• Volume 95, Issue 1

      March 1986,   pages  1-141

    • Power spectrum analysis of geomagnetic indices

      R P Kane

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      Using the method of maximum entropy spectral analysis (MESA) by Burg and the least-squares linear prediction (lslp) (also calledFABNE) by Barrodale and Erickson, the spectra of geomagnetic indices Ap, AN, AS, AE, AU, AL, Dst and cosmic ray neutron intensity at Deep River are obtained for 1965 and 1969 from daily means and for longer periods from monthly means. A large number of peaks of periodicities from 2 days to several years is obtained, many of which are shown by all the indices. Some of these are probably harmonics of the 27 days solar rotation period and the 20–22 years double sunspot cycle period. Comparison is made with results of earlier workers who reported fewer peaks.

    • Wave packet structure and phase jumps in low latitude Pc3 geomagnetic pulsation

      I A Ansari B J Fraser K J McLean

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      An array of four low latitude induction coil magnetometer stations has been used to study the spatial and temporal characteristics of Pc3 pulsations over a longitudinal range of 17° atL=1·8 to 2·7 in south-east Australia. A preliminary study of individual Pc3 wave packet structure at the azimuthal stations has established the existence of phase jumps between wave packets at low latitudes similar to those observed at synchronous orbit and at higher latitude ground stations. However there did not appear to be any obvious pattern in phase jump occurrences between stations or signal components.

    • Spectral characteristics of low latitude Pc3 geomagnetic pulsations in south-east Australia

      I A Ansari B J Fraser K J McLean

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      Geomagnetic pulsations in the Pc3 period range (10–45 sec) were recorded simultaneously in south-east Australia using an array of four low latitude induction coil magnetometers over a longitudinal range of 17° atL=1·8 to 2·7. Geomagnetic data from March 25 to September 21, 1982 were digitized and spectral studies were carried out from Grey scale digital (GSD) dynamic spectra and contour sonagrams. Similar wave spectra were generally seen at the three stationsWM, BH andNC in the same latitude with Pc3 waves mostly being observed in the 40–45 mHz band. In addition the higher latitude station (LN) also showed lower frequency Pc4 bands (≈ 10mHz) on some days but higher frequency bands (60–80mHz) on others. Sometimes completely different wave frequencies were observed on theX andY components with the stations in the same latitude generally showing similar spectra on the corresponding components. Dynamic spectra also indicated that wave energy often turned off and on abruptly over short intervals of time implying solar wind control and Pc3 pulsation activity. A band harmonic structure was occasionally seen in wave spectra at the azimuthal stations mainly on theX component during the local afternoon with a harmonic spacing (Δf) of ≈ 10 mHz.

    • Sub-seasonal scale fluctuations of theITCZ over the Indo-Pacific region during the summer monsoon season. Part I: Features over the Indian region

      D R Sikka D K Paul V R Deshpande V R Mujumdar P V Puranik

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      Monex-79 andISMEX-73 data have been analysed to study the sub-seasonal scale fluctuation of near equatorial oceanic intertropical convergence zone (ITCZ) over the North Indian ocean during the summer monsoon of 1979 and 1973. The oceanicITCZ is characterised by a narrow shear zone between the equatorial westerlies and the tropical easterlies, associated with organised convective clouds. Synoptic analysis presented in this paper shows the steady northward propagation of the oceanicITCZ from its near equatorial position (5–10°N) to the continental position (20–25°N) during the onset and mid-season revivals of monsoon after breaks. The northward propagation is initiated by the strengthening of the equatorial westerlies which result in the intensification of the shear zone and the embedded disturbances. The establishment of the northward propagating mode near normal monsoon trough position over the continent characterises the active phase of monsoon. As the monsoon cycles from active to weak/break phase, the monsoon trough (continentalITCZ) dissipates near the foothills of the Himalayas and the oceanicITCZ gets emphasised once again near the equatorial region. The major phase changes in theITCZ occur at an interval of about 30–50 days which dominantly control the intra-seasonal fluctuation of the Indian summer monsoon. The paper also discusses the characteristic features of the oceanicITCZ during different phases of the monsoon.

