GEOPHYSICAL RESEARCH LETTERS, VOL. 25, NO. 11, 1837-1840, 1998.

A New Method to Determine Solar Cycle Length

Kalevi Mursula1 and Thomas Ulich2

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


The length of the solar cycle and its long-term variation have recently received additional significance due to their suggested connection to the Earth's climate. The cycle length is conventionally defined as the time difference between two successive epochs of minimum sunspot activity. However, the calculation of times of sunspot minima sensitively depends, on one hand, on the way the sunspot numbers are averaged, i.e. whether one uses daily, monthly or annually averaged sunspot data and, on the other hand, on whether and how the data are further smoothed. Using differently processed sunspot data, the times of sunspot minima vary typically by a few months, leading to a corresponding inaccuracy in solar cycle length. Here we propose and discuss a new method which defines the solar cycle length as a difference between the median activity times of two successive sunspot cycles. The great advantage of this method is that the median times are almost completely independent of how the sunspot minima are determined, and therefore the method allows the solar cycle lengths to be calculated with a very small inaccuracy of a few days only. We show that the individual cycle lengths calculated according to the conventional and the median method may differ by nearly a year. However, the long-term trend of cycle lengths remains roughly the same during modern times.

© 1998 by the American Geophysical Union. Further electronic publication not allowed.

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