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The chromosphere of the quiet Sun is an important stellar atmospheric regionwhose thermal and magnetic structure we need to decipher for unlocking newdiscoveries in solar and stellar physics. To this end, we must identify andexploit observables sensitive to weak magnetic fields (B<100 G) and to thepresence of cool and hot gas in the bulk of the solar chromosphere. Here wereport on an investigation of the Hanle effect in two semi-empirical models ofthe quiet solar atmosphere with different chromospheric thermal structures. Ourstudy reveals that scattering polarization in the Ca II IR triplet has thermaland magnetic sensitivities potentially of great diagnostic value. The linearpolarization in the 8498 A line shows a strong sensitivity to inclined magneticfields with strengths between 0.001 and 10 G, while the emergent linearpolarization in the 8542 A and 8662 A lines is mainly sensitive to magneticfields with strengths between 0.001 and 0.1 G. The reason for this is that thescattering polarization of the 8542 A and 8662 A lines, unlike the 8498 A line,is controlled mainly by the Hanle effect in their (metastable) lower levels.Therefore, in regions with magnetic strengths sensibly larger than 1 G, theirStokes Q and U profiles are sensitive only to the orientation of the magneticfield vector. We also find that for given magnetic field configurations thesign of the Q/I and U/I profiles of the 8542 A and 8662 A lines is the same inboth atmospheric models, while the sign of the linear polarization profile ofthe 8498 A line turns out to be very sensitive to the thermal structure of thelower chromosphere. We suggest that spectropolarimetric observations providinginformation on the relative scattering polarization amplitudes of the Ca II IRtriplet will be very useful to improve our empirical understanding of thethermal and magnetic structure of the quiet chromosphere.

Scattering Polarization of the Ca II IR Triplet for Probing the Quiet Solar Chromosphere