“The 27-day periodicity in geomagnetic activity and solar wind parameters over solar cycle 23”,

A.G.ELIAS; V.M.SILBERGLEIT; A.CURCIO; P.L.LAROCCA

Solar Wind Effects Dynamics and Interactions,

Nova Science Publishers, Inc

Lugar: New York; Año: 2009; p. 391 - 402

Geomagnetic activity and solar wind parameters are analyzed in terms of the periodicity linked to solar rotation, that is the 27-day cycle. Its fluctuation in frequency and time is studied using the wavelet power spectrum. For this purpose we used the geomagnetic activity aa index and three solar wind parameters: magnetic field magnitude (B), density (d) and velocity (v). The sunspot number, Rz, is also analyzed to have a solar activity reference. The study was carried out for the period July 1996 – December 2005, which corresponds to solar cycle 23, except for the last years corresponding to its final minimum level. For the time period and parameters here analyzed, the 27-day periodicity is observed to have enhanced power during maximum and falling phase of the solar activity cycle, with no significant power during the ascending phase, not even in solar activity. Besides the time evolution, a periodicity variation is also noticed along the solar cycle. In some cases the period decreases as the solar cycle approaches minimum levels, as expected from the meridional movement of active regions towards lower solar latitudes during this time. However, periodicites lower than 27.27 days (synodic period at the solar equator ) are also observed, pointing out inner regions of the sun as possible sources of the active regions, or a s