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Astron. Astrophys. 347, L27-L30 (1999)

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Letter to the Editor

Expansion of solar magnetic flux tubes large and small

S.K. Solanki 1, W. Finsterle 2, I. Rüedi 1 and W. Livingston 3

1 Institute of Astronomy, ETH-Zentrum, CH-8092 Zürich, Switzerland
2 Physikalisch Meteorologisches Observatorium, World Radiation Center, Dorfstr. 33, CH-7260 Davos Dorf, Switzerland
3 National Solar Observatory, National Optical Astronomy Observatories *, P.O. Box 26732, Tucson, AZ 85726, USA

Received 30 April 1999 / Accepted 14 June 1999

Abstract

In the solar photosphere the magnetic field of magnetic elements and sunspots is known to expand with height. In the case of sunspots this expansion is known to be very rapid, with the field forming an almost horizontal canopy. In this contribution we present new results on the superpenumbral canopy of sunspots based on fits to Stokes I and V profiles of infrared spectral lines. The new models take pressure balance across the boundary of the canopy field into account, which leads to significantly lower canopy base heights than previously determined from similar data.

Due to the lower canopy base height, the density above the canopy base is larger, so that estimates of the mass transported by the Evershed effect in the canopy need to be revised upwards: approximately 15-50% of the mass flowing through the penumbra travels beyond the sunspot boundary above the canopy base.

A comparison with small flux tubes leads to the surprising result that although the two types of features have magnetic fluxes that differ by 5-6 orders of magnitude, their relative rate of expansion with height is very similar, suggesting that at least in this respect sunspots can be described by the thin-tube approximation.

The remaining small differences between the relative expansion of the two types of flux tubes is qualitatively compatible with the presence of magnetic flux that returns into the solar interior at the spot boundary, as has been proposed by Westendorp Plaza et al. (1997).

Key words: magnetic fields – Sun: sunspots – Sun: faculae, plages – Sun: infrared

* Operated by the Association of Universities for Research in Astronomy, Inc. (AURA), under cooperative agreement with the National Science Foundation

Send offprint requests to: S.K. Solanki

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© European Southern Observatory (ESO) 1999

Online publication: June 30, 1999
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