Issue |
A&A
Volume 432, Number 1, March II 2005
|
|
---|---|---|
Page(s) | 295 - 305 | |
Section | The Sun | |
DOI | https://doi.org/10.1051/0004-6361:20035773 | |
Published online | 22 February 2005 |
Interpretation of second solar spectrum observations of the Sr I 4607 Å line in a quiet region: Turbulent magnetic field strength determination*
1
Laboratoire d'Étude du Rayonnement et de la Matière en Astrophysique, CNRS UMR 8112 – LERMA, Observatoire de Paris, Section de Meudon, 92195 Meudon, France e-mail: v.bommier@obspm.fr
2
Università degli Studi di Firenze, Dipartimento di Astronomia e Scienza dello Spazio, Largo E. Fermi 2, 50125 Firenze, Italy
3
Laboratoire d'Études Spatiales et d'Instrumentation en Astrophysique, CNRS UMR 8109 – LESIA, Observatoire de Paris, Section de Meudon, 92195 Meudon, France
Received:
1
December
2003
Accepted:
23
October
2004
This paper presents and interprets some observations of
the limb polarization of 4607 Å obtained with the
spectropolarimeter of the French-Italian telescope THEMIS in quiet
regions close to the solar North Pole on 2002 December 7–9.
The linear polarization was measured for a series of limb
distances ranging from 4 to 160 arcsec, corresponding to heights of
optical depth unity in the line core ranging from about 330 to
220 km, respectively, above the
level. To
increase the polarimetric sensitivity, the data were averaged
along the spectrograph slit (one arcmin long) set parallel to the
solar limb. Since the data show no rotation of the linear
polarization direction with respect to the limb direction, the
observed depolarization is ascribed to the Hanle effect of a turbulent
weak magnetic field, the zero-field polarization being derived from a
model. The interpretation is performed by means of an algorithm which
describes the process of line formation in terms of the atomic density
matrix formalism, the solar atmosphere being described by an
empirical, plane-parallel model. The collisional rates entering the
model (inelastic collisions with electrons, elastic depolarizing
collisions with neutral hydrogen), have been computed by applying fast
semi-classical methods having a typical accuracy of the order of 20%
or better (see Derouich [CITE]),
leading to 6% inaccuracy on the magnetic field strength determination.
We assume a unimodal distribution for the intensity of the turbulent field.
The computed intensity profile has been adjusted to the observed one in
both depth and width, by varying both microturbulent and macroturbulent
velocities. The best adjustment is obtained for respectively 1.87 km s-1
(micro) and 1.78 km s-1 (macro). The evaluation of the magnetic
depolarization leads then to the average value of 46 Gauss for the
turbulent magnetic field strength, with a gradient of -0.12 Gauss/km.
Our results are in very good agreement with the value of 60 Gauss determined
at large μ, in the volume-filling field case, by Trujillo Bueno et al.
([CITE], Nature, 430, 326), using a 3D magneto-convective simulation.
This validates our method.
Key words: atomic processes / line: formation / line: profiles / Sun: magnetic fields / polarization / radiative transfer
© ESO, 2005
Current usage metrics show cumulative count of Article Views (full-text article views including HTML views, PDF and ePub downloads, according to the available data) and Abstracts Views on Vision4Press platform.
Data correspond to usage on the plateform after 2015. The current usage metrics is available 48-96 hours after online publication and is updated daily on week days.
Initial download of the metrics may take a while.