Determination of solar cycle length variations using the continuous wavelet transform

The length of the sunspot cycle determined by Friis-Christensen & Lassen (1 991) correlates well with indicators of terrestrial climate, but has been c riticized as being subjective. In the present paper we present a more objec tive and general cycle-length determination. Objectivity is achieved by usi ng the continuous wavelet transform based on Morlet wavelets and carrying o ut a careful error analysis. Greater generality comes from the application of this technique to different records of solar activity, e.g. sunspot numb er, sunspot area, plage area or Be-10 records. The use of different indicat ors allows us to track cycle length variations back to the 15th century. Al l activity indicators give cycle length records which agree with each other within the error bars, whereby the signal due to the solar cycle is weaker within Be-10 than in the other indicators. In addition, all records exhibit cycle length variations which are, within the error bars, in accordance with the record originally proposed by Friis- Christensen & Lassen (1991). In the 16th century, however, the Be-10 record suggests a much longer cycle than the auroral record used by Friis-Christe nsen & Lassen. Also, the presence of a distinct 11-year cycle in the Be-10 record during the Maunder Minimum is confirmed. By combining the results fr om all the indicators a composite of the solar cycle length is constructed, which we expect to be more reliable than the length derived from individua l records.