.5� E), Minicoy (8.5� N, 73.0� E), and Trivandrum (8.5� N, 77.0� E). In addition, the top of the atmosphere fluxes were measured simultaneously by the CERES radiation budget instrument. Long-term observations (more than 15 years) over Trivandrum show that there is a gradual increase in aerosol visible optical depth from � 0.2 in 1986 to � 0.4 in 1999. Pre- and post-monsoon aerosol characteristics are examined to study the seasonal variations. The impact of aerosols on short-wave radiation budget is estimated using direct observations of solar radiation using several independent ground-based radiometers and satellite data as well as from modeled aerosol properties. It was observed that ‘‘excess absorption’’ is not needed to model diffuse fluxes. The lower atmospheric heating due to absorbing aerosols was as high as � 20 W m � 2 which translates to a heating rate perturbation of � 0.5� K/day. The effect of aerosol mixing state (internally and externally) on aerosol forcing appears to be negligible. A sensitivity study of the effect of aerosols over land in contrast to that over the ocean shows an enhancement in lower atmosphere heating by about 40% simultaneous with a reduction of � 33% in surface cooling. Increasing sea-surface winds increase aerosol cooling due to increased sea salt aerosol concentrations, which can partly offset the heating effect due to absorbing aerosols. INDEX TERMS: 0305 Atmospheric Composition and Structure: Aerosols and particles (0345, 4801); 1610 Global Change: Atmosphere (0315, 0325); 1704 History of Geophysics: Atmospheric sciences; 4801 Oceanography: Biological and Chemical: Aerosols (0305); KEYWORDS: aerosols, radiative forcing, climate, chemical composition

Characterization of low dielectric constant polyaniline thin film synthesized by ac plasma polymerization technique

Abstract: Polyaniline thin films were prepared by ac plasma polymerization technique. Capacitance, dielectric loss, dielectric constant and ac conductivity of these films were investigated in the frequency range from 100 Hz to 1 MHz and in the temperature range from 300 to 373 K. Capacitance and dielectric loss decreased with frequency and increased with temperature. This type of behaviour was found to be in good agreement with an existing model. The ac conductivity σ(ω) was found to vary as ωs with the index s≤1. Annealing of polyaniline thin films in high vacuum at 373 K for 1 h was found to reduce the dielectric loss. FTIR studies reveal that the aromatic ring is retained in the polyaniline thin films, which enhances the thermal stability of the polymer films.

Response of the equatorial ionosphere in the Indian (midnight) sector to the severe magnetic storm of July 15, 2000

Abstract: [1] During the severe magnetic storm of July 15, 2000, an impulsive and remarkably large downward movement of F region occurred simultaneously at locations throughout the equatorial region (dip 0.3–20°N) in the Indian sector, in close association with episodes of rapid ring current intensification. The abnormal midnight descent of equatorial F region (maximum amplitude close to the magnetic equator, 215 km/hr) indicative of a short-lived westward electric field disturbance (peak amplitude ≈4.6 mV/m) is interpreted as the signature of prompt penetration electric fields associated primarily with impulsive ring current injections. The westward electric field disturbance in the Indian (midnight) sector occurred near simultaneous with the eastward electric field disturbances in the dusk sector, reported by Basu et al. [2001a]. Moreover, the prompt electric field penetration to the magnetic equator both in the dusk and midnight sectors occurred in an environment already under the influence of ionospheric disturbance dynamo (IDD) electric fields, illustrating the profound manner in which the equatorial F region plasma dynamics can get modified globally during the main phase of severe magnetic storms.

Discovery of Soft X-Ray Emission from Io, Europa, and the Io Plasma Torus

Abstract: We report the discovery of soft (0.25-2 keV) X-ray emission from the Galilean satellites Io and Europa, probably Ganymede, and from the Io Plasma Torus (IPT). Bombardment by energetic (greater than 10 keV) H, O, and S ions from the region of the IPT seems to be the likely source of the X-ray emission from the Galilean satellites. According to our estimates, fluorescent X-ray emission excited by solar X-rays, even during flares from the active Sun, charge-exchange processes, previously invoked to explain Jupiter's X-ray aurora and cometary X-ray emission, and ion stripping by dust grains fail to account for the observed emission. On the other hand, bremsstrahlung emission of soft X-rays from nonthermal electrons in the few hundred to few thousand eV range may account for a substantial fraction of the observed X-ray flux from the IPT.

