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We investigate the main physical mechanisms that shape the intensity andpolarization of the Ba II D1 line at 4934 angstroms via radiative transfernumerical experiments. We focus especially on the scattering linearpolarization arising from the spectral structure of the anisotropic radiationin the wavelength interval spanned by the line's hyperfine structure (HFS)components in the odd isotopes of barium. After verifying that the presence ofthe low-energy metastable levels only impacts the amplitude, but not the shape,of the D1 linear polarization, we relied on a two-term atomic model thatneglects such metastable levels but includes HFS. The D1 fractional linearpolarization shows a very small variation with the choice of atmospheric model,enhancing its suitability for solar magnetic field diagnostics. Tangledmagnetic fields with strengths of tens of gauss reduce the linear polarizationand saturation is reached at roughly 300 G. Deterministic inclined magneticfields produce a U/I profile and, if they have a significant longitudinalcomponent, a V/I profile, whose modeling requires accounting for HFS and thePaschen-Back effect. Because of the overlap between HFS components, themagnetograph formula cannot be applied to infer the longitudinal magneticfield. Accurately modeling the D1 intensity and polarization requires an atomicsystem that includes the metastable levels and the HFS, the detailed spectralstructure of the radiation field, the incomplete Paschen-Back regime formagnetic fields, and an accurate treatment of collisions.

The polarization of the solar Ba II D1 line with partial frequency redistribution and its magnetic sensitivity