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.