Use of Uniform and Inert Beads for the Determination of Shrinkage Limit of Fine Grained Soils

Shrinkage limit is one of the important index properties determined in the laboratory, which has practical applications such as classification of soils based on their degree of expansivity and as a measure of volumetric stability of clayey soils in the field. The conventional method of its determination requires the use of mercury, which is a heath hazardous substance. This paper proposes and discusses the use of beads of an inert material having uniform grain size for the determination of volume of dry soil pat, which is required for calculating the shrinkage limit of the soil. The shrinkage limit values obtained from the use of uniform beads of an inert material compare very well with those obtained from the conventional mercury displacement method and are within the acceptable limits.     

High resolution 2-D maps of OI 630.0 nm thermospheric dayglow from equatorial latitudes

The first-ever high resolution 2-D maps of OI 630.0 nm dayglow obtained from equatorial latitudes clearly reveal the movement as a large-scale feature of the equatorial ionization anomaly (EIA). These also show the presence of wave-like features classified as gravity waves presumably originating at the crest of the EIA, similar to the equatorial electrojet acting as a source of these waves. These results are presented and discussed.     

Vertical structure and characteristics of 23–60 day (zonal) oscillations over the tropical latitudes during the winter months of 1986 — Results of equatorial wave campaign—II

The equatorial wave campaign-II which formed a part of the Indian Middle Atmosphere Programme (IMAP), was conducted from SHAR (13.7°N, 80.2°E) from 15 January to 28 February 1986. Winds were measured from ground to 60 km by means of high altitude balloon and a meteorological rocket (RH-200), once everyday, for 45 days. The frequencies of the oscillations in the deviations of the east-west component of the winds from its mean at each height with one kilometer interval were obtained by the maximum entropy (ME) method and phases/amplitudes of these frequencies were determined by the least squares technique on the wind variation time series. The ME method has the inherent advantage of providing periodicities up to 1.5 times the data length. The height structure of the long period waves of > 23 day periodicities that have larger amplitudes nearly by a factor of 2 as compared to the medium (9 to 22 day) or shorter period (4 to 8 day) ones, reveal two height regions of enhanced amplitudes, one in the troposphere and another in the upper stratosphere/lower mesosphere, that too, mostly in the regions of positive (westerly increasing or easterly decreasing with height) wind shears. The waves are seen to be inhibited in the negative wind shear regions. From the abrupt changes in the altitude variation of phase, the possible source region has been identified. The vertical wavelengths have been estimated to be 34 km and 19 km in the troposphere and lower stratosphere respectively and 8 km in the upper stratosphere and lower mesosphere. Around 56 km the wave amplitude is reduced to 1/4 of its value below, while the vertical shear strength in the mean wind doubled up. The tropospheric waves are suggested to be Rossby waves of extratropical origin penetrating to tropical latitudes. The stratospheric/mesospheric waves however appear to emanate from a source around the stratopause.     

Dynamic instability of frames having thin-walled columns

Dynamic instability of single storey frames having thin-walled columns has been investigated. The lateral loads sustained by the frame are dynamic in character, while the axial loads are deemed to be quasi-statically applied. The analytical model employed by the authors has the capability of modelling the combined action of the two ‘companion’ local modes whose amplitudes are variable along the length of the column and any type of end conditions of the members. For given levels of axial loads sustained by the columns, the magnitudes of lateral loads causing instability can be significantly smaller than those corresponding to static buckling, provided the dynamic load is of sufficient duration. There exists, however, a threshold value of axial force carried by the columns, below which there is no elastic instability—static or dynamic. For columns with overall critical loads several times greater than the local critical load, there is no danger of elastic instability, but the deflections under dynamic lateral loads of less than 1 per cent of the axial load may reach such huge values that there is a serious danger of localized plastic collapse. It is also shown that moment frames having thin-walled columns such as those fabricated out of cold formed steel are extremely vulnerable to moderate seismic excitations.     

