Periodicity and hemispheric phase relationship in high-latitude solar activity

The counts of the monthly averaged polar faculae, from observations of the National Astronomical Observatory of Japan (NAOJ), are examined by using linear and nonlinear approaches to find the periodicity characteristics of the polar faculae in the northern and southern hemispheres and the phase relationship between them. Both the cross-wavelet transform (XWT) and wavelet coherence (WTC) indicate the prominent period with 95% confidence level, namely the Schwabe cycle of about 11 years. The Schwabe cycle is in phase in the two hemispheres. Within the 11-year frequency band, there is a small phase difference during the period of 1966-1975 when the activity of polar faculae in the northern hemisphere slightly leads the one in the southern hemisphere. A cross-recurrence plot analysis and the line of synchronization (LOS) extracted from the cross-recurrence plot show further the phase difference between the two hemispheres. The LOS deviates significantly from the main diagonal during the period of 1965-1970 and LOS > 0, showing that the activity of polar faculae in the northern hemisphere leads in phase, which is in accordance with XWT and WTC analyses. Moreover, asynchronization is highest (about 30 months) during this period.