M.Sc. Thesis, Dept of Physical Sciences, University of Oulu, Oulu, Finland
The Varying Length of the Solar Cycle
as Determined by Different Methods
Geophysical Observatory, Sodankylä, Finland
The sunspot numbers have traditionally been used as a measure for solar activity. They are, e.g., well correlated to the solar 10.7 cm radio flux emission rate. Recently, solar activity has been related to the long-term evolution of global temperature [Friis-Christensen and Lassen, 1991] (see figure below); the key question being whether this connection dominates over anthropogenic effects on climate.
First, this work briefly introduces the sunspots and solar activity as well as the sunspot counts in past and present. The processing of the sunspot data by running mean filters is explained, and the extrema of solar activity are determined using the filtered data and thereafter compared to the offcially published extrema.
Subsequently, a novel technique based on determining the time of half of the integrated sunspot number per cycle (Median Time) is introduced. Compared to traditional methods, which are shown to yield uncertainties of several month for the solar cycle length due to uncertainties in the determination of the solar maxima and minima, the median method is found to be very stable to these differences, reducing the uncertainty in cycle length by two orders of magnitude.
Finally, the estimates of the solar cycle length based on the median method are compared to the Northern Hemisphere Land Surface Temperature Anomalies, and to the local temperature record of Oulu, Finland. The comparisons fully support the finding of Friis-Christensen and Lassen .
References:: Friis-Christensen, Eigil, and Knud Lassen, Length of the Solar Cycle: An Indicator of Solar Activity Closely Associated with Climate, Science, 254, 698-700, 1991.
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