Shock-heated NH3 in a Molecular Jet Associated with a High-Mass Young Star

We present the discovery of shock-excited NH3 in a well-collimated jet associated with the extremely young high-mass star IRAS 20126+4104. The NH3 (3, 3) and (4, 4) emission is dominated by three clumps along the SiO jet. At the end of the jet, there exists strong and broad (+/-10 km s-1) NH3 (3, 3) emission. With typical brightness temperatures greater than 500 K, the overall emission indicates a weakly inverted population and appears in an arc, consistent with the excitation by bow shocks. There are two bright spots in the NH3 (3, 3) emission with brightness temperatures of approximately 2000 K. The narrow line width (1.5 km s-1 FWHM), the small sizes (<0&farcs;3), and the unusually high brightness temperature of the features are indicative of maser emission. Our observations provide clear evidence that NH3 (3, 3) masers are excited in shock regions in molecular outflows.     

First results of warm mesospheric temperature over Gadanki (13.5°N, 79.2°E) during the sudden stratospheric warming of 2009

Rayleigh lidar observations at Gadanki (13.5°N, 79.2°E) show an enhancement of the nightly mean temperature by 10–15 K at altitudes 70–80 km and of gravity wave potential energy at 60–70 km during the 2009 major stratospheric warming event. An enhanced quasi-16-day wave activity is observed at 50–70 km in the wavelet spectrum of TIMED–SABER temperatures, possibly due to the absence of a critical level in the low-latitude stratosphere because of less westward winds caused by this warming event. The observed low-latitude mesospheric warming could be due to wave breaking, as waves are damped at 80 km.     

Measurements of atmospheric turbulence parameters at Vainu Bappu Observatory using short-exposure CCD images

We report atmospheric turbulence parameters, namely atmospheric seeing, tilt-anisoplanatic angle(θ_0) and coherence time(Τ_0), measured under various sky conditions, at Vainu Bappu Observatory in Kavalur. Bursts of short exposure images of selected stars were recorded with a high-speed, frame-transfer CCD mounted on the Cassegrain focus of a newly commissioned 1.3 m telescope. The estimated median seeing is ≈ 1.85 " at wavelength of ～ 600 nm, the image motion correlation between different pairs of stars is ～44% for θ0≈ 36" and mean Τ_0 is ≈ 2.4 ms. This work was motivated by the design considerations and expected performance of an adaptive optics system that is currently being planned for the telescope.     

Estimation of Fried's Parameter from Long-Exposure Solar Images

We propose a criterion for extending the parameter search method (Krishnakumar and Venkatakrishnan, 1997) of estimating the point spread function to solar data. In the parameter search method, the number of pixels with negative intensity values in the restored object is used as an estimator for determining the unknown parameters of the point spread function. As a solar image has a high background, the restored object does not contain negative values, thereby making the method unsuitable for solar data. We propose to use the intrinsic contrast of solar features as a criterion for identifying the unknown parameter. We validate our method through simulations. This method can not be used for image restoration but can be used for monitoring daytime seeing.     

SBAS-derived TEC maps: a new tool to forecast the spatial maps of maximum probable scintillation index over India

With the increase in the use of satellite-based navigation services, the forecasting of L band scintillation has turned out to be of paramount importance as it affects their accuracy and availability. Forecasting the time of occurrence or non-occurrence, strength and probable location of scintillation enables the service providers and users to take appropriate action to mitigate the effects and optimize the services. We use the recently developed method to retrieve TEC from the ionospheric correction data transmitted by the Indian satellite-based augmentation system (SBAS)–GAGAN. By making use of the established linear relation between the dusk time TEC and the maximum probable scintillation intensity (S_4max), scintillation forecast maps have been generated as early as 1930 LT. The superposition of actual S₄ measurements, obtained from the GAGAN network of receivers, on the forecasted S_4max map shows that the actual measurements are less than the predicted S_4max except on very few occasions. The potential of the simple technique to predict the 2 D maps of maximum probable scintillation index for the whole night has been demonstrated which with more refinements could evolve into a viable forecast or forewarning system.