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Evidence for a downward mass flux in the penumbral region of a sunspot

Nature volume 389, pages 47–49 (1997)Cite this article

Sunspots were the first extraterrestrial phenomenon found to harbour magnetic fields1,2. But the physical nature of sunspots and their relationship to the Sun's global magnetic field are still poorly understood3. Perhaps the largest uncertainty is related to the outermost region of sunspots (the penumbra) and, in particular, the nature of the so-called Evershed4 flow—a stream of material emanating radially from sunspots at velocities of up to ∼6 km s−1(ref. 5), before vanishing abruptly at the outer penumbral edges. Here we make use of a recently developed optical tomographic technique6 to obtain a three-dimensional model of the magnetic field and mass flow in the vicinity of a sunspot. We find that some of the magnetic field lines, together with a significant part of the Evershed mass flux, flow back towards the Sun in the deepest atmospheric layers at the outer edge of the sunspot and its surroundings. This observation should provide an important clue to our understanding of the appearance, stability and decay of sunspots, the most conspicuous tracers of the solar activity cycle.

The Evershed effect consists of a shift of the absorption line profiles towards shorter wavelengths (that is, blueshift) in the penumbra closest to the centre of the solar disk, and a redshift on the opposite side farthest from the disk centre. The effect produces asymmetric absorption line profiles, in the sense that the line wings are more shifted than the line core. This asymmetry in turn results in a net circular polarization when integrated over the full line profile, and has been linked to a gradient of the line-of-sight component of the velocity7,8,9. The decrease in velocity with height has also been determined by simultaneously observing several spectral lines which probe different heights in the solar atmosphere. Spectral lines forming at very high levels (in the solar chromosphere) even show that the direction of the motion is an inflow rather than an outflow at these heights. The Evershed velocities disappear when the spot is close to the disk centre, and steadily grow as the spot approaches the solar limb, suggesting that the flows are nearly parallel to the solar surface at average photospheric levels.

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Figure 1: Simple round sunspot near the centre of the solar disk.
Figure 2: Diagram of penumbral structure as inferred from our results.

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Acknowledgements

This work was supported in part by the Spanish DGES. The NCAR is sponsored by the National Science Foundation.

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Authors and Affiliations

  1. Instituto de Astrofísica de Canarias, La Laguna, E-38200, Tenerife, Spain

    C. Westendorp Plaza, J. C. del Toro Iniesta, B. Ruiz Cobo & V. Martínez Pillet

  2. High Altitude Observatory, NCAR, Boulder, PO Box 300, Colorado, 80307, USA

    B. W. Lites & A. Skumanich

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  1. C. Westendorp Plaza
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Plaza, C., del Toro Iniesta, J., Cobo, B. et al. Evidence for a downward mass flux in the penumbral region of a sunspot. Nature 389, 47–49 (1997). https://doi.org/10.1038/37933

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