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Interpreting the Stokes profiles observed in quiet regions of the solarchromosphere is a challenging task. The Stokes Q and U profiles are dominatedby the scattering polarisation and the Hanle effect, and these processes canonly be correctly quantified if 3D radiative transfer effects are taken intoaccount. Forward-modelling of the intensity and polarisation of spectral linesusing a 3D model atmosphere is a suitable approach in order to statisticallycompare the theoretical and observed line profiles. Our aim is to present novelobservations of the Ca 8542 line profiles in a quiet region at the centre ofthe solar disc and to quantitatively compare them with the theoretical Stokesprofiles. We aim at estimating the reliability of the 3D model atmosphere usingnot only the line intensity but the full vector of Stokes parameters. We useddata obtained with the ZIMPOL instrument at the IRSOL and compared theobservations with the theoretical profiles computed with the PORTA radiativetransfer code, using as solar model atmosphere a 3D snapshot taken from aradiation-magnetohydrodynamics simulation. The synthetic profiles were degradedto match the instrument and observing conditions. The degraded theoreticalprofiles of the Ca 8542 line are qualitatively similar to the observed ones. Weconfirm that there is a fundamental difference in the widths of all Stokesprofiles: the observed lines are wider than the theoretical lines. We find thatthe amplitudes of the observed profiles are larger than those of thetheoretical ones, which suggests that the symmetry breaking effects in thesolar chromosphere are stronger than in the model atmosphere. This means thatthe isosurfaces of temperature, velocity, and magnetic field strength andorientation are more corrugated in the solar chromosphere than in the currentlyavailable 3D radiation-magnetohydrodynamics simulation.

Comparison of theoretical and observed Ca~{\sc ii}~8542 Stokes profiles in quiet regions at the centre of the solar disc