• G K Rangarajan

      Articles written in Journal of Earth System Science

    • Periodicity in rainfall at Madras

      G K Rangarajan K N Rao

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      Monthly rainfall data from Madras for the period 1813–1976 are spectrally analysed for detection of significant oscillations using the new data adaptive technique of maximum entropy developed by Burg. Spectra for annual, NE monsoon and SW monsoon rainfall are separately presented and compared with the spectra derived from Fourier transform using FFT. The long series of data are divided into several subsets and analysed to determine the stability of the periodicities. The effect of changing the length of PEF on the resultant spectra is also discussed. The analysis reveals that longer period variations (>20 years) in the rainfall is either absent or non-stationary. Solar-cycle periodicity of 11 years is also missing in the rainfall. A quasibiennial oscillation in the annual and monsoon rainfall is observed in all the subsets and in the full series. Stacked spectrum derived from the spectra of monthly rainfall series for 30-year spans is dominated by the annual line and its harmonics. The biennial pulse is absent in the monthly rainfall series.

    • Daily variation of geomagnetic field at low latitudes associated with stable solar wind flow

      G K Rangarajan

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      Mean diurnal variation ofH at low and equatorial latitudes is computed for days in the vicinity of passage of ‘quiet’ solar wind. It is shown that the prepassage magnitude of the diurnal variation is appreciably larger when compared to post-passage intervals at low latitudes but the difference vanishes in the electrojet region. It is suggested that the Sq current system moves towards dip equator immediately following quiet wind conditions relative to earlier periods. It is also shown that during conditions of stable solar wind, the solar wind proton density is inversely related to the electrojet strength, while at low latitudes outside the jet influence, there is no clear association.

    • Worldwide features of the strength of recurrent geomagnetic activity

      G K Rangarajan A Bhattacharyya

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      Variation of the strength of recurrent geomagnetic activity, which occurs just before a sunspot minimum, with local time is studied for a network of observatories covering different latitude and longitude zones. For this purpose, hourly averages of horizontal intensity(H) for each UT hour for 173 days, which are totally free of disturbances due to solar transients, have been subjected to spectral analysis. Well-defined spectral peaks associated with periodicities of 28, 14 and 9 days were present in almost all the spectra. The pattern of daily variation of the strength of the 27-day signal changes from a diurnal one at low latitudes to a semi-diurnal one near the Sq focus and in this region, the 14-day signal appears to have an independent origin irrespective of the longitude zone. A study of 27-day oscillation in mean dailyH field also indicates that apart from ring current modulation, both Sq and electrojet fields also undergo 27-day oscillations during the declining phase of a solar cycle possibly through the ionospheric wind system.

    • Quasi-biennial oscillation in geomagnetic disturbance field

      G K Rangarajan

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      Using the techniques of second derivatives, digital filtering and power spectral analysis, the fine structure of the quasi-biennial oscillation (qbo) in the equatorial geomagnetic field in the Indian zone is studied. The salient results are: (i) in the quiet day field,qbo is weak and the signal strength has no local time dependence; (ii) in the disturbance field the signal is strongest in the afternoon and evening hours and (iii) larger magnitudes are manifested closer to the dip equator. The difference in dawn and dusk amplitudes of the signal and the absence of significant strength close to noon in the electrojet region rules outeuv flux as the responsible agent.

    • Application of the Linear Prediction filters in equatorial electrojet studies

      G K Rangarajan

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      Wiener filters are derived from the horizontal field data of two adjacent equatorial electrojet stations, Adis Ababa and Trivandrum from several sequences of quiet days. The time invariant property of the filter is established and the filter is applied to conditions marked by afternoon counter electrojet events. The prediction efficiency is shown to be consistently high. Possible uses of this technique in studies related to generating mechanisms of counter electrojet events and the day-to-day variability in electrojet currents are indicated. Inter-relationships and day-to-day variability of different components of the fields at the two stations are highlighted.

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