ESP Online Seminar: Formation and morphology of solar circular polarization with dichroism, atomic polarization and velocity gradients (Edgar S. Carlin)

Since the work of Carlin et al. (2012), more studies have investigated, with the help of MHD models of the solar chromosphere, the behavior of scattering linear polarization (LP) in presence of macroscopic motions and weak magnetic fields. The results of the spatio-temporal simulations revealed a variable spectral morphology in the polarization of chromospheric lines, as well as modulations of LP amplitude that affect Hanle diagnosis, and also several situations with clear potential for diagnosing solar and stellar atmospheres. While much of these results resorts in the dynamic variations of radiation field anisotropy along the (also dynamic) formation region, they also pose new questions that seem to transcend the role of the anisotropy. On the other hand, two problems slow down further research in this area. One is the SNR reached with current telescopes, which is insufficient to compare observations with time-resolved simulations of scattering polarization. And the other is the extension and complexity of the formation region of the polarization signals, which often demands analysis of multidimensional simulations to extract conclusions. In this regard, I seek to develop a standard model for polarization that is precise enough to explain the signals but simple enough to provide analyses without relying in MHD models. We will start this seminar by summarizing the physical situation concerning the formation of dynamical LP in the external solar layers and by supporting the need of investigating the circular polarization. We will then focus on the novel exploration of NLTE circular polarization considering atomic orientation and velocity gradients. This approach leads to a better understanding of the formation of polarization by avoiding the limb-darkening modulation (introduced by the anisotropy in LP) and by allowing effective comparisons with observations. To explain this I will advance some observational results and I will present the first version of a simple and insightful model for explaining NLTE polarization signals. A particularity of the approach here proposed is the association of the zeroes of the emergent polarization spectrum with its morphology and with the properties of the scattering medium.