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Astron. Astrophys. 345, 1006-1010 (1999)

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1. Introduction

Sunspots are among the solar active phenomena, which were earliest observed. Sporadic naked-eye observations exist in chinese dynastic histories since 165 BC (Wittmann & Xu, 1987). The sunspot number is dominated by an 11-year periodicity, which was first reported by Wolf from his examination of the Zurich observatory sunspot records. Earlier, Schwabe had announced an apparent systematic fluctuation with a shorter periodicity (see Meadows, 1970; Shove, 1983; Hathaway, et al. 1994). At present, it is known that sunspot cycles are not strictly periodic, and periods of inactivity like the Maunder minimum from 1645 to 1715 and the Sp[FORMULA]rer minimum in the 15th century (Maunder, 1922; Eddy, 1976; Ding Youji, 1978) are known to have occurred. For details, see the paper by Polygiannakis, et al.(1996), and references herein Several different approaches have been attempted for understanding and, if possible, predicting evolution of the sunspot cycle by studying the spectral, statistical, morphological and chaotic properties of the sunspot index, area or latitudes (Vigouroux & Delache, 1994; Leftus, 1994; Hong, 1990, 1994; Elling & Schwentek, 1992; Polygiannakis, et al., 1996; Feynman & Gabriel, 1990). In this paper, we describe our efforts to find a simpler function that might be used to reproduce the shape of the sunspot area cycles 12-22, simpler in terms of having fewer free parameters. We examine the relationships between the various parameters and assess the potential for an early determination of the relevant parameters for use in predicting future solar activity. The analysis reveals that, indeed, the temporal behavior of the sunspot area can be adequately described by a simple function of only two parameters for each cycle and these parameters can be determined fairly early in the cycle. Characteristics of the sunspot cycle shape are investigated in this paper too.

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© European Southern Observatory (ESO) 1999

Online publication: April 28, 1999