Abstract
A new type of two-dimensional polarimeter is used to measure the center-to-limb variation of the scattering induced polarization in a narrow continuum window up to the extreme limb. The polarimeter is set on the Tenerife Gregory Coude telescope's hour axis, where the two folding flat mirrors cancel their polarizing eects for zero solar declination at the equinox. The short CCD exposure of only 5 ms allows high spatial resolution images in polarized light. A beam switching technique together with an integration parallel to the solar limb over 20 00 , yields a high polarimetric accuracy with an rms noise of 2 10 4 . Our results for a continuum window at 4506-4508 A agree with model calculations down to limb distances of 0: 00 32 (i.e. cos#<0:025).
Highlights
Scattering processes in the solar atmosphere produce deviations from a Planckian source function
Our results for a continuum window at 4506–4508 Å agree with model calculations down to limb distances of 0. 32
A much stronger test would be the comparison of data with theory at the very limb where the rise of the polarization is very steep at blue wavelengths. Such observations are rather difficult since the continuum limb polarization (i) is of very small amount, (ii) is superposed with the strong intensity gradient at the very limb, and (iii) interferes with the usually much larger instrumental polarization
Summary
Scattering processes in the solar atmosphere produce deviations from a Planckian source function. The Gregory Coude Telescope at Locarno enabled first measurements of the continuum polarization in spectrally well defined narrow windows free from absorption lines: at 5 limb distance (cos θ = 0.23) the continuum polarization decreases from 10−3 at 4234.8 Å to 5 × 10−5 at 6578.0 Å (Wiehr 1975). Such observations have been made by Leroy (1972) who, used broader filters covering more lines, and did not reach the very limb. The much longer exposures with “ZIMPOL” do not allow a determination of the strong increase of the linear polarization at the very limb, presented in this paper
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