The 1.3-year and 156-day periodicities in sunspot data: Wavelet analysis suggests a common origin

Helioseismic data have revealed a 1.3-year periodicity in the solar rotation rate near the bottom of the solar convection zone. In order to test whether these rotation rate variations have a significant impact on the solar dynamo, we search for such a periodicity in tracers of relatively freshly emerged flux at the solar surface, namely sunspots. Sunspot areas and sunspot number time series are studied with the help of the wavelet transform. Significant power at this period (1.28 years) is indeed found and is observed to vary strongly with time. This provides independent support for the presence of a 1.3 year periodicity in solar data. The power at the 154-158-day Rieger period of solar flares is seen to vary approximately in phase with the 1.28-year period. Based on this we propose that the Rieger period is the third harmonic (3 x 156 days = 1.28 years) of the 1.3-year period. If the rotation rate of the Sun does vary with 1.3 years then the enhanced flaring with the Rieger period may finally be driven by the 1.3 year periodicity. However, the power in both periods is also found to approximately follow the total number of sunspots. Therefore we cannot rule out that the 1.3-year and 156-day periods are harmonics of the solar activity cycle. Finally, our analysis of a calibrated sunspot area record reveals that the 156-day period continues into the most recent cycles, in contrast to earlier results.