Long-term variations in the mean meridional motion of the sunspot groups

Abstract

Aims. We seek the long-term variations close to the length of a solar cycle in the mean meridional motion of sunspot groups (a proxy of the meridional plasma flow). Methods. Using the largest set of available reliable sunspot group data, the combined Greenwich and Solar Optical Observation Network sunspot group data during the period 1879 – 2008, we determined variations in the mean meridional motion of the sunspot groups in the Sun’s whole northern and southern hemispheres and also in different 10 latitude intervals.We determined the variations from the yearly data and for the sake of better statistics by binning the data into 3 – 4 year moving time intervals (MTIs) successively shifted by one year. We determined the periodicities in the mean meridional motion from the fast Fourier transform (FFT) power spectrum analysis. The values of the periodicities are determined from the maximum entropy method (MEM) and the temporal dependencies of the periodicities are determined from the Morlet-wavelet analyses. Results. We find that the mean meridional motion of the spot groups varies considerably on a time scale of about 5 – 20 years. The maximum amplitude of the variation is about 10 – 15 m s−1 in both the northern and the southern hemispheres. Variation in the mean motion is considerably different during different solar cycles. At the maximum epoch (year 2000) of the current cycle 23, the mean motion is relatively strong in the past 100 years and northbound in both the northern and the southern hemispheres. This abnormal behavior of the mean motion may be related to the low strength and the long duration of the current cycle, and also to the violation of the Gnevyshev and Ohl rule by the cycles pair 22,23. The power spectral analyses suggest the existence of 3.2- and 4.3-year periodicities in the mean motion of the spot groups in the southern hemisphere, whereas a 13 – 16 year periodicity is found to exist in the mean motion of the northern hemisphere. There is strong evidence for a latitude-time dependency in the periodicities of the mean motion. The north-south difference in the mean motion also varies by about 10 m s−1. During the recent cycles, the northsouth difference is negligibly small. Approximate 12- and 22-year periodicities are found to exist in the north-south difference. The implications of all these results are briefly discussed.