Optical interferometric observations of a transient event of 1986 March 13 in the coma of Comet Halley     

C. Debi Prasad  T. Chandrasekhar  J. N. Desai  N. M. Ashok  Vinod Krishan 《Journal of Astrophysics and Astronomy》1989,10(1):1-20

During the recent apparition of Comet Halley in 1985-86 a transient ionic event in the form of a blob of H₂ O⁺ emission was recorded in thecoma at ～ 0^h UT on 1986 March 13. Observations were carried out using a special IHW filter for H₂ O⁺ emission at 7000 å/175 å, a 35 cm telescope, a Fabry-Perot interferometer and an image intensifier camera from Gurushikhar, Mt Abu. (24?39’N,72? 47’E). A Fabry-Perot inter-ferogram in Ha taken a few minutes later at the same location reveals strong hydrogen emission (Hα) associated with the blob. The velocity field in the blob is structured with relative velocities upto ～ 35km s^?1. The event is interpreted as arising due to the sector boundary crossing of the interplanetary magnetic field by the comet  相似文献   

Room temperature solid-state detectors for Gamma ray burst spectroscopy     

U. D. Desai  J. Pantazis  K. Shah 《Astrophysics and Space Science》1995,231(1-2):487-491

We have evaluated several solid state detectors which offer excellent energy resolution at room temperature for soft X-rays. For soft X-rays (< 1 keV to 20 keV), silicon P-intrinsic-N (PIN) and avalanche-mode photodiodes (APD's) have been studied. Using commercially available charge sensitive pre-amplifiers, these photodiodes provide 1 keV resolution without cooling. Their detection efficiencies are limited to about 20 keV and 15 keV, respectively. To overcome this constraint, we have studied thick (1.5 mm) PIN detectors made by Micron Semiconductor Ltd., U.K., as well as compound semiconducting materials with high Z constituents such as CZT and PbI₂. PbI₂ allows high detection efficiencies of photons up to 100 keV with detectors 100–300 microns thick. These new detectors offer the capability to study the low-energy spectral evolution of Gamma ray bursts (GRBs). A matrix of these detectors could be used as an image plane detector with moderate spatial resolution for imaging.  相似文献   

Near-infrared observations of the peculiar binary system epsilon Aurigae     

H. C. Bhatt  T. Chandrasekhar  N. M. Ashok  J. N. Desai 《Astrophysics and Space Science》1984,104(2):293-296

Near-infrared photometric and low-resolution spectroscopic observations of Aurigae at two phases during the current eclipse are presented. The eclipse depths are found to be wavelength-independent in the infrared right up to 2.5 m. There is no infrared excess at wavelengths shorter than 2.5 m. The light absorbing particles in the eclipsing body must be larger than 10 m in size.  相似文献   

Identification of Suitable Natural Calibration Test Site for the Calibration of IRS Optical Sensors     

Shailendra S. Srivastava  Yogdeep Desai  K. S. S. Sekhar  V. R. Bhavsar  Shambhu Singh Tak  Maneesha Gupta  N. K. Vyas  K. V. S. Badarinath  B. Kartikeyan 《Journal of the Indian Society of Remote Sensing》2016,44(3):307-319

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An effective stress-based DSC model for predicting the coefficient of lateral soil pressure in unsaturated soils     

Akbari Garakani  Amir  Pirjalili  Ali  Desai  Chandrakant S. 《Acta Geotechnica》2021,16(12):3813-3830

Acta Geotechnica - In this study, an analytical model is developed to establish a framework for predicting the coefficient of lateral soil pressure in unsaturated soils. To this end, the disturbed...  相似文献   

Letter to editor single surface yield and potential function plasticity models: A review     

C. S. Desai 《Computers and Geotechnics》1989,7(4):319-333

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Global spectra of energy and enstrophy and their fluxes during July 1979     

