How accurately do we know the lengths of the sunspot cycles?

Thomas Ulich1,2

1Space Physics Group, Dept. of Physical Sciences, FIN-90014 University of Oulu, Finland
2Sodankylä Geophysical Observatory, FIN-99600 Sodankylä, Finland


The sunspot numbers have traditionally been used as a measure for the solar activity. Solar activity, in turn, has been related to the long-term evolution of global temperature and other climate parameters. Esp. the time series of the durations of the Schwabe Cycles correlates well with northern hemispheric air temperatures. The key question is whether this connection dominates over the anthropogenic effects on climate.

In this work I re-analyse the sunspot cycle lengths using traditional methods and a novel technique based on determining the time ('median time') of half of the integrated sunspot number per cycle. While traditional methods are sensitive to the different estimations of sunspot minima or maxima, the median method is very stable to these differences. Furthermore, possible gaps in the time series of sunspot counts can safely be interpolated with only little effect on the median cycle length.