North-south asymmetry of solar activity as a superposition of two realizations – the sign and absolute value

Abstract

Context. Since the occurrence of north-south asymmetry (NSA) of alternating sign may be determined by different mechanisms, the frequency and amplitude characteristics of this phenomenon should be considered separately. Aims. We propose a new approach to the description of the NSA of solar activity. Methods. The asymmetry defined as A = ( N − S )/( N + S ) (where N and S are, respectively, the indices of activity of the northern and southern hemispheres) is treated as a superposition of two functions: the sign of asymmetry (signature) and its absolute value (modulus). This approach is applied to the analysis of the NSA of sunspot group areas for the period 1874–2013. Results. We show that the sign of asymmetry provides information on the behavior of the asymmetry. In particular, it displays quasi-periodic variation with a period of 12 yr and quasi-biennial oscillations as the asymmetry itself. The statistics of the so-called monochrome intervals (long periods of positive or negative asymmetry) are considered and it is shown that the distribution of these intervals is described by the random distribution law. This means that the dynamo mechanisms governing the cyclic variation of solar activity must involve random processes. At the same time, the asymmetry modulus has completely different statistical properties and is probably associated with processes that determine the amplitude of the cycle. One can reliably isolate an 11-yr cycle in the behavior of the asymmetry absolute value shifted by half a period with respect to the Wolf numbers. It is shown that the asymmetry modulus has a significant prognostic value: the higher the maximum of the asymmetry modulus, the lower the following Wolf number maximum. Conclusions. A fundamental nature of this concept of NSA is discussed in the context of the general methodology of cognizing the world. It is supposed that the proposed description of the NSA will help clarify the nature of this phenomenon.