The periodic behaviour of the North-South asymmetry of sunspot areas revisited

Abstract

Up to now, the periodic behaviour of the N-S asymmetry of solar activity has been analyzed by applying the power spectrum analysis to the time series generated from a normalized definition of the asymmetry. Using sunspot areas and the properties of the discrete Fourier transform, we show that the use of the normalized time series leads to misleading results, and that the correct asymmetry time series to be used is generated from the difference between the values of solar activity indicators in the Northern and Southern solar hemispheres. However, in this case the found significant periodicities correspond to periodicities already present in the hemispheric sunspot areas time series, not providing with any interesting information about the asymmetric behaviour of the solar activity. Of course, our conclusions are extensive to all the studies of the periodic behaviour of the N-S asymmetry of solar activity time series computed by means of the normalized definition of the asymmetry, and independent of the considered solar activity feature.