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Astron. Astrophys. 329, 704-720 (1998)

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Complex magnetic fields in an active region

P.N. Bernasconi 1, C.U. Keller 2, S.K. Solanki 1 and J.O. Stenflo 1

1 Institut für Astronomie, ETH-Zentrum, CH-8092 Zürich, Switzerland
2 NOAO*/National Solar Observatory, P.O.Box 26732, Tucson, AZ 85726, USA

Received 23 December 1996 / Accepted 22 May 1997

Abstract

High-resolution observations of the full Stokes vector in Fe i spectral lines around 5250 Å obtained at the Swedish Vacuum Solar Telescope on La Palma with the ZIMPOL I Stokes polarimeter in a complex active region reveal the presence of anomalously shaped Stokes profiles indicating the coexistence of at least two magnetic components within the same spatial resolution element. These Stokes profiles have been analyzed with an inversion code based on a 3-component atmospheric model with two magnetic and one field-free component. The fits to the observations in a magnetic region that resembles a small penumbra reveal the presence of a horizontal magnetic field component with an average field strength of [FORMULA]  G, a mean filling factor of [FORMULA], and an average temperature [FORMULA]  K at [FORMULA] embedded in the main "penumbral" magnetic field that has [FORMULA]  G, [FORMULA], and [FORMULA]  K. The horizontal component exhibits a mean outflow of 2.7 km s-1 which is mainly due to the Evershed flow. In a region where there are strong downflows up to 7 km s-1, we infer the possible presence of a shock front whose height changes along the slit. The height variation can be explained by a change of the gas pressure at the base of the photosphere below the shock front as proposed by Thomas & Montesinos (1991). Small plages with field strengths below 900 G have been observed in the vicinity of some pores. Finally, we present a puzzling field structure at the boundary between two adjacent pores. Ambiguous results suggest that although the inversion code is able to successfully invert even very complex Stokes profiles, we are far from a complete description of the field structure in complex magnetic regions. We warn that magnetograms and fits to data involving only a single magnetic component may hide the true complexity of the magnetic structure in at least some parts of active regions.

Key words: Sun: magnetic fields – flux tubes – active regions – polarimetry

* The National Optical Astronomy Observatories are operated by the Association of Universities for Research in Astronomy, Inc., under cooperative agreement with the National Science Foundation.

Send offprint requests to: P.N. Bernasconi

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© European Southern Observatory (ESO) 1998

Online publication: December 8, 1997
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