• Volume 88, Issue 1

      March 1979,   pages  1-75

    • A qualitative appraisal of geomagnetic manifestations of the geological structures in the Indian equatorial region

      A V S Murty D R K Rao

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      From the data of the geomagnetic survey of the Indian equatorial region conducted during 1971, it has been possible to deduce the subsurface magnetic anomalies at each survey station. The vertical magnetic anomaly profiles seem to reflect some of the known geological structures in the region. The association between magnetic anomalies and the deep-seated structures in this area emphasises the importance of future intensive studies.

    • Rb-Sr age of Godhra and related granites, Gujarat, India

      K Gopalan J R Trivedi S S Merh P P Patel S G Patel

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      Rubidium and strontium determinations are reported for Godhra and geographically related granites from central Gujarat. The whole rock data define a Rb-Sr isochron corresponding to a common age of 955±20 m.y. and initial Sr ratio of 0·7130±0·001. This age is distinctly older than the age of 735 m.y. reported for the Erinpura suite of rocks from Mount Abu in western Rajasthan and from Idar in northern Gujarat. There are at least two generations of post-Delhi intrusive rocks in the Gujarat precambrian. Biotites associated with these granites have the same age as the whole-rocks within experimental error indicating the absence of significant metamorphic heating since the time of emplacement. It is significant that rocks of similar age occur in the Rajasthan Precambrian mainly in the axial zone of the Aravalli Mountains.

    • Vanadium, manganese and iron in the carbonate rocks of the Rohtas formation

      Rajendra P Sharma

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      The environmental conditions that prevailed during the formation of the Rohtas carbonates have been delineated on the basis of the Eh-pH diagrams for V, Mn, Fe+2 and Fe+3 compounds. The high content of vanadium in the insoluble residue is indicative of the prevalence of reducing environment. During early-diagenesis manganese seems to have been mobilised from the soft sediments. Higher manganese content in the carbonates is a result of late-diagenesis. Prior to late diagenesis, ferric iron appears to have been precipitated from the waters while manganese remained in solution, and this process accounts for the low iron content of the carbonates.

    • Annual fallout of32Si,210Pb,22Na,35S and7Be in rains in India

      D Lal V N Nijampurkar G Rajagopalan B L K Somayajulu

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      The concentration of radioisotopes7Be,35S were measured in Bombay since 1956 and22Na,210Pb,32Si since 1963. In Khandala and other stations such measurements have been made at irregular periods since 1961. In addition several measurements especially that of32Si were made in 1970. Data available todate from Indian stations is summarised and critically analysed.

      We conclude that appreciable amounts of35S,22Na and32Si, over and above their production by cosmic rays, were produced during the high yield Russian tests as evidenced by their fallout between 1962–66. Based on the bomb produced excess the half period for their removal from the stratosphere is deduced to be less than 1 year. The ‘excess’ contribution of32Si due to bombs is, however, small; about 1% of its inventory in the oceans.

      The present study shows that for stations where orogeny is the principal mechanism of precipitation, the annual fallout is independent of the annual rainfall.

    • On the interpretation of velocities derived from spaced fading records

      H Chandra

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      A number of spaced fading records obtained at Buckland Park, South Australia, using partial reflection technique (at 80–100 km) have been subjected to different methods of analysis, viz., time delay methods, the full correlation method and the cross-spectrum method. True drift velocities derived from the full correlation method appear to be the most correct. The variation of velocity with fading frequency obtained from cross-spectrum method is most likely to be due to the variation of velocity with time within the record itself.

    • Solar zenith angle and frequency dependence of ionosphericD-region absorption

      S L Jain R K Rai

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      In this paper, the results of a series of extensive measurements of multifrequency radio wave absorption in theD-region of the ionosphere during the epoch of the minium period of the solar cycle are presented. Experimental results for Udaipur, a low latitude station (24°35′N) are compared with the theoretically expected results. It is found that the experimental values of frequency indexm and cos χ indexn agree well with those obtained theoretically. The mean values ofm andn for the whole period of observation are found to be 1·76 and 1·4 respectively. The seasonal variation of the values ofm andn is also discussed.

    • Remote probing ofD-region irregularities

      H Chandra R A Vincent

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      The nature of theD-region irregularities responsible for partial reflections is studied using amplitude, phase and range information taken at Buckland Park, South Australia, utilising a 178-dipole receiving array. Stratifications with distinct layer type structures below 80 km are noticed. A major part of the echo amplitude consists of specularly reflected signals.

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

      T N Rajaraman J N Desai S S Degaonkar

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      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.

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