  Measurable linear scattering polarization signals have been predicted and
detected at the solar disk center in the core of chromospheric lines. These
forward-scattering polarization signals, which are of high interest for
magnetic field diagnostics, have always been modeled either under the
assumption of complete frequency redistribution (CRD), or taking partial
frequency redistribution (PRD) effects into account under the angle-averaged
(AA) approximation. This work aims at assessing the suitability of the CRD and
PRD-AA approximations for modeling the forward-scattering polarization signals
produced by the presence of an inclined magnetic field, the so-called
forward-scattering Hanle effect, in the chromospheric CaI 4227 A line.
Radiative transfer calculations are performed in semi-empirical 1D solar
atmospheres, out of local thermodynamic equilibrium (LTE). A two-step solution
strategy is applied: the non-LTE RT problem is first solved considering a
multilevel atom and neglecting polarization phenomena. The same problem is then
solved including polarization, considering a two-level atom and keeping fixed
the lower-level population calculated at the previous step. The emergent linear
polarization signals calculated under the CRD and PRD-AA approximations are
analyzed and compared to those obtained by modeling PRD effects in their
general angle-dependent (AD) formulation. With respect to the PRD-AD case, the
CRD and PRD-AA calculations significantly underestimate the amplitude of the
line-center polarization signals produced by the forward-scattering Hanle
effect. The results of this work suggest that a PRD-AD modeling is required in
order to develop reliable diagnostic techniques exploiting the
forward-scattering polarization signals observed in the CaI 4227 A line. These
results need to be confirmed by full 3D calculations including non-magnetic
symmetry-breaking effects.
