The secular variation of the magnetic field and its cyclic components

Abstract

The application of effective digital filters to separate and compute the amplitude of cyclic components of the observed secular variation (Bhargava and Yacob, 1968) has been extended to a series of mean annual values of horizontal force at Alibag derived from night-time field on magnetically quiet days as well as to series of mean annual values, one for every month. It is shown that the 11-yr cyclic component of secular variation is not associated with the greater incidence of D_st during high solar activity and that the amplitude of this component is largest in Winter and smallest in Summer. The method has also been applied to a long series of data from Oslo, Norway. Amplitudes of 11-yr cyclic component at Oslo have been computed both from power density in the autospectra as well as by application of a band-pass filter. Cross spectra of the computed 11-yr cyclic components ΔH, ΔX, ΔY, ΔZ and ΔF with sunspot numbers show that at Oslo the minima of ΔH, ΔX, ΔZ and ΔF precede the maxima of solar activity by an interval of 0.3 to 1.4 yr. In the case of ΔY the maximum in this component precedes the solar activity maximum by 0.6 yr. The coherence between these components and sunspot numbers is found to vary from 0.70 for ΔZ to 0.89 for ΔH.