Relying on the assumption that the interchange convection of magnetic flux tubes is the physical cause for the existence of sunspot penumbrae, we propose a model in which the dynamical evolution of a thin magnetic flux tube reproduces the Evershed effect and the penumbral fine structure such as bright and dark filaments and penumbral grains. According to our model, penumbral grains are the manifestation of the footpoints of magnetic flux tubes, along which hot subphotospheric plasma flows upwards with a few km/s. Above the photosphere the hot plasma inside the tube is cooled by radiative losses as it flows horizontally outwards. As long as the flowing plasma is hotter than the surroundings, it constitutes a bright radial filament. The flow confined to a thin elevated channel reaches the temperature equilibrium with the surrounding atmosphere and becomes optically thin near the outer edge of the penumbra. Here, the tube has a height of approximately 100 km above the continuum and the flow velocity reaches up to 14 km/s. Such a flow channel can reproduce the observed signatures of the Evershed effect.
This paper describes the main characteristics of GRIS (GREGOR Infrared Spectrograph), the grating spectrograph installed in the recently inaugurated (May 2012) 1.5-meter GREGOR telescope located at the Observatorio del Teide in Tenerife. The spectrograph has a standard Czerny-Turner configuration with parabolic collimator and camera mirrors that belong to the same conic surface. Although nothing prevents its use at visible wavelengths, the spectrograph will be initially used in combination with the infrared detector of the Tenerife Infrared Polarimeter (TIP-II) in standard spectroscopic mode as well as for spectropolarimetric measurements.
Abstract. 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 56• a Doppler-shift as high as 5 km s (1998b), 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.
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