Solar coronal magnetic field diagnostics through polarimetric forward modelling of the Hanle effect

Abstract

Context. Progress in the solution to some of the most outstanding open problems of solar physics, such as coronal heating, solar wind acceleration, the generation and triggering of explosive events like flares and CMEs, hinges on the provision of a more stringent estimate of the solar magnetic field coordinates. Aims. We seek a way to infer the magnetic field of the solar atmosphere. A very promising way of doing this is by using the Hanle effect in resonance scattering in the Lα line of the solar atmosphere. Methods. By forward modelling the known scattering effects in the presence of magnetic fields, i.e. rotation of the plane of polarisation and depolarisation of the linear polarisation parameters, and by comparing them to observations, one could potentially uncover the magnetic morphology and restrict its intensity range. We simulate the effects of simple dipole configurations along the coordinate axes and analyse the outcome through two kinds of graphs (i.e. the difference in angle of the plane of linear polarisation with respect to the field-free case, and the relative depolarisation). Results. The graphs are either symmetric, anti-symmetric or asymmetric with respect to the (y,z) plane. This is explained by invoking two symmetry operations and taking into account that the magnetic field is a pseudovector. We also show the polarimetric effects of active regions and use them pairwise with the magnetic field due to dipoles to analyse the polarimetric signatures of magnetic field line loops. Inspired by the famous TRACE image, we finally show what one could expect from polarimetry performed on the region of the solar atmosphere displayed in the image. Conclusions. By combining the two complementary remote sensing techniques, i.e. the Zeeman and the Hanle effect, in all thinkable ways with tracers such as the images revealed by TRACE, SOHO, STEREO, etc., we hope one day to be able to infer the solar magnetic field coordinates. Much theoretical and instrumental work still lies ahead, however.