The Magnetic Sensitivity of the Second Solar Spectrum

This paper reviews some of the developments that over the last 10 years have allowed us to go from deciphering the physical origin of several of the enigmatic features of the second solar spectrum to discovering unknown aspects of the Sun’s hidden magnetism via sophisticated radiative transfer modeling. The second solar spectrum is the observational signature of radiatively induced quantum coherences in the atoms and molecules of the solar atmosphere. Magnetic fields produce partial decoherence via the Hanle effect, giving rise to fascinating observable effects in the emergent spectral line polarization. Interestingly, these effects allow us to “see” magnetic fields to which the Zeeman effect is blind within the limitations of the available instrumentation. In the coming years, the physical interpretation of observations of the spectral line polarization resulting from the joint action of the Hanle and Zeeman effects might lead to a new revolution in our empirical understanding of solar magnetic fields. 1. Historical Introduction “Recent accurate measurements of the wavelength dependence of linear polarization of the solar limb radiation has resulted in the discovery of what may be called the second solar spectrum, with dozens of polarization (Q/I) features seen both ‘in absorption’ and ‘in emission’, i.e., with correspondingly smaller and larger polarization than in the adjacent continuum.” (Ivanov