Quasi-Periodicities, Magnetic Clusters and Solar Activity
To investigate quasi-periodicities, 12 hour averages of the radial component of the interplanetary magnetic field and the solar wind speed, covering ~ 42 (1963-2005) years were analyzed. A Lomb periodogram for data up to 1998 showed a dominant period of 27.03 days as fpund in earlier results. Including cycle 23, a dominant period of 27.06 days was identified. Analysis of the solar cycles independently showed a dominant period of 27.03 days in solar cycle 20, but not in the other cycles. To investigate the degree of persistency of a particular signal, the technique of complex demodulation was applied since it permits the determination of continuous changes in time of the amplitude and frequency of the signal relative to the test signal. It was found that a period of ~27.6 days gave an overall flat phase function in time, while other periods < ~0.5 day shorter and longer, with comparable but lesser amplitude, come and go. To investigate the solar sources of these periods, the method of principal component analysis (PCA) was applied to ~ 27 years (1976-2003) of synoptic maps obtained with the NSO Kitt Peak Vaccum Telescope. Before the analysis, the original synoptic maps were shifted relative to the previous maps using the period under investigation. Using PCA the Empirical Orthogonal Functions (EOFs) and Pricipal Components (PCs) were found for the set of synoptic maps rescaled to the rotation rate 27.03 days in 1999-2003. The patterns characterized by EOFs 1 and 2 are mostly axisymmetric and PCs 1 and 2 show solar cycle variability. EOF3 shows only one well-localized pattern in the Southern Hemisphere which is markedly non-axisymmetric and PC3 has peaks at times when fast CMEs occur.