Bibcode
Schlichenmaier, R.; Collados, M.
Bibliographical reference
Astronomy and Astrophysics, v.381, p.668-682 (2002)
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1
2002
Journal
Citations
112
Refereed citations
95
Description
Stokes profiles of sunspot penumbrae show distinct asymmetries, which
point to gradients in the velocity field and in the magnetic field. We
present spectropolarimetric measurements of the Stokes vector in the
neutral iron triplet at 1564.8 nm taken with the Tenerife Infrared
Polarimeter (TIP) at the German Vacuum Tower Telescope (VTT) in
Tenerife. We report on the peculiarities of the profiles of circularly
and linearly polarized light for spots at different heliocentric angles.
We elaborate on the spatial dependence of Stokes asymmetries within the
penumbra and find for profiles of circularly polarized light: (1) In the
center-side penumbra the amplitude difference of Stokes-V exhibits a
sign reversal on a radial cut, i.e., in the inner (outer) penumbra the
red (blue) lobe is broader and shows a smaller amplitude than the blue
(red) lobe. (2) In the outer limb-side penumbra (beyond the magnetic
neutral line) the red lobe is broader and of less amplitude than the
blue lobe. (3) Along the magnetic neutral line we find abnormal Stokes-V
profiles, which consist of more than 2 lobes. This indicates the
presence of two polarities. For small heliocentric angles abnormal
profiles are also seen beyond the magnetic neutral line in the outer
penumbra. (4) Maps of the net circular polarization have the tendency to
be antisymmetric with respect to the axis that connects disk center with
spot center. This finding is striking, because corresponding maps for Fe
I 630.25 are symmetric. For linearly polarized profiles we extract the
following features: (5) On the center-side penumbra at a heliocentric
angle of 56o a Doppler-shift as high as 5 km s-1
can be directly measured by the splitting of the pi -component of the
linearly polarized component. (6) In limb-side penumbrae, the profiles
of the pi -component show the typical asymmetry properties of the
Evershed flow as observed in Stokes-I of magnetically insensitive lines.
(7) In the outer center- and limb-side penumbrae the center of the pi
-component is blue-shifted relative to the zero-crossing of the
V-profile. Motivated by the moving tube model of Schlichenmaier et al.
(cite{schlichenmaier+jahn+schmidt1998b}), we construct simple model
atmospheres featuring hot upflows and cool outflows and calculate
corresponding synthetic V-profiles. These profiles are compared with our
measured ones and with observed V-profiles in Fe I 630.25 from other
authors. We find that the synthetic V-profiles can reproduce all
essential characteristics of observed V-profiles for both lines.