CENTER-TO-LIMB VARIATION OF THE 2ND 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 b ut quite different in appearance and information contents. One of the most used and theoretically best understood lines in the second solar spectrum is the Sr I 4607 Angstrom line, which has served as a diagnos tic tool for determinations of spatially unresolved, turbulent magneti c fields via the Hanle effect. Here we present the detailed center-to- limb variation of the scattering polarization in this line for a numbe r of new data sets obtained both with an electrooptical modulation sys tem (ZIMPOL) and a non-modulating beam splitter system (at IRSOL, Loca rno), to provide improved observational constraints for theoretical mo delling. The amplitude and width of the polarization profile, the amou nt of continuum polarization, as well as the depth and width of the in tensity profile have been evaluated and carefully corrected for spectr al 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 spatia lly varying Hanle depolarization across the solar disk. A number of ot her spectral lines have been observed with the ZIMPOL system at two di fferent limb distances (mu = 0.1 and 0.2) to allow us to compare the s teepness of the center-to-limb variation of their polarization amplitu des. The steepest variation is exhibited by the continuum polarization , which declines by approximately a factor of 6 when going the 15 arcs ec distance from mu = 0.1 to mu = 0.2. The spectral lines with the ste epest center-to-limb variation are molecular lines, the Ca II infrared triplet, and H alpha. In contrast the Sr I 4607 and Ba II 4554 Angstr om lines have only moderately steeper center-to-limb variations than t hat of an ideal, purely dipole-scattering atmosphere, for which the po larization ratio between mu = 0.1 and mu = 0.2 is 1.38. These center-t o-limb variations may be used to constrain temperature-density models of the upper photosphere and chromosphere.