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Astron. Astrophys. 350, 1060-1070 (1999)


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Hanle effect observations with the Ca I 4227 Å line

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

1 Istituto Ricerche Solari Locarno (IRSOL), Via Patocchi, 6605 Locarno-Monti, Switzerland
2 Institute of Astronomy, ETH Zentrum, 8092 Zürich, Switzerland (stenflo@astro.phys.ethz.ch)

Received 6 July 1999 / Accepted 10 September 1999

Abstract

The Hanle effect in the Ca I 4227 Å line has been explored through the analysis of a large number of Stokes profile recordings obtained on the quiet Sun with the beam-splitter polarimeter system at IRSOL (Istituto Ricerche Solari Locarno). In contrast to previous Hanle observations with this line, which were limited to the Stokes I and Q parameters, we are now in a position to study the combined effects of Hanle depolarization (via Stokes Q) and rotation of the plane of linear polarization (via Stokes U) with the same methods that we recently applied to the Sr II 4078 Å line. The Hanle histograms for the distributions of the depolarization and rotation parameters are very similar for the two lines and show that there must be mixed contributions to the Hanle signals from spatially unresolved magnetic fields with random orientations (which do not contribute to Stokes U) and partially resolved magnetic fields with a net orientation of the field vectors. Field strengths in the range 5-10 G are preferred. We also determine the "Hanle efficiency profile", which shows how the Hanle effect is confined to the Doppler core but vanishes in the line wings. It is wider than the corresponding profile for the Sr line, as expected from the difference in atomic weight and wavelength between the two lines. The [FORMULA] profiles of the Ca I 4227 Å line have minima around the Doppler core which turn negative (polarization perpendicular to the limb) for limb distances [FORMULA], a likely signature of partial redistribution effects.

Key words: atomic processes – polarization – scattering – techniques: polarimetric – Sun: magnetic fields

Send offprint requests to: J.O. Stenflo

© European Southern Observatory (ESO) 1999

Online publication: October 14, 1999

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