2006A&A...447..735T

From sunspot drawings provided by the Kanzelhoehe Solar Observatory, Austria, and the Skalnate Pleso Observatory, Slovak Republic, we extracted a data catalogue of hemispheric Sunspot Numbers covering the time span 1945-2004. The validated catalogue includes daily, monthly-mean, and smoothed-monthly relative sunspot numbers for the northern and southern hemispheres separately and is available for scientific use. These data we then investigated with respect to north-south asymmetries for almost 6 entire solar cycles (Nos. 18-23). For all the cycles studied, we found that the asymmetry based on the absolute asymmetry index is enhanced near the cycle maximum, which contradicts to previous results that are based on the normalized asymmetry index. Moreover, the weak magnetic interdependence between the two solar hemispheres is confirmed by their self-contained evolution during a cycle. For the time span 1945-2004, we found that the cycle maxima and also the declining and increasing phases are clearly shifted, whereas the minima seem to be in phase for both hemispheres. The asymmetric behavior reveals no obvious connection to either the sunspot cycle period of ∼11- or the magnetic cycle of ∼22-years. The most striking excess of activity is observed for the northern hemisphere in cycles 19 and 20.