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Astron. Astrophys. 331, 760-770 (1998)

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Hanle depolarisation in the solar chromosphere

M. Bianda 1, S.K. Solanki 2 and J.O. Stenflo 2

1 Istituto Ricerche Solari Locarno (IRSOL), Via Patocchi, CH-6605 Locarno-Monti, Switzerland
2 Institute of Astronomy, ETH-Zentrum, CH-8092 Zürich, Switzerland

Received 17 April 1997 / Accepted 3 November 1997


A new polarimeter and an extension of the polarimetric technique of Donati et al. (1990) to higher polarisation values are presented. With this system, which allows low-noise polarimetric observations in the blue and near-UV part of the solar spectrum, we have recorded more than 200 Q/I profiles of Ca I 4227 Å with an accuracy of 2-3 [FORMULA].

We find that while the [FORMULA] line shape outside the line core is very stable from one part of the solar surface to the next, the line core exhibits large variations relative to the rest of the profile. These variations are best interpreted in terms of partial depolarisation produced by the Hanle effect in the presence of weak fields. We obtain a quantitative and model-independent estimate of the depolarisation, from which we deduce field strengths of 5-15 G at the level of line-core formation, i.e. in the low to mid chromosphere. We also find evidence that the field strength is distributed around these average values with a FWHM that is approximately the same as the average value.

Key words: polarization – scattering – polarimeter – polarimetric technique – Sun: magnetic field – Sun: chromosphere – Hanle effect

© European Southern Observatory (ESO) 1998

Online publication: February 16, 1998