Melt rheology and morphology of LLDPE/EVA blends: Effect of blend ratio, compatibilization, and dynamic crosslinking

Abstract: The melt rheological properties of linear low-density polyethylene (LLDPE)/ethylene vinyl acetate (EVA) blends were investigated with special reference to the effect of blend ratio, temperature, shear rate, compatibilization, and dynamic vulcanization. The melt viscosity of the blends determined with a capillary rheometer is found to decrease with an increase of shear rate, which is an indication of pseudoplastic behavior. The viscosity of the blend was found to be a nonadditive function of the viscosities of the component polymers. A negative deviation was observed because of the interlayer slip between the polar EVA and the nonpolar LLDPE phases. The melt viscosity of these blends decreases with the increased concentration of EVA. The morphology of the extrudate of the blends at different shear rates and blend ratios was studied and the size and distribution of the domains were examined by scanning electron microscopy. The morphology was found to depend on shear rate and blend ratio. Compatibilization of the blends with phenolic- and maleic-modified LLDPE increased the melt viscosity at lower wt % of compatibilizer and then leveled off. Dynamic vulcanization is found to increase the melt viscosity at a lower concentration of DCP. The effect of temperature on melt viscosity of the blends was also studied. Finally, attempts were made to correlate the experimental data on melt viscosity and cocontinuity region with different theoretical models. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 86: 3210–3225, 2002

Poly(arylene ether nitriles)

Abstract: Polyaryl ethers are a well known class of engineering thermoplastics. The aromatic portion of the poly(arylene ether) is believed to contribute to thermal stability and good mechanical properties o...

Atmospheric Surface-Layer Processes During the Total Solar Eclipse Of 11 August 1999

Abstract: The central path of the total solar eclipse (TSE) of 11 August 1999 mostly passedthrough a region of active monsoon in India, with the eclipse ending around localsunset. Measurements in the surface layer (SL) were carried out close to the eclipseaxis at Akola (20°42' N, 77°2' E) in central India. The globalradiation flux reaching the surface vanishes around totality at 1803:24 (LT), followedby a small recovery before again dipping to zero at sunset. The temperatures in the SL, and subsoil at 50-mm depth, show a local minimum with a lag of about 10 min after the second contact, whereas the lag appears to vanish when the temperature series is detrended. The SL exhibits near-neutral, though generally stable, conditions from about 1500 hr itself due to heavy cloud cover followed by the eclipse-induced cooling of the surface. The wind component across the eclipse axis vanishes at totality, the wind vector aligning with the azimuth of the traversing moon shadow. The deceleration of the mean flow can be due to the combined effect of the colder surface and downward heat flux, where the locally altered horizontal temperature gradients may cause, as in this instance, the cessation of the cross flow. The oscillations in temperature and wind that show significant peaks, around 90–100 min as well as the semi-period of the eclipse near 60 min, persist for several hours past the eclipse event. A fresh analysis of the published data on the TSE of 7 March 1970 shows spectral peaks in the temperature nearly coincident with those already reported from the surface pressure records. The oscillations in the SL fields during the two TSE events are very similar implying that the source mechanism is also likely to be the same in both the cases.

