LONGITUDINALLY ASYMMETRIC SUNSPOT DISTRIBUTION: A SYSTEMATIC DIFFERENCE BETWEEN THE TWO SOLAR HEMISPHERES

摘要

We study the solar cycle evolution during the last 8 solar cycles using a vectorial sunspot area called the LA (longitudinal asymmetry) parameter. This is a useful measure of solar activity in which the stochastic, longitudinally evenly distributed sunspot activity is reduced and which therefore emphasizes the more systematic, longitudinally asymmetric sunspot activity. Interesting differences are found between the LA parameter and the more conventional sunspot activity indices like the (scalar) sun-spot area and the sunspot number. E.g., cycle 19 is not the highest cycle according to LA. The separate LA parameters for the northern and southern hemisphere depict a systematic dipolar-type oscillation in the dominating hemisphere during high solar activity times which is reproduced from cycle to cycle. We have analysed this oscillation during cycles 16-22 by a superposed epoch method using the date of magnetic reversal in the southern hemisphere as the zero epoch time. According our analysis, the oscillation starts by an excess of the northern LA value in the ascending phase of the solar cycle which lasts for about 2.3 years. Soon after the maximum northern dominance, the southern hemisphere starts dominating, reaching its minimum some 1.2-1.7 years later. The period of southern dominance lasts for about 1.6 years and ends, on an average, slightly before the end of magnetic reversal.