Astronomy and Astrophysics – Astrophysics
Scientific paper
Jun 1995
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995a%26a...298..289f&link_type=abstract
Astronomy and Astrophysics, v.298, p.289
Astronomy and Astrophysics
Astrophysics
71
Mhd, Magnetic Fileds, Sun: Magnetic Field, Sun: Photosphere, Turbulence
Scientific paper
Turbulent magnetic fields in the solar photosphere may be determined by their depolarizing Hanle effect on the linear polarization of some solar absorption lines formed outside active regions (Stenflo 1982). In Faurobert-Scholl (1993) the center-to-limb variation of the linear polarization in the SrI 4607 A line was analyzed. It was shown that it is affected by the Hanle effect due to a weak magnetic field with mixed polarity at small scales. The accurate determination of the turbulent magnetic field strength requires radiative transfer calculations taking into account the effect of collisions and magnetic fields on the frequency and angular redistribution of the light. Here we present the different steps in the theory of redistribution, together with the first precise quantum calculations of the collisional cross-sections between hydrogen and SrI and CaI atoms. These new results are then used to perform a more accurate determination of the turbulent magnetic field strength in the solar photosphere. It is shown that the center-to-limb variation of the linear polarization in the SrI line is mainly sensitive to the average value of the field strength between the altitudes 200 and 400km. According to the observations this average value is between 20 and 10G. A theoretical interpretation is given in terms of a one-dimensional passive magnetic field transport equilibrium model with first-order smoothing. The resulting <|B_x_|>(z) equilibrium distribution reproduces the observations rather well, but the uncertainty in the amplitude, height-dependence and interpretation of microturbulence suggests that the present model could be brought to an even better agreement with the observations if a microturbulence different from the VAL3C model is used. It is finally pointed out that in the upper photosphere the turbulent field becomes force-free instead of being passively transported. It is found that this does not seriously modify our conclusions.
Faurobert-Scholl Marianne
Feautrier N.
Machefert F.
Petrovay Kristof
Spielfiedel A.
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