Scale-resolved phase coherence analysis of hemispheric sunspot activity: a new look at the north-south asymmetry

Aims. The sunspot activity on both solar hemispheres is known to show time intervals of different phase relationships. In this letter, we introduce an advanced statistical framework that allows an appropriate analysis of these relationships. Methods. We combine wavelet analysis with statistical measures of phase coherence to obtain well-defined phase differences in dependence on the selected oscillation period. Results. Our approach characterises the temporally varying phase difference of the hemispheric sunspot activity over the past 130 years, which is a major contribution to the north-south asymmetry. Coherent phase variables are found to exist for a frequency band with periodicities around the dominating 11-year cycle. We observe significant continuous long-term variability of interhemispheric phase shifts, which is not reproduced by solar activity records from previous centuries.