Observations of scattering polarization and the diagnostics of solar magnetic fields

The scattering polarization that constitutes the so-called "second solar spectrum" contains a multitude of magnetic-field signatures in parameter regimes that are poorly accessed by the Zeeman effect. In the present brief overview of the various physical effects we show how lower-level polarization, which is sensitive to milligauss magnetic fields, can be unambiguously distinguished from the excited-state Hanle effect, which responds to field strengths in the 1-100 G range. We indicate how the scattering polarization can be used to diagnose chromospheric magnetic fields and the magnetoturbulent spectrum of fields that fill most of the photospheric volume. We also show how Hanle observations have ruled out the existence of magnetic canopies in quiet solar regions.