    • Quasi-specular sea scatter with modified reflection coefficient

      Raj Kumar Abhijit Sarkar

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      The paper deals with the relationship between the sea surface wind speed (U) and the backscattering coefficient (σ0) for microwave frequencies and quasi specular range of angles. Valenzuela’s effective reflection coefficient based on the theory of scattering of electromagnetic waves from rought dielectric surfaces has been introduced in the expression of backscattering coefficient and the final result compared with the σ0-U relationship derived fromSEASAT scatterometer and concurrent sea truth data.

    • The effect of filtering on drift and anisotropy parameters determined by full correlation analysis

      B M Vyas D V Sardesai R K Rai H Chandra G D Vyas

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      Ionospheric drift records obtained by the closely spaced receiver technique at Udaipur are subjected to low and high pass filtering with varying cutoff frequencies and the effects of filtering on various drift and anisotropy parameters as determined by full correlation analysis are studied. Limits in which filtering could be employed are suggested on the basis of results obtained.

    • Geochemistry of coexisting hornblende and biotite from the Ambalavayal granite, Kerala

      M Santosh

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      Major and trace element geochemistry of coexisting hornblendes and biotites from the Ambalavayal granite, northern Kerala, are presented. The hornblendes correspond to edenitic composition, whereas the biotites correspond to annite. The hornblendes typically show high Al2O3 contents (9·69–11·89%) comparable with those from anorogenic granites. The biotites are characteristically low Mg-type, similar to those reported from alkaline rocks. The distribution coefficients calculated for all the major and trace elements are presented and an evaluation of the nature of variation indicate near-chemical equilibrium conditions during the crystallization of the two minerals. The hornblende-biotite tie lines in the Fe3+−Fe2+−Mg compositional triangle, lie parallel to those of buffered biotites, indicating crystallization in an environment closed to oxygen and well above the Ni−NiO buffer. It is inferred that thefH2O increased towards the residual stage andfO2 values were high, in the range of 10−15 bars.

    • Radioelement geochemistry of alkali granites of the Kerala region, south-west India

      M Santosh K Vijaya Kumar

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      Th, U and K abundances in four alkali granites of the Kerala region, south-west India, are presented. The plutons show high radioelement levels, correlatable with those of alkali granites in other regions. The nature of variation is consistent with the correlation of Th and U with accessory phases like sphene, zircon, allanite, apatite and monazite. A geochronologic correlation is also observed between the alkali granites and the Th-bearing beach placers of the region. The petrogenetic features of the alkali plutons, their taphrogenic association, Pan-African affiliation and high Th/U levels suggest that the alkali plutons are favourable locales for radioelement exploration.

    • The petrology and geochemistry of massif anorthosites and gabbros of the Bavali fault zone, North Kerala

      G R Ravindra Kumar

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      The massif anorthosites and gabbros occurring in the north Kerala region are localised along the Bavali fault. Several other bodies of granophyres and syeites also occupy this fault. All these bodies are considered to be a continuous graduating igneous series. This paper aims at delineating the field relations, petrochemistry and origin of massif anorthosites and gabbros occurring in the vicinity of the fault.

      Anorthosites (An 70-55) are nonlayered and highly deformed with no igneous structure present. Gabbros show relict poikilitic texture with a mineralogy of plagioclase, clinopyroxene, hornblende and biotite. Geochemical study suggests dominantly calc-alkaline characteristics for both anorthosites and gabbros. However, their chemical variation is not transitional to indicate evolution by continuous differentiation from a single parent magma. The rock types seem to be chemically unrelated and appear to have evolved independently from discreet magma sources and the magma migration might have only been promoted by the Bavali lineament.

    • Base metal exploration by gravity surveys in a part of the Cuddapah basin (India)—A case study

      G Ramadass S M Varaprasada Rao N L Mohan

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      A closely spaced gravity survey was conducted in the Chikativaripally area in the Cuddapah basin, Andhra Pradesh, India for base metal exploration. Analysis of the data using two-dimensional prismatic models suggest a doubly plunging folded structure in the area and also its associated mineralization.

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