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Astron. Astrophys. 333, 721-731 (1998)


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On the reliability of Stokes diagnostics of magnetic elements away from solar disc centre

S.K. Solanki 1, O. Steiner 2, 3, M. Bünte 1, G. Murphy 4 and S.R.O. Ploner 1

1 Institute of Astronomy, ETH-Zentrum, CH-8092 Zürich, Switzerland
2 Kiepenheuer Institut für Sonnenphysik, Schöneckstrasse 6, D-79104 Freiburg i.Br., Germany
3 High Altitude Observatory, National Center for Atmospheric Research, Boulder, CO 80307, USA
4 Applied Physics Laboratory, Johns Hopkins University, Laurel, MD 20702, USA

Received 16 June 1997 / Accepted 22 December 1997

Abstract

The spectropolarimetric signature of models of small-scale magnetic features is well understood at the centre of the solar disc, but has been little studied near the solar limb, mainly because the detailed geometry of the flux tubes must then be taken into account in a realistic analysis. We present multi-ray calculations of Stokes profiles through arrays of 2-D magnetohydrostatic models of small flux tubes. We compare the Stokes profile shapes and Stokes based diagnostics (Q to V ratio, V amplitude, magnetic line ratio, centre of gravity wavelengths, etc.) resulting from plane-parallel and 2-D flux-tube models at different limb distances for two lines in the visible and an infrared H-band line. For the visible lines around 5250 Å all the diagnostics we have studied, with the exception of the Q and U to V ratio, are significantly affected by the finite size of the flux tubes and the passage of the rays through non-magnetic material near the limb. We show that magnetic filling factors and the global magnetic flux may be underestimated using the usual calibration techniques. In addition, near the solar limb the magnetic line ratio can move into a regime that is forbidden according to simpler models. The spatially averaged longitudinal field derived from the centre-of-gravity method also becomes strongly model dependent. The thermal structure of the non-magnetic atmosphere appears to play a particularly important role in determining the values of these diagnostic parameters. The infrared line at 1.5648 µm, on the one hand, reacts little to the external atmosphere and provides superior diagnostics of both the magnetic field strength and filling factor. On the other hand, for a flux tube with a sharp boundary this line shows little dependence on the flux-tube diameter, in contrast to the earlier findings of Zayer et al. (1989).

Key words: Sun: magnetic fields – Sun: infrared – Sun: faculae, plages – polarization – line: profiles – line: formation – radiative transfer

Send offprint requests to: S.K. Solanki

© European Southern Observatory (ESO) 1998

Online publication: April 20, 1998
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