S S Desai  S K Mishra 《Journal of Earth System Science》1993,102(2):329-350

Transient and stationary spectra of kinetic energy (KE), available potential energy (APE) and enstrophy (EN), and their spectral fluxes as a function of the two-dimensional wavenumbern were computed for July 1979. Triangular truncation at zonal wavenumber 42 was used for computation. The slopes of various spectra in the wavenumber range 14≤n≤25 were obtained by fitting a straight line in log-log scale by the least square method. The transientKE, APE andEN spectra in the lower (upper) troposphere had slopes −2·21 (−2·30), −2·65 (−2·64) and −0·36 (−0·46), respectively. The effect of stationary and divergent motion on the slope values was investigated. The possible correlation between the slope and percentage of transient component in the combined energy and enstrophy was examined to identify the transient motion of the atmosphere with the two-dimensional homogeneous isotropic turbulence. The vertically averaged slope of kinetic energy and enstrophy in the lower (upper) troposphere was close to the value at 700 (200) hPa level. The spectral fluxes of kinetic energy and enstrophy in the wavenumber range 14≤n≤25 satisfied, to a very rough approximation, the criteria of inertial subrange. The stationary fluxes were small. The estimated stationary-transient component of flux was larger, comparable and less than the corresponding transient flux of APE, KE and EN. Representative levels for computation of energy and enstrophy spectra and their fluxes in the lower and upper troposphere were identified.  相似文献   

Fabry-Perot spectroscopy in space science     

J. N. Desai 《Journal of Earth System Science》1984,93(3):189-200

The paper describes use of Fabry-Perot (fp) systems in spectroscopic studies of astronomy and earth’s upper atmosphere. Factors influencing the resolution, luminosity and instrumental line shape offp spectrometer are discussed and the need for their optimization is emphasised, if desired luminosity gain is to be realised in practice. Use offp as imaging spectrometer, scanning spectrometer and as a narrow band filter are illustrated. Tandemfp systems are briefly dealt with.  相似文献   

Equatorial Indian Ocean evaporation estimates from operational meteorological satellites and some inferences in the context of monsoon onset and activity     

Baby Simon  Pranav S. Desai 《Boundary-Layer Meteorology》1986,37(1-2):37-52

The estimation of evaporation from the sea surface is not yet achieved adequately by remote sensing techniques, in general. However, for approximate averaged estimates over moderate space and time scales over a specific tropical region, e.g., weekly values over the Indian Ocean as needed in monsoon moisture diagnosis, it may be possible to extrapolate satellite wind and humidity data to the ocean surface and then use bulk aerodynamic parameterization for estimating evaporation. In the present investigation, GOES low-level cloud winds and TIROS-N moisture profiles over the Indian Ocean are extrapolated to the ocean surface. The planetary boundary layer (PBL) wind shear is obtained over different sub-regions and periods during the monsoon season, by reference to objectively analysed fields. These shear values are applied to GOES satellite winds to obtain sea-surface winds. The humidity extrapolation was based on (i) an exponential fit for water vapour density and (ii) a vertical distribution of relative humidity approximately proportional to atmospheric pressure. The exchange coefficient is varied slightly depending on wind speed and boundary-layer stability inferred approximately from TIROS-N sea surface temperatures and temperature profiles. The evaporation estimate as based on these satellite parameters is assessed by comparison with ships' surface observations. Sensible heat exchange is also estimated and assessed. Some inferences based on these estimates are also presented, in relation to monsoon onset and activity.  相似文献   

Consequences of deviatoric normality in plasticity with isotropic strain hardening     

R. Baker  C. S. Desai 《国际地质力学数值与分析法杂志》1982,6(3):383-390

For many soils experimental evidence suggests that deviation from normality between the plastic strain increments and the yield function is exhibited mainly in the volumetric behaviour, whereas the deviatoric behaviour exhibits normality. An analysis of the consequence of this observation is presented, and it leads to an expression for the plastic potential function as composed of two functions, a yield function and another dependent only on the hydrostatic pressure; the latter is considered to constitute the concept of correction functions presented recently by Desai and Siriwardane¹³.  相似文献