Observation and modeling of scattering polarization signals sensitive to unresolved photospheric magnetic fields via Hanle and magneto-optical effects

The main focus of this Thesis is to explore and apply techniques based on scattering polarization to study certain aspects of the solar magnetism that are not accessible through more widespread techniques. Using the ZIMPOL instrument at the GREGOR solar telescope, we measured the spatial variation of the scattering polarization signals of the Sr I 4607 Å line, whose sensitivity to Hanle effect encodes information on the small-scale magnetic fields present in the solar photosphere. Performing new observations using ZIMPOL at IRSOL and at GREGOR, we found clear observational proof that the scattering polarization wings of the chromospheric Ca I 4227 Å line are sensitive to photospheric magnetic fields via magneto-optical effects. We finally explored the diagnostic potential of such sensitivity by theoretically modeling the Ca I 4227 Å line and we identified the linear polarization angle as an observable of interest for diagnosing solar magnetic fields