ANALYSIS OF THE FORWARD-SCATTERING HANLE EFFECT IN THE Ca I 4227 A LINE

Journal Article · Sat Aug 20 00:00:00 EDT 2011 · Astrophysical Journal

DOI:https://doi.org/10.1088/0004-637X/737/2/95· OSTI ID:21579938

Anusha, L S; Nagendra, K N; Sampoorna, M; Smitha, H N ^[1]; Bianda, M; Stenflo, J O; Ramelli, R ^[2]; Holzreuter, R ^[3]; Frisch, H ^[4]

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1. Indian Institute of Astrophysics, Koramangala, Bangalore 560 034 (India)
2. Istituto Ricerche Solari Locarno, Via Patocchi, 6605 Locarno-Monti (Switzerland)
3. Institute of Astronomy, ETH Zuerich, CH-8093 Zuerich (Switzerland)
4. UNS, CNRS, OCA, Laboratoire Cassiopee, F-06304 Nice Cedex (France)

Coherent scattering of limb-darkened radiation is responsible for the generation of the linearly polarized spectrum of the Sun (the Second Solar Spectrum). This Second Solar Spectrum is usually observed near the limb of the Sun, where the polarization amplitudes are largest. At the center of the solar disk the linear polarization is zero for an axially symmetric atmosphere. Any mechanism that breaks the axial symmetry (like the presence of an oriented magnetic field, or resolved inhomogeneities in the atmosphere) can generate a non-zero linear polarization. In the present paper we study the linear polarization near the disk center in a weakly magnetized region, where the axisymmetry is broken. We present polarimetric (I, Q/I, U/I, and V/I) observations of the Ca I 4227 A line recorded around {mu} = cos {theta} = 0.9 (where {theta} is the heliocentric angle) and a modeling of these observations. The high sensitivity of the instrument (ZIMPOL-3) makes it possible to measure the weak polarimetric signals with great accuracy. The modeling of these high-quality observations requires the solution of the polarized radiative transfer equation in the presence of a magnetic field. For this we use standard one-dimensional model atmospheres. We show that the linear polarization is mainly produced by the Hanle effect (rather than by the transverse Zeeman effect), while the circular polarization is due to the longitudinal Zeeman effect. A unique determination of the full B vector may be achieved when both effects are accounted for. The field strengths required for the simultaneous fitting of Q/I, U/I, and V/I are in the range 10-50 G. The shapes and signs of the Q/I and U/I profiles are highly sensitive to the orientation of the magnetic field.

• OSTI ID: 21579938
• Journal Information: Astrophysical Journal, Vol. 737, Issue 2; Other Information: DOI: 10.1088/0004-637X/737/2/95; ISSN 0004-637X
• Country of Publication: United States
• Language: English

Related Subjects

79 ASTROPHYSICS
COSMOLOGY AND ASTRONOMY
AXIAL SYMMETRY
COHERENT SCATTERING
MAGNETIC FIELDS
ONE-DIMENSIONAL CALCULATIONS
POLARIZATION
RADIANT HEAT TRANSFER
SIMULATION
SOLAR ATMOSPHERE
SUN
ZEEMAN EFFECT
ATMOSPHERES
ENERGY TRANSFER
HEAT TRANSFER
MAIN SEQUENCE STARS
SCATTERING
STARS
STELLAR ATMOSPHERES
SYMMETRY