Segregation of dust grains in dark clouds

Gravitational settling of dust grains in dark clouds has been considered. It has been shown that such a process gives rise to a modification of the grain size distribution. Starting with a simple model of uniform spherical cloud and normal interstellar grain size distribution for the dust we derive expressions for the modified grain size distribution function, average grain size and extinction as functions of distance from the cloud's center and the age of the cloud. The mean grain size increases towards the center of the cloud as does the extinction. Results of the numerical evaluation of these quantities have been discussed with their implications for the observations of anomalous reddening and polarization within dark clouds and Bok globules.     

The excitation mechanism and the line-to-continuum intensity ratio of the [Fexiv] 5303 Å line in various coronal regions

The excitation mechanism of the coronal green line in various coronal regions is studied. The line-to-continuum intensity ratio is calculated using the model electron density values given by Newkirk (1961) and is compared with the observed values. Reasonably good agreement is found between the model calculations and observed values. The model calculations show that collisional excitation is more important in the innermost regions while radiative contribution progressively increases towards outer regions. The individual contributions depend strongly on the activity of the particular coronal region. An increased contribution from collisional excitation is seen in coronal active regions owing to the large electron density. A contour map of the line-to-continuum intensity ratio as derived from the observations, is plotted. At 1.10R the value is about 80 in active regions, 30–40 in coronal streamers, while it is less than 10 in polar regions.     

The plasma condensation region in the coma of Halley's Comet

The cometary images taken on 1986 January 8.590 and 8.638 UT (R-0.9 AU, ~ 1.29 AU) at Gurushikhar, Mt. Abu, India (24 °39 N, 72 °43 E alt: 1700 m) show distinct condensation region in the tail direction. The size of the condensation region is 4 × 10³ km. The condensation region showed up strongly in the blue emission, implying the abundance of CO⁺. It was inferred to be moving with a velocity of 37 ± 3 km/s relative to the comet at a distance of 2.3 × 10⁵ km from the nucleus in the tailward direction.The analysis show that the condensation was a result of rapid ionization mechanism, with a time scale of \~10³ to 10⁴ sec. The most probable mechanism for producing the ionization region was found to be the discharge of cross tail electric current passing through the neutral sheet in the near nucleus region followed by an outburst observed in IR wavelengths at 8.1 UT. It was accelerated by J × B drift at a rate of ~24 cm/sec² to the position observed by us.This feature, most probably is the precursor of the first dramatic Disconnection Event (DE) observed in Halley's Comet at Jan.10.375 UT. This supports the conjecture that the tail features originate in the coma with a velocity of ~20–40 km/s.     

A high-resolution Fabry-Pérot spectrometer for emission line studies in planetary nebulae and other extended astronomical objects

We describe here a scanning piezo-electric Fabry-Pérot spectrometer operating in the photoncounting mode whose plate spacing and parallelism are maintained by a servo-controlled system, ensuring high accuracy, for the study of emission lines from extended astronomical objects in the spectral range 4500–7000 Å. Details of the optical set-up and the Data Acquisition System (DAS) are described. Its performance at the Cassegrain focus of the 1 m telescope at Kavalur is discussed. Some line profiles on planetary nebulae studied with the above spectrometer are also presented.     

Ichnological analysis of transgressive marine tongue in prograding deltaic system: Evidences from Ukra Hill Member, Western Kachchh, India

The Ukra Hill Member exposed only in Western part of Kachchh basin comprises of fossiliferous shales, limestones and glauconitic sandstones in between thick unfossiliferous sandstone. The present study documents abundance of trace fossil Teredolite, occurring as driftwood and bedload log ground as well as re-worked log ground from the basal part of the Ukra Hill Member. These Teredolite bearing horizons are underlain by coarse grained cross bedded sediment with monodominant Psilonichnus tubiformis, while abundant Gyrolithes cycloides along with Rhizocorallium irregulare, R. Jenense occurs in overlying horizons. Three ichnofabrics recognized within the Ukra Hill Member are inferred to represent near shore high energy conditions (Psilonichnus ichnofabric), transgressive condition with subsequent erosion (Teredolites Ichnofabric) and shelf environment (Gyrolithes-Rhizocorallium Ichnofabric). TheTeredolite bearing horizons also occurs with abundant belemnites Neohibolithes ewaldi, which is considered to be index fossil for the Early Aptian sediments. Thus, based on the ichnofabric analysis, the basal part of the Ukra Hill Member represents an Early Aptian transgressive event in Kachchh basin.     

