Evidence for Ground-Level Atomic Polarization in the Solar Atmosphere

Abstract

Recent observations of linear polarization at the limb of the quiet Sun, performed with the high sensitivity polarimeter ZIMPOL (Zürich IMaging PO Larimeter), have revealed a large number of unexpected phenomena and, in particular, have shown the existence of a complicated linear polarization pattern across the NaI doublet and of a sharp polarization peak in the core of the D₁ line (Stenflo and Keller, 1996, 1997). These phenomena were interpreted as due to the presence of atomic polarization in the hyperfine-structured ground level 3s ²S_1/2 of the Na atom, a finding that contradicts the existence of turbulent and canopy-like magnetic fields in the solar chromosphere (Landi Degl’Innocenti, 1998) . In this paper we review the theoretical aspects of the problem (already presented in an abridged form in the paper just quoted) and we comment on some alternative explanations of the polarization pattern of the Na doublet.