Center-to-limb variation of the second solar spectrum.


The linear polarization that is caused by scattering processes in the solar atmosphere has been refered to as the ``second solar spectrum’’, since it is structurally as rich as the ordinary intensity spectrum but quite different in appearance and information contents. One of the most used and theoretically best understood lines in the second solar spectrum is the SrI 4607̊A line, which has served as a diagnostic tool for determinations of spatially unresolved, turbulent magnetic fields via the Hanle effect. Here we present the detailed center-to-limb variation of the scattering polarization in this line for a number of new data sets obtained both with an electrooptical modulation system (ZIMPOL) and a non-modulating beam splitter system (at IRSOL, Locarno), to provide improved observational constraints for theoretical modelling. The amplitude and width of the polarization profile, the amount of continuum polarization, as well as the depth and width of the intensity profile have been evaluated and carefully corrected for spectral broadening and stray light. While there is generally good agreement between the five data sets, some systematic differences are shown to be of solar rather than instrumental origin, most likely due to spatially varying Hanle depolarization across the solar disk. A number of other spectral lines have been observed with the ZIMPOL system at two different limb distances (ensuremathμ=0.1 and 0.2) to allow us to compare the steepness of the center-to-limb variation of their polarization amplitudes. The steepest variation is exhibited by the continuum polarization, which declines by approximately a factor of 6 when going the 15 arcsec distance from ensuremathμ=0.1 to ensuremathμ=0.2. The spectral lines with the steepest center-to-limb variation are molecular lines, the CaII infrared triplet, and Hensuremathα. In contrast the SrI 4607 and BaII 4554Å lines have only moderately steeper center-to-limb variations than that of an ideal, purely dipole-scattering atmosphere, for which the polarization ratio between ensuremathμ=0.1 and ensuremathμ=0.2 is 1.38. These center-to-limb variations may be used to constrain temperature-density models of the upper photosphere and chromosphere.