G K Rangarajan
Articles written in Journal of Earth System Science
Volume 91 Issue 3 November 1982 pp 235-239
Long period variations in the occurrence of prolonged intervals of calm magnetic field conditions are studied using index Ap of magnetic activity. The solar-cycle variation in occurrence is compared with the sunspot number. Anomalous behaviour for solar cycle 20, observed in other solar parameters, are shown to be manifested in the occurrence frequency of quiet intervals. Spectral characteristics of occurrence indicates a dominant long period variation of about 30 years and a more feeble 11-year oscillation
Volume 93 Issue 4 November 1984 pp 343-351
Solar wind velocity control of low latitude geomagnetic field both on long and short term basis is studied. It is shown that semiannual averages of the low latitude field is inversely related to solar wind velocity and that there is a dominant local time dependence of the relationship. Strongest correlation are confined to the local afternoon hours. It is also shown that for a duration when the solar wind velocity exhibits significant recurrent pattern the low latitude geomagnetic field also depicts strong solar synodic rotation periodicity of 27 days with significant coherence with velocity. The low latitude field on a short term basis is influenced by variable solar wind velocity with a delay of about 1–2 days. During the period of systematic recurrent pattern in solar wind velocity even the quiet-time night field at equatorial and low latitudes show a strong dependence on velocity indicative of the solar wind control of the quiet-time proton belt encompassing the earth.
Volume 96 Issue 2 September 1987 pp 195-202
Study of the response of geomagnetic activity to five different kinds of solar events reveals that an average north-south asymmetry of about 15% exists which diminishes with enhanced geomagnetic activity. The response of the geomagnetic field is quite significant only when high speed solar wind stream in association with sector boundary of interplanetary magnetic field (IMF) or solar proton streams near sector boundary sweeps past the earth. When the frequency of occurrence of indices of geomagnetic activity is considered, the index zero shows a marked difference in its response characteristics for the southern hemisphere. This appears to be a real feature and not attributable to any artefact of the index or its derivation.
Volume 100 Issue 1 March 1991 pp 49-54
From maximum entropy spectral analysis (MESA) of short lengths of
Volume 100 Issue 4 December 1991 pp 361-368
A ground geomagnetic survey conducted in peninsular India has demarkated the dip equator for the epoch 1991.0. A well-defined southward migration in the last two decades is evident. The secular trend in the vertical component and the direction of the migration in the India zone are shown to be consistent. The migratory movement of the dip equator in different longitude sectors is compared to highlight the complex nature of the secular trends.
Volume 103 Issue 4 December 1994 pp 439-448
Homogeneous Indian Monsoon region rainfall for the epoch 1871–1990 has been analysed using Singular Spectral Analysis. It is shown that the HIM time series is simple in structure with only the annual oscillation and its first two harmonics accounting for almost the entire variability. Longer period oscillations related to lunar tidal forcing, solar activity and quasibiennial variation are conspicuously absent. It is also shown that the singular spectral decomposition is closely similar to complex demodulation and thus provides variations in the signals which evolve only slowly with time. As the rainfall series is marked by several jerky changes, predictability of HIM rainfall through the principal components derived from SSA appears impossible.
Volume 107 Issue 3 September 1998 pp 217-224
Singular spectrum isolates significant principal components in a time series from the embedded noise. This tool-kit is used to reconstruct trend-free individual time series, formed by restricting the mean monthly hourly values of geomagnetic field to one hour at a time at a low latitude station Alibag (dipole latitude 9.5°N). Each reconstructed component is extrapolated over the next 12 values using an autoregressive model based on Burg’s maximum entropy algorithm. Details of a numerical approach to increase the reliability of extrapolation are highlighted. The extrapolated reconstructed components are then combined to generate predicted monthly values for each hour. The mean diurnal variation for any month obtained from the extrapolated individual hourly time series compares favorably with the observations. This approach to Sq(H) modelling incorporating both long and short term variations will be beneficial in the derivation of Dst index.