Frequency-modulated solar rotational periodicity of geomagnetic indices

Many attempts have been made to search for various timescales in the power spectrum of geomagnetic indices so that common periodicities in the solar activity and geomagnetic activity indices are identified. The spectral behavior of geomagnetic activity parameters may also provide invaluable information about physical processes involved. In this study we attempt to demonstrate that the frequency modulation associated with a long-term variation may cause extra sidelobes around the principal peak with a periodicity of ∼27 d in the observed power spectrum of geomagnetic activity indices, and/or may even split the peak into two adjacent peaks. We employ a straightforward model of an oscillation frequency-modulated by an arbitrary agent to consider the solar rotational periodicity of geomagnetic indices. As a result, we have found that the peak with the periodicity of ∼27 d in the observed power spectrum of geomagnetic indices seems likely frequency-modulated by the amount of 0.0026 d-1 which corresponds to a ∼1 yr period. We thus suggest that the fundamental period of the periodic perturbative agent is much longer than a year according to our analysis. Finally, we conclude by discussing the implications of what we have found.