• B R Arora

      Articles written in Journal of Earth System Science

    • Analytical representation of spatial and temporal variations of the geomagnetic field in the Indian region

      B R Arora V H Badshah B P Singh M G Arur P S Bains Jeevan Lal

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      The magnetic measurements of declination (D), horizontal (H) and vertical (Z) components of earth’s magnetic field, collected from ground surveys between 1962 and 1966, are used to develop an analytical model of geomagnetic field variations over Indian region for the epoch 1965. In order to reflect spatial features with wavelengths of approximately 1000 km, sixth degree polynomial as a function of differential latitude and longitude is calculated by the method of least squares. The root mean square fit of the model to the input data is better than that accounted by the International Geomagnetic Reference Field for 1965.0. Isomagnetic charts drawn forD, H, Z and total force (F) reflect more details than that shown on world magnetic charts. Further, the values of the field at common repeat stations recorded between 1962 and 1974, after eliminating the field values for the epoch 1965.0, are used to get the secular variation as well as its spatial dependence again by means of polynomial which now includes coefficients which are functions of time and of geographical locations. The accuracy of coefficients is tested against the behaviour of secular variation at permanent magnetic observatories. The merits and limitations of the model are discussed.

    • Indices of equatorial electrojet and counter-electrojet in the Indian region: Evolution of the indices and their authenticity

      B N Bhargava B R Arora N S Sastri

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      Indices of the equatorial electrojet and counter-electrojet in the Indian region have been evolved based on certain reasonable assumptions, by a criterion depending on the difference in the horizontal field strength between a station under the electrojet axis (Trivandrum) and a station outside the jet influence (Alibag). The indices enable one to characterize each day of the year by an appropriate index. Distribution, in the months, seasons and over the years, of the electrojet and counter-electrojet frequencies at Trivandrum during a 20-year period, 1959–78, is examined. The salient features of the distributions are: (i) Strong electrojets (range 50 nT) and the afternoon counter-electrojets occur, on an average, on about 50% of the days in the year; (ii) For the counter-electrojet, the monthly frequencies show an annual variation with a summer maximum, and for the strong electrojet, a semi-annual variation with maxima in the equinoxes; (iii) A secondary maximum is noted in January in the occurrence frequency of both counter-electrojet and strong electrojet events; (iv) Intense electrojets occur with greater frequency in September.

    • Rock magnetic and palaeomagnetic studies on the alkaline complexes of western Rajasthan, India

      C LAKSHMI NARASIMHAN B R ARORA S K PATIL

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      Forty-five oriented block samples were collected from 12 sites of Mundwara and Sarnu-Dandali alkaline complexes of western Rajasthan for palaeomagnetic and rock magnetic investigations to constrain the timing of the emplacement of alkaline suites in relation to the time-frame of main Deccan volcanism. The rock magnetic studies indicated (titano) magnetite as the main magnetic carrier of stable remanence. The Lowrie–Fuller (L–F) test revealed SD and PSD type magnetic carrier in samples. AF and thermal demagnetizations were piloted on the samples to isolate Characteristic Remanent Magnetisation (ChRM) directions. The samples exhibited stable remanence between 5 and 35 mT during AFD. Thermaldemagnetizations unblocked remanence between $350^{\circ}$ and $500^{\circ}C$. The mean ChRM directions with $\rm{Decl_{m} = 342, Incl_{m} = –35 (\alpha_{95} = 4.39, K = 121, N = 9)}$ corresponds to paleopole position at $42^{\circ}\rm{N}$ and $274^{\circ}\rm{E}$ with a palaeo-latitude of $24.5^{\circ}\rm{S}$ is coincident with the Deccan Super pole position. The comparison of ChRM in alkaline complexes with those documented in DVP implies coeval emplacement of alkaline suites with Deccan eruption. Further, the ChRM marked largely by normal polarity suggests that alkaline intrusions were accomplished within the magnetic Chron C30N, during the onset of Deccan volcanism. The results also indicate that Deccan volcanism extended far beyond the present day boundaries of the traps, especially in the north.

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