  The linear polarization pattern produced by scattering processes in the Ca I
resonance line at 4227 angstroms is a valuable observable for probing the solar
atmosphere. Via the Hanle effect, the very significant Q/I and U/I line-center
signals are sensitive to the presence of magnetic fields in the lower
chromosphere with strengths between 5 and 125 G, approximately. On the other
hand, partial frequency redistribution (PRD) produces sizable signals in the
wings of the Q/I profile, which have always been thought to be insensitive to
the presence of magnetic fields. Interestingly, novel observations of this line
revealed a surprising behavior: fully unexpected signals in the wings of the
U/I profile and spatial variability in the wings of both Q/I and U/I. Here we
show that the magneto-optical (MO) terms of the Stokes-vector transfer equation
produce sizable signals in the wings of U/I and a clear sensitivity of the Q/I
and U/I wings to the presence of photospheric magnetic fields with strengths
similar to those that produce the Hanle effect in the line core. This radiative
transfer investigation on the joint action of scattering processes and the
Hanle and Zeeman effects should facilitate the development of more reliable
techniques for exploring the magnetism of stellar atmospheres. To this end, we
can now exploit the circular polarization produced by the Zeeman effect, the
magnetic sensitivity caused by the above-mentioned MO effects in the Q/I and
U/I wings, and the Hanle effect in the line core.