Paleoecological significance of the trace fossils of Dhosa Oolite Member (Jumara Formation), Jhura dome,Mainland Kachchh,Western India

Dhosa Oolite Member of the Jumara Formation comprises alternating bands of oolitic limestones and shales, exposed in Jhura dome of Mainland Kachchh, Western India. This sequence is highly bioturbated and exhibits a moderate diversity and behaviourally complex assemblage of ichnospecies. The rhythmically bedded sequence shows three different levels of preservation of traces. Epichnial tiering consists of moderately bioturbated oolitic limestone exhibiting horizontal or low-angle protrusive/retrusive biogenic laminae, commonly dominated by feeding structures like Rhizocorallium jenense, R. irregulare, Zoophycos brianteus and Zoophycos isp. The endichnial structures within the oolitic limestone can be separated into two different preservational trace fossil suites. The endichnial shallow suites consist chiefly of deposit feeders like Chondrites intricatus, C. targionii, Planolites beverleyensis, Taenidium cameronensis, Thalassinoides isp., Z. brianteus, Z. cf circinnatus and Zoophycos isp. and few suspension feeder forms like Palaeophycus tubularis; while endichnial deep suites consist of Chondrites intricatus, Skolithos linearis and Zoophycos isp. Hypichnial structures consists abundant, cylindrical, branched, horizontal, large-sized three dimensional feeding burrows of Thalassinoides isp. and somewhat irregular, obtuse angle ramification burrows of Phycodes isp., which are attached to the lower surface of the casting medium. The trace fossil association indicates Cruziana ichnofacies and abundance of Zoophycus species below the fair weather wave base level is largely a preservational artifact. The preservational processes of the trace fossils indicate soft substrate and diversity and their abundance reflects the other palaeoecological parameters of the open shallow marine environments.     

Implementation of hierarchical single surface δ0 and δ1 models in finite element procedure

Hierarchical single surface (HISS) δ₀ and δ₁ models are briefly discussed. An algorithm for the implementation of δ₀ and δ₁ models in a finite element code and the associated subroutines, written in FORTRAN 77, are presented. The algorithm is general and can be adopted for other computer codes with minor changes. Detailed steps and subroutines are included for easy implementation of the models in specific codes. Two example problems are also presented to enable the user to verify the implementation.     

A residual flow procedure and application for free surface flow in porous media

A finite element procedure based on the extension of the saturated flow domain into the partially saturated zone above the free surface is proposed. The finite element equations are derived by using a pseudo variational principle which results into a residual or correction load vector. The steady or transient free surface is corrected by using the residual load in an iterative scheme. The proposed procedure uses only one (initial) mesh and does not require modification of the mesh during iterations. It is compared (qualitatively) with other procedures such as variable mesh and variational inequalities. The procedure provides satisfactory comparisons with a number of closed-form solutions and laboratory test results; two applications involving the latter are described in the paper.     

Variation ofF-region nighttime neutral temperatures with equatorial spread-F activity

NighttimeF-region temperatures have been obtained over Mount Abu (24·6°N, 72·7°E geographic, 15·0° geomagnetic latitude) by monitoring OI 6300 Å line widths, using Fabry-Perot spectrometer. Enhancement in neutral temperatures associated with spread-F activity over Thumba (8·5°N, 76·8°E geographic, 0·6°S geomagnetic), which is on the magnetic equator were reported earlier. Additional data over Mt Abu for seven days bring out the following major features. (i) In theF-region over Mt Abu, the neutral temperature enhancements upto 200°K are observed on most of the nights when there is spread-F activity over the magnetic equator. (ii) There are no occasions which show temperature enhancements over Mt Abu without spread-F activity over the equator. On such occasions the observed atmospheric temperatures agree very well with those calculated on the basis of the Jachhia atmospheric model, both for magnetically quiet as well as disturbed days. (iii) There is a good indication that the increase inF-region temperature over Mt Abu is delayed with respect to the onset of spread-F over Thumba by approximately 15–30 min.