Evolution and rotation of large-scale photospheric magnetic fields of the Sun during cycles 21–23

Periodicities, north-south asymmetries and r-mode signatures

¹ Institute of Astronomy, ETH-Zentrum, 8092 Zurich, Switzerland e-mail: [rknaack;stenflo;sveta]@astro.phys.ethz.ch
² Faculty of Mathematics and Science, University of Zurich, 8057 Zurich, Switzerland
³ Astronomy Division, PO Box 3000, 90014 University of Oulu, Finland

Received: 29 September 2004
Accepted: 24 March 2005

Abstract

We present the results of an extensive time series analysis of longitudinally-averaged synoptic maps, recorded at the National Solar Observatory (NSO/Kitt Peak) from 1975 to 2003, and provide evidence for a multitude of quasi-periodic oscillations in the photospheric magnetic field of the Sun. In the low frequency range, we have located the sources of the , , and periodicities that were previously detected in the north-south asymmetry of the unsigned photospheric flux (Knaack et al. 2004, A&A, 418, L17). In addition, quasi-periodicities around and have been found. The period is most likely related to large-scale magnetic surges toward the poles and appeared in both hemispheres at intermediate latitudes ~– during the maxima of all three cycles 21–23, being particularly pronounced during cycle 22. Periods near have recently been reported in the rotation rate at the base of the convection zone (Howe et al. 2000, Science, 287, 2456), in the interplanetary magnetic field and geomagnetic activity (Lockwood 2001, J. Geophys. Res., 106, 16021) and in sunspot data (Krivova & Solanki 2002, A&A, 394, 701). In the intermediate frequency range, we have found a series of quasi-periodicities of 349–, , 249–, 222–, , 158–, 129– and 103–, which are in good agreement with period estimates for Rossby-type waves and occurred predominantly in the southern hemisphere. We provide evidence that the best known of these periodicities, the Rieger period around , appeared in the magnetic flux not only during cycle 21 but also during cycle 22, likely even during cycle 23. The high frequency range, which covers the solar rotation periods, shows a dominant (synodic) periodicity in the southern hemisphere during cycles 21 and 22. A periodicity around 25.0– occurred in the south during all three cycles. The large-scale magnetic field of the northern hemisphere showed dominant rotation periods at during cycle 21, at 28.3– during cycle 22 and at during cycle 23.