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Context. The correct modeling of the scattering polarization signals observedin several strong resonance lines requires taking partial frequencyredistribution (PRD) phenomena into account. Aims. This work aims at assessingthe impact and the range of validity of the angle-averaged AA approximationwith respect to the general angle-dependent (AD) treatment of PRD effects inthe modeling of scattering polarization in strong resonance lines, with focuson the solar Ca i 4227 {\AA} line. Methods. Spectral line polarization ismodeled by solving the radiative transfer problem for polarized radiation,under nonlocal thermodynamic equilibrium conditions, taking PRD effects intoaccount, in static one-dimensional semi-empirical atmospheric models presentingarbitrary magnetic fields. The problem is solved through a two-step approach.In step 1, the problem is solved for intensity only, considering a multi-levelatom. In step 2, the problem is solved including polarization, considering atwo-level atom with an unpolarized and infinitely sharp lower level, and fixingthe lower level population calculated at step 1. Results. The results for theCa i 4227 {\AA} line show a good agreement between the AA and AD calculationsfor the Q/I and U/I wings signals. However, AA calculations reveal anartificial trough in the line-core peak of the linear polarization profiles,whereas AD calculations show a sharper peak in agreement with observations.Conclusions. An AD treatment of PRD effects is essential to correctly model theline-core peak of the scattering polarization signal of the Ca i 4227 {\AA}line. By contrast, in the considered static case, the AA approximation seems tobe suitable to model the wing scattering polarization lobes and their magneticsensitivity through magneto-optical effects.

Modeling the scattering polarization of the solar Ca i 4227 {\AA} line with angle-dependent partial frequency redistribution