Magnetic Diagnostics of the Chromosphere with the MG II H–K Lines

Wednesday, August 31, 2016

We developed a numerical code for polarized radiative transfer in a plane-parallel geometry that implements a recent formulation of partially coherent scattering by polarized multi-term atoms in arbitrary magnetic field regimes.

Stokes profiles of the Mg II h–k doublet image
Stokes profiles of the Mg II h–k doublet modeled in a weakly magnetized FAL-C atmosphere (B = 20 G, ϑB = 30◦, ϕB = 180◦) and for various directions of the LOS (corresponding to µ = 0.1, 0.3, 0.5, 0.8, respectively, for the black, red, blue, and green curves). Left: note the remarkable presence of broadband Stokes-U polarization due to the combination of upper-term quantum interferences and magneto-optical effects. Right: finer details of the polarization of the h and k line cores and of the quantum interference pattern between them. We note in particular the reversal of the sign of Stokes V (and more subtly, of Stokes U) for µ = 0.8, in accordance with the sign of the LOS projection of the magnetic field vector. We also note the complete absence of a magnetic signature in the core of the h-line at 280.35 nm, as expected for an intrinsically non-polarizable transition in the weak-field limit.

This code allows the realistic modeling of the scattering polarization of important chromospheric lines, such as the Mg II h–k doublet, the Ca II H–K doublet and IR triplet, and lines of the H I Lyman and Balmer series. We present explicit results of the Mg II h– k doublet in a weakly magnetized atmosphere (20–100G). These confirm the importance of partial redistribution effects in the formation of these lines, as pointed out by previous work in the non- magnetic case. We show that the presence of a magnetic field can produce measurable modifications of the broadband linear polarization even for relatively small field strengths (∼10 G), while the circular polarization remains well represented by the classical magnetograph formula. Both these results open an important new window for the weak-field diagnostics of the upper solar atmosphere.

This manuscript was submitted to the Astrophysical Journal Letters in July 2016.