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Astron. Astrophys. 339, 610-614 (1998)

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1. Introduction

Sunspots are exceedingly complex magnetic structures in the photosphere. See, for example, Stanchfield et al. (1997). The diameter of the central dark region, called the umbra , varies widely, but it is typically 5-10 Megameters, and the magnetic field at the centre is a few tenths of a tesla.

Innumerable papers have reported observations on sunspots. For example, Koutchmy & Adjabshirizadeh (1981), and later Adjabshirizadeh & Koutchmy (1983), reported careful observations on a single spot. More recently, Solanki & Schmidt (1993) summarized the observations of several groups and Stanchfield et al. (1997) reported on an approximately axisymmetric spot.

Equally innumerable theoretical papers have been written on sunspots. To our knowledge, none account for the existence of the azimuthal currents that necessarily accompany the axial magnetic field. The INSPEC compilation lists about 150 papers per year on sunspots.

The model discussed here was first proposed by Hénoux & Somov (1987), and later developed by Heyvaerts & Hagyard (1991), by Hirayama (1991), and by Lorrain & Koutchmy (1993), for magnetic elements. These latter authors later used it in their model for solar spicules (Lorrain & Koutchmy 1996). This model accounts for the existence of the azimuthal current. However, it does not account for the penumbra, nor for dots, light bridges, etc. in the umbra, nor for the Evershed flow or the Wilson depression.

This dynamo is axisymmetric, which is forbidden by the Cowling "theorem" (Cowling 1934, 1957, 1976). However, it has long been clear that the "theorem" is a misconception (Kolm & Mawardi 1961, Shercliff 1965, Fearn et al. 1988, Alexeff 1989, Lorrain, 1991, Ingraham 1995).

We assume a single, vertical, axisymmetric magnetic flux tube. This is an approximation. For example, Lites et al. (1993) studied an umbral magnetic field that comprised five maxima, all of the same polarity.

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© European Southern Observatory (ESO) 1998

Online publication: October 21, 1998