1 Introduction

The relative sunspot numbers R are a measure of solar activity on the entire disk of the Sun. The relevance of the relative sunspot numbers lies in particular in the fact that they represent one of the longest time series of solar activity indices available. Thus, relative sunspot numbers provide the foundation of a continuous data set for research on the solar cycle and its long-term variations. R is defined by

 

R = k   (10 g + f), (1)

where g is the number of observed sunspot groups, f the number of spots and k is an observatory-related correction factor (the details depending on the actual seeing conditions and the instrument used).

Historically it can be noted that R is the modified form of the so-called Wolf index or Wolf number, which was defined without the correction factor k. The Wolf number was introduced in 1848 by Rudolph Wolf, who was the first to compile daily Sunspot Numbers. The original intention of introducing the correction factor k in about 1882 by Wolf's successors at the Zürich Observatory was to convert the actual daily measurements to the scale originated by Wolf (cf. Waldmeier 1961). Waldmeier (1961) compiled actual records from the Zürich Observatory in coordination with various additional observing stations as well as data collected by Wolf (1858). Beginning from 1700, yearly means of relative sunspot numbers are listed; starting with 1749 monthly mean Sunspot Numbers also are given. This compilation of the so-called Zürich relative sunspot numbers is one of the most commonly used databases of solar activity (see also Hoyt & Schatten 1998a,b). Beginning with 1981, the Zürich relative sunspot program was replaced by the Sunspot Index Data Center (SIDC) in Brussels, which is the World Data Center for Sunspot Indices (see also Rishbeth 1991; Ruttenberg & Rishbeth 1994).

In contrast to the relative sunspot numbers, the hemispheric Sunspot Numbers $R_{\rm n}$ and $R_{\rm s}$ were not compiled on a regular basis and are not available before 1992. From January 1992, the daily $R_{\rm n}$ and $R_{\rm s}$ as well as monthly means have been compiled by the SIDC (Cugnon 1997). However, for several research purposes, in particular the study of North-South (N-S) asymmetries of solar activity, as it is presented in this paper (see Sect. 4), hemispheric Sunspot Numbers are needed for longer periods. With the present catalogue we aim to provide hemispheric Sunspot Numbers for at least two full solar cycles (21 and 22). In general, we derived the hemispheric Sunspot Numbers for the period 1975-2000. The data beginning from January 1992 are used for an estimation of the consistency of the derived $R_{\rm n}$ and $R_{\rm s}$ with respect to the International $R_{\rm n}$ and $R_{\rm s}$ compiled by the SIDC.

At the Kanzelhöhe Solar Observatory (KSO), Austria, daily sunspot drawings have been provided since 1947 within the framework of solar surveillance programs. The daily provisional relative sunspot numbers that are derived from the drawings are compiled and reported to the SIDC in Brussels. The KSO is part of collaborating observatories all over the world from which provisional Sunspot Numbers are collected and averaged in an advanced form. The finally derived numbers are published as the definitive International Sunspot Numbers $R_{\rm i}$ by the SIDC. A detailed description of the calculation of the definitive International Sunspot Numbers, which in particular has to ensure that the scaling with respect to the Zürich Sunspot Numbers is maintained, is given in Cugnon (1997) as well as online at http://sidc.oma.be/index.php3. Details concerning the KSO and its observing capabilities can be found in Steinegger et al. (2001). Furthermore, the sunspot drawings and other observational data are online available at the KSO web page at http://www.kso.ac.at/.

The plan of the paper is as follows. In Sect. 2 the gathering and validation of the hemispheric Sunspot Numbers is described. Section 3 gives a description of the online catalogue. In Sect. 4´, an analysis of the N-S asymmetry based on the hemispheric Sunspot Numbers is presented. In Sect. 5 we discuss our results and in Sect. 6 the conclusions are drawn.