The scientific case for quantum spectropolarimetry from space

One of the greatest future challenges in cosmic physics is the empirical investigation of the magnetic field vector in a variety of astrophysical plasmas, including the solar corona. The chances of attaining this goal would be dramatically increased if we could carry out spectropolarimetric observations from space in order to measure the polarization signals that scattering processes induce in permitted UV/EUV spectral lines, such as those of the Lyman series of hydrogen. The physical interpretation of this type of observations would allow us to infer the magnetic field vector via the Hanle effect, which consists in the modification of scattering polarization signals due to the presence of a magnetic field. In particular, the Hanle effect either in forward scattering or at 90° scattering is a unique and powerful tool for the "measurement" of magnetic fields in the solar transition region and corona. In this paper1 we highlight the great diagnostic potential of spectropolarimetry and argue that ESA should take advantage of the present European leadership in this field to open this new diagnostic window on the Universe.