The Length of the Sunspot Cycle
Determined by Different Methods

Thomas Ulich(1) and Kalevi Mursula(2)

(1) Geophysical Observatory, FIN-99600 Sodankylä, Finland
(2) Dept. of Physical Sciences, University of Oulu, FIN-90570 Oulu, Finland


The sunspot numbers have traditionally been used as a measure for the solar activity. The sunspot number is, e.g., well correlated to the 10.7cm radio flux emission rate.
Recently, solar activity has been related to the long-term evolution of global temperature (Friis-Christensen and Lassen, 1991). The key question is whether this connection dominates over the anthropogenic effects on climate.
In this work we re-analyse the solar 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.
Finally we correlate the revised solar cycle data with the Oulu temperature record, which is one of the longest available in Finland.