EDP Sciences
Free access
Volume 367, Number 3, March I 2001
Page(s) 1033 - 1048
Section The Sun
DOI http://dx.doi.org/10.1051/0004-6361:20000495

A&A 367, 1033-1048 (2001)
DOI: 10.1051/0004-6361:20000495

Influence of magnetic fields on the coherence effects in the NaI D1 and D2 lines

J. O. Stenflo1, A. Gandorfer1, T. Wenzler1 and C. U. Keller2

1  Institute of Astronomy, ETH Zentrum, 8092 Zurich, Switzerland
2  National Solar Observatory, PO Box 26732, Tucson, AZ 85726-6732, USA

(Received 1 September 2000 / Accepted 12 December 2000 )

To clarify the physical nature of the enigmatic scattering polarization in the Na I D1 and D2 line cores we have explored their behavior with full Stokes vector polarimetry in regions with varying degree of magnetic activity near the solar limb. These observations represent the first time that ZIMPOL II, the second generation of our CCD based imaging polarimeter systems, has been used for a scientific program. With ZIMPOL II the four Stokes images can be demodulated and recorded with a single CCD sensor such that the resulting images of the fractional polarization Q/I, U/I, and V/I are entirely free from spurious features due to seeing or flat-field effects. The polarization in the cores of the lines, in particular in D2, exhibits dramatic and unexpected spatial variations in both Q/I and U/I, including polarization self-reversals of the D2 Q/I core peak. As the fluctuations in the Q, U, and V parameters appear to be relatively uncorrelated, we have parametrized the profiles and made scatter plots of the extracted parameters. Comparison with synthetic scatter plots based on different theoretical models suggests that the polarization signals in the cores of the D2 and D1 lines have different physical origins: While the D1 core is likely to be governed by ground-state atomic polarization, the D2 core is dominated by the alignment of the excited state and by effects of partial frequency redistribution.

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

Offprint request: J. O. Stenflo, stenflo@astro.phys.ethz.ch

SIMBAD Objects in preparation

© ESO 2001