Effects of solar eclipse on the electrodynamical processes of the equatorial ionosphere: a case study during 11 August 1999 dusk time total solar eclipse over India

Abstract: . The effects on the electrodynamics of the equatorial E- and F-regions of the ionosphere, due to the occurrence of the solar eclipse during sunset hours on 11 August 1999, were investigated in a unique observational campaign involving ground based ionosondes, VHF and HF radars from the equatorial location of Trivandrum (8.5° N; 77° E; dip lat. 0.5° N), India. The study revealed the nature of changes brought about by the eclipse in the evening time E- and F-regions in terms of (i) the sudden intensification of a weak blanketing ES-layer and the associated large enhancement of the VHF backscattered returns, (ii) significant increase in h' F immediately following the eclipse and (iii) distinctly different spatial and temporal structures in the spread-F irregularity drift velocities as observed by the HF radar. The significantly large enhancement of the backscattered returns from the E-region coincident with the onset of the eclipse is attributed to the generation of steep electron density gradients associated with the blanketing ES , possibly triggered by the eclipse phenomena. The increase in F-region base height immediately after the eclipse is explained as due to the reduction in the conductivity of the conjugate E-region in the path of totality connected to the F-region over the equator along the magnetic field lines, and this, with the peculiar local and regional conditions, seems to have reduced the E-region loading of the F-region dynamo, resulting in a larger post sunset F-region height (h' F) rise. These aspects of E-and F-region behaviour on the eclipse day are discussed in relation to those observed on the control day. Key words. Ionosphere (electric fields and currents; equatorial ionosphere; ionospheric irregularities)

Development and validation of a processing map for zirconium alloys

Abstract: For the development of processing maps to zirconium alloys, a simple instability condition based on the Ziegler's continuum principles as applied to large plastic flow is extended for delineating the regions of unstable metal flow/occurrence of fracture or defects, utilizing the flow stress data of Zr-2.5Nb-0.5Cu. An attempt is made to fit the measured flow stress data in a constitutive equation, useful in the finite element process models. Instability maps at different strain levels were superimposed while delineating the unstable regions in the processing maps. This phenomenon takes into account the dependence of strain rate sensitivity and strain hardening coefficient of the material on the plastic instability during hot deformation. The applicability of the developed processing map has been examined by comparing with the reported microstructural observations of the deformed compression specimens of various zirconium alloys. It is found that the processing map is practically usable in the real fabrication process for the zirconium alloys.

Numerical analysis of pressure oscillations over axisymmetric spiked blunt bodies at Mach 6.80

Abstract: The pressure oscillations over a forward facing spike attached to an axisymmetric blunt body are simulated by solving time-dependent compressible Navier–Stokes equations. The governing fluid flow equations are discretized in spatial coordinates employing a finite volume approach which reduces the equations to semidiscretized ordinary differential equations. Temporal integration is performed using the two-stage Runge–Kutta time stepping scheme. A global time step is used to obtain a time-accurate numerical solution. The numerical computation is carried out for a freestream Mach number of 6.80 and for spike length to hemispherical diameter ratios of 0.5, 1.0 and 2.0. The flow features around the spiked blunt body are characterized by a conical shock wave emanating from the spike tip, a region of separated flow in front of the hemispherical cap, and the resulting reattachment shock wave. Comparisons of the numerical results are made with the available experimental results, such as schlieren pictures and the surface pressure distribution along the spiked blunt body. They are found to be in good agreement. Spectral analysis of the computed pressure oscillations are performed employing fast Fourier transforms. The surface pressure oscillations over the spike and phase plots exhibit a behaviour analogous to that of the Van der Pol equation for a self-sustained oscillatory flow.

Evidence of intermediate layer characteristics in the Gadanki radar observations of the upper E region field-aligned irregularities

Abstract: [1] The Gadanki VHF radar observations of the upper E region field aligned irregularities are presented. These are the first observations of low-latitude upper E region irregularities that resemble the characteristics of intermediate layers observed over Arecibo. The most interesting aspect of these observations is their occurrence at altitudes as high as 160 km, which requires an interpretation in terms of their source mechanism. These irregularities were found to trigger about 21 LT first at higher altitude (140–160 km) and propagate downward with time. The signal intensities are found to be lower by about 12 dB and the Doppler spectra narrower by more than a factor of two compared to that of the normal E region echoes. The Doppler velocities are found to be both upward and downward with values less than 40 m s−1.

