The amplitude and length of sunspot cycles are known to vary within wide ranges. The quantitative data of sunspot numbers exist from around 1700, or the end of the Maunder minimum. Earlier records can be produced from ice cap drillings (e.g. Blunier et al., 1998) that indirectly reveal solar activity variations through their influence on Earth's climate. Only since the records by Carrington (1853-1861), have the heliographic coordinates of individual sunspot measurements been included. Sunspot cycles are variable with respect to magnitude, as measured by the numbers of spots, duration, as measured by the time between successive minima, and also in the latitudinal distribution of the spots. It is the latter which is the particular object of this study, with special attention being given to asymmetries between the north and south hemispheres.
Variations in all these three measures do not appear to be purely random, but rather to display long term patterns of behaviour. This has encouraged people to look for stable quantities or longer cycles than the 11-year one. Already Wolf (1861) proposed the constancy of the product of sunspot cycle length and amplitude which turned out to be, however, incorrect. Gleissberg (1967) introduced a long-time period, the Gleissberg cycle, having a period of about 80 years. This rather freely defined period is roughly estimated as the time lapse between groups of inactive cycles with active cycles in between. Later Yoshimura (1979) reported a modulation of the solar cycle length which repeated every 5 cycles. This cycle of approximately 55 years is difficult to reconcile with Gleissberg's result. Recently, growing interest has been shown to the possible solar impact on the observed global warming, in particular to evidence that long-term mean temperature variations are correlated strongly with variations in the length of the solar cycle (see Friis-Christensen & Lassen, 1991).
The north-south asymmetry of sunspots has been studied for the last five decades and its existence has proved to be real (Newton & Milson, 1955; Roy, 1977; Vizoso & Ballester, 1989). Waldmeier (1957 and 1971) pointed out a phase shift between hemispheres and suggested a period over 8 eleven-year cycles. Verma (1993) studied long-term north-south sunspot asymmetry and found a characteristic period of 110 years. Oliver & Ballester (1994) reported on a long-term trend in the asymmetry on a time-scale of years, and also on a shift of the dominance of solar activity from the north to the south during sunspot cycle 22. In this letter we consider the activity variation and north-south asymmetry through sunspot latitudes and compare results with those in earlier studies.
The Sun's magnetic activity is generally believed to be supplied by a hydromagnetic dynamo operating either in or at the base of the solar convective zone. Dynamo models predict the possibility of mixed parity solutions where the field has both dipolar and quadrupolar components. Such fields would be asymmetric with respect to the equator. Indeed there is evidence that the Sun's field was highly asymmetric as it emerged from the Maunder Minimum (Ribes and Nesme-Ribes 1993, Sokoloff and Nesme-Ribes 1994). It is often stated that the solar field has been dipolar since then (e.g. Tobias 1996, 1998). The observed asymmetries would thus not be intrinsic to the dynamo and could be regarded as a noisy signal imposed on the dipolar field. However, it has recently been proposed that the dipolar and quadrupolar components could both be oscillating coherently (although the quadrupolar component would be much weaker) and that therefore the observed asymmetry should have a periodic component to its signal (Brooke et al. 1998).
© European Southern Observatory (ESO) 1999
Online publication: December 4, 1998