Testing spectropolarimetry in the extreme ultraviolet to infer the solar coronal magnetic field

摘要

Context. Spectropolarimetry has the potential to provide us with important coronal plasma parameters. Aims. We test spectropolarimetric forward modelling by investigating whether it is possible to reproduce the only linear polarisation measurement made for the optically thin 1032?? UV O?vi line detected by the Solar Ultraviolet Measurements of Emitted Radiation (SUMER) instrument, operating aboard the SOlar and Heliospheric Observatory (SOHO) spacecraft located 1.29 R⊙ above the southern solar rotation axis. Methods. Through the realistic synthesis of line-of-sight integrated emission coefficients in the four Stokes parameters, we explore subsets of the ten-dimensional parameter space, (ne, nO5?+, αc, Ichr, T, B, v, w⊥, w∥,δ), i.e., the number density of electrons and O5+ ions, respectively, the electronic collision rate, the chromospheric intensity of the O?vi line, the electronic temperature, the magnetic and solar outflow-velocity vectors, the perpendicular and parallel (with respect to the magnetic field) parameters of the anisotropic velocity-distribution functions, and finally the tilt of the solar rotation axis, non-rigorously in search for agreement between the forward-modelled linear polarisation parameters and the observed values. Results. The most interesting result is that the tilt of the solar rotation axis creates non-radial fields, for both the magnetic field and velocity, above the Sun in the plane of the sky, thus transforming this previously rather uninteresting area from the polarimetric point of view into a highly exciting one. Our findings show that if the magnetic field intensity lies in the range 10–45 G and the solar outflow velocity in the range 20–100 km s-1, we are able to reproduce the full range of observed values plus uncertainties in the rotation angle of 9° ± 6°. The second observable, i.e., the fractional linear polarisation is somewhat harder to bring into alignment with our forward modelling efforts in that one has to decrease the electron density in most current models by an order of magnitude. Conclusions. It is indeed very encouraging to note how this single measurement of the linear polarisation parameters in the ultraviolet virtually steers the forward modeller in the right direction of reproducing the physical environment that gave rise to the observed values. This bodes well for spectropolarimetry because it provides the basis for the hope that these observations will aid the forward modeller in determining how and where to start searching in the possibly terribly complicated maze of parameter space.