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Astron. Astrophys. 357, 351-358 (2000)

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2. Observations

The observational data we refer to were obtained in 1996 with the HAO/NSO Advanced Stokes Polarimeter (ASP; Elmore et al. 1992) at the NSO/Sacramento Peak Richard B. Dunn Solar Telescope. The data consist of high resolution Stokes [FORMULA] and V spectra of FeI  6301.5 Å and FeI  6302.5 Å from quiet regions at disc center. The RMS noise of Stokes [FORMULA] in the continuum is [FORMULA] [FORMULA], so that it is possible to analyze very weak signals with amplitudes down to 0.15%. A detailed description of the observations as well as an account of data reduction and analysis is given in Sigwarth (1999) and Sigwarth et al. (1999). About 10% of all analyzed Stokes V profiles of FeI 6302.5 have an `abnormal' shape that deviates strongly from the nearly antisymmetric form found in low-resolution observations.

About 35% (1051 spectra) of the abnormal Stokes V profiles are of the one-lobe type, i.e., they have virtually one lobe only. In 835 cases the lobe is at the blue side (blue-only type) and in 216 cases at the red side (red-only type) of the profile with the opposite lobe virtually absent. Fig. 1 shows a few Stokes V profiles of this type. Most of the one-lobe Stokes V profiles are not accompanied by significant linear polarization (Stokes Q and U), so that the magnetic field at these positions is most likely not strongly inclined with respect to the vertical.

[FIGURE] Fig. 1. Examples of observed one-lobe Stokes V profiles. Profiles with blue lobe only are shown in the upper, those with red lobe only in the lower row. Vertical solid lines mark the wavelength of the center of Stokes I and stars mark the Stokes V zero-crossing position (determined by extrapolation).

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© European Southern Observatory (ESO) 2000

Online publication: May 3, 2000