The Hanle effect of Ly$\alpha$ in an MHD model of the Solar Transition   Region
  In order to understand the heating of the solar corona it is crucial to obtain empirical information on the magnetic field in its lower boundary (the transition region). To this end, we need to measure and model the linear polarization produced by scattering processes in strong UV lines, such as the hydrogen Ly$\alpha$ line. The interpretation of the observed Stokes profiles will require taking into account that the outer solar atmosphere is highly structured and dynamic, and that the height of the transition region may well vary from one place in the atmosphere to another. Here we report on the Ly$\alpha$ scattering polarization signals we have calculated in a realistic model of an enhanced network region, resulting from a state-of-the-art radiation MHD simulation. This model is characterized by spatially complex variations of the physical quantities at transition region heights. The results of our investigation lead us to emphasize that scattering processes in the upper solar chromosphere should indeed produce measurable linear polarization in Ly$\alpha$. More importantly, we show that via the Hanle effect the model's magnetic field produces significant changes in the emergent $Q/I$ and $U/I$ profiles. Therefore, we argue that by measuring the polarization signals produced by scattering processes and the Hanle effect in Ly$\alpha$ and contrasting them with those computed in increasingly realistic atmospheric models, we should be able to decipher the magnetic, thermal and dynamic structure of the upper chromosphere and transition region of the Sun. 
Jiri Stepan, Javier Trujillo Bueno, Mats Carlsson, Jorrit Leenaarts
astro-ph.SR