Influence of Atomic Polarization and Horizontal Illumination on the Stokes Profiles of the Hei10830 A Multiplet

The polarization observed in the spectral lines of the He I 10830 multipletcarries valuable information on the dynamical and magnetic properties of plasmastructures in the solar chromosphere and corona, such as spicules, prominences,filaments, emerging magnetic flux regions, etc. Here we investigate theinfluence of atomic level polarization on the emergent Stokes profiles for abroad range of magnetic field strengths, in both 90 degree and forwardscattering geometry. We show that, contrary to a widespread belief, theselective emission and absorption processes caused by the presence of atomiclevel polarization may have an important influence on the emergent linearpolarization, even for magnetic field strengths as large as 1000 G.Consequently, the modeling of the Stokes Q and U profiles should not be done bytaking only into account the contribution of the transverse Zeeman effectwithin the framework of the Paschen-Back effect theory, unless the magneticfield intensity of the observed plasma structure is sensibly larger than 1000G. We point out also that in low-lying optically thick plasma structures, suchas those of active region filaments, the (horizontal) radiation field generatedby the structure itself may substantially reduce the positive contribution tothe anisotropy factor caused by the (vertical) radiation field coming from theunderlying solar photosphere, so that the amount of atomic level polarizationmay turn out to be negligible. Only under such circumstances may the emergentlinear polarization of the He I 10830 multiplet in such regions of the solaratmosphere be dominated by the contribution caused by the transverse Zeemaneffect.Comment: Accepted for publication in The Astrophysical Journal (It is tentatively scheduled for the ApJ January 20, 2007 issue