Synthesis, characterization and thermal properties of boron and silicon containing preceramic oligomers

Abstract: In recent years, considerable interest has been shown towards developing boron and silicon containing polymers as they possess certain technological advantages when compared to polymers containing only boron or silicon [1, 2]. These polymers on pyrolysis in inert atmosphere give mixed non-oxide ceramics such as B4CSiC, B4C-Si3N4 and BN-SiC. These mixed non-oxide ceramics are reported to possess superior high temperature properties, oxidation resistance and thermal shock resistance when compared to boron and silicon based simple non-oxide ceramics [1, 3, 4]. Boron and silicon containing polymers have also gained importance as protective coatings for materials which are constantly exposed to oxidizing environment [5–8]. When these coatings are exposed to oxidizing environment at elevated temperatures, a protective passive borosilicate glassy layer is formed on the surface which prevents further oxidation of the protective coating and the substrate. Boric acid is considered to be the cheapest source of boron for making boron and silicon containing polymers. Yajima and coworkers [9] synthesized poly (borodiphenylsiloxane) through the reaction of boric acid with diphenyldichlorosilane in n-butylether at 300 ◦C. Hoshii et al. [10–12] prepared borodiphenylsiloxane oligomer by reacting boric acid and diphenylsilanediol. However, these polymers give relatively low ceramic residue on pyrolysis (<50%) and also require high temperature (∼300◦) for their synthesis. In this communication, we report the synthesis of boron and silicon containing oligomers, capable of giving a high ceramic yield, through the condensation of boric acid with phenyltrimethoxysilane and vinyltriethoxysilane at a relatively low temperature (150–160 ◦C). A mixture containing 57 g (0.3 mol) of vinyltriethoxy-silane, 9.27 g (0.15 mol) of boric acid, 50 ml of diglyme and 1 ml of concentrated hydrochloric acid was taken in a three necked round bottom flask equipped with a mechanical stirrer, an inlet and outlet for nitrogen. The contents of the flask were heated under the flow of nitrogen in an oil bath maintained at 150–160 ◦C. The reaction mixture was stirred for 3 h at this temperature, after which it was allowed to cool to room temperature. Ethanol which was formed as a by product was distilled off. Diglyme was distilled off

Equatorial waves in temperature in the altitude range 4 to 70 km

Abstract: Using altitude profiles of temperature in the range 4 to 70 km derived from Mesosphere–Stratosphere–Troposphere radar and lidar observations at Gadanki (13.5°N,79.2°E) from 18 January 1999 to 5 March 1999, characteristics of equatorial waves are studied. Two-dimensional Fourier-transform analysis of the temperature profiles is carried out to identify the periodicities and their vertical wave numbers. From the characteristics obtained, equatorial slow Kelvin waves with periodicities 15.7 d, 9.4 d, 7.8 d and 6.7 d are identified in the troposphere and stratosphere regions and among these 7.8 d and 6.7 d periodicities are found to penetrate into the mesosphere. Equatorial waves with smaller periodicities in the range 5.2 d to 3.6 d are also observed. The vertical flux of horizontal momentum (zonal) of the identified slow Kelvin-wave periodicities in the altitude region 4–25 km is estimated. It is found that equatorial waves modulate tropical tropopause temperature and altitude. Copyright © 2002 Royal Meteorological Society.

Evidence of kilometer-scale waves in the lower E region from high resolution VHF radar observations over Gadanki

Abstract: [1] High resolution VHF radar observations made at Gadanki revealed the kilometer-scale waves in the low latitude lower E region. These waves are found to occur in a narrow altitude range of 92–97 km and have vertical wavelengths of ∼0.5 km and wave periods of ∼40 seconds. They resemble quite closely to that reported recently for midlatitude using the MU radar. The signal intensities associated with these waves, shown in SNR, are found to be as high as 25 dB, quite comparable to that observed at higher altitudes. It is in contrast to the much weaker midlatitude lower E region echoes reported by the MU radar. The wave structures show both positive and negative altitude rates with values in the range of 15–20 m s−1, which are in good agreement with the observed Doppler velocities. The spectral widths are found to be relatively lower, implying the lower E region wave structures to be less turbulent compared to their counterpart at higher altitudes. The observed wave structures bear remarkable resemblance to that simulated for the equatorial E region through nonlocal nonlinear gradient drift instability.