Moments of the latitudinal dependence of the sunspot cycle: a new diagnostic of dynamo models

¹ Max-Planck-Institut für Sonnensystemforschung, 37191 Katlenburg-Lindau, Germany e-mail: [solanki;schmitt]@mps.mpg.de
² Institute of Astronomy, ETH Zentrum, 8092 Zurich, Switzerland e-mail: wenzler@astro.phys.ethz.ch

Received: 30 September 2005
Accepted: 6 March 2008

Abstract

Aims. The latitude-distribution of solar activity as represented by sunspots is studied.

Methods. We first determined the latitudinal distribution of a sunspot cycle by integrating the butterfly diagram at each latitude over the length of each cycle. We then formed the five lowest moments of the latitudinal distribution of all complete sunspot cycles since 1874 and compared these moments with each other.

Results. The three lowest moments correlate remarkably well with each other. For example, the mean latitude of the sunspots during a cycle and the latitude range are correlated at the 0.96 level. A clear asymmetry is seen between the two hemispheres, with the southern solar hemisphere showing consistently stronger and more positive correlations than the northern hemisphere. When applied to different simple dynamo models, the same analysis reveals significant differences between the models and demonstrates that such moments are a useful diagnostic in distinguishing between dynamo models. Remarkably, dynamos without a meridional flow provide results closer to those of the Sun's northern hemisphere, while a dynamo with a meridional flow produces fields more like those in the Sun's southern hemisphere. This may provide a clue to the cause of the well-known north-south asymmetry of solar activity.