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Astron. Astrophys. 364, 829-834 (2000)

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1. Introduction

Important physical processes in sunspots occur on very small spatial scales, at or below the resolution limit of the instruments that are currently available. In the past, there have been conflicting results concerning a relationship between the magnetic field strength, the material flow and the brightness of the narrow, elongated penumbral filaments (see review of Wiehr 1999). Some observers reported a correlation between the intensity and the magnetic field strength, whereas Schmidt el al. (1992) found a correlation between the magnetic field inclination and the intensity, and no variation of the field strength. From a statistical analysis of two-dimensional data, Title et al. (1993) find a correlation between brightness and the velocity of the Evershed flow, and a difference of some 5o between the inclination angle of the bright and dark component.

From the analysis of one-dimensional slit spectra, observers could not establish a clear relationship between brightness and inclination angle, of either the material flow or the magnetic field. This has often been correctly addressed to insufficient spatial resolution. On the other hand, slit spectra always intersect the penumbra at different spot radii, thus mixing the properties of inner, central and outer penumbra.

Neglecting the small-scale structure, Schlichenmaier & Schmidt (2000, hereafter referred to as SS2000) have described the overall geometry of the Evershed flow in the deepest observable layers of a sunspot penumbra. They find an upflow in the inner part of the penumbra, followed by nearly horizontal motion in the middle penumbra. At the outer edge of the spot, but still inside the spot, the flow is slightly inclined downwards.

In the present work we apply a refined analysis to the same data, which allows us to distinguish between the material flow in the bright and the dark penumbral elements. We compare our findings with the predictions of the moving tube model (Schlichenmaier et al. 1998): This model describes the penumbral filaments as magnetic flux tubes in which hot material is moving upwards and outwards due to an internal gas pressure gradient. In the outflow phase the material cools by radiation. Therefore a penumbral filament that appears bright in the inner penumbra may become dark towards the outer penumbra.

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© European Southern Observatory (ESO) 2000

Online publication: January 29, 2001