Abstract
The plasma thermodynamics in the solar upper atmosphere, particularly in the corona, are dominated by the magnetic field, which controls the flow and dissipation of energy. The relative lack of knowledge of the coronal vector magnetic field is a major handicap for progress in coronal physics. This makes the development of measurement methods of coronal magnetic fields a high priority in solar physics. The Hanle effect in the UV and IR spectral lines is a largely unexplored diagnostic. We use magnetohydrodynamic (MHD) simulations to study the magnitude of the signal to be expected for typical coronal magnetic fields for selected spectral lines in the UV and IR wavelength ranges, namely the H I Ly-$\alpha$ and the He I 10830 {\AA} lines. We show that the selected lines are useful for reliable diagnosis of coronal magnetic fields. The results show that the combination of polarization measurements of spectral lines with different sensitivities to the Hanle effect may be most appropriate for deducing coronal magnetic properties from future observations.
Highlights
Our understanding of coronal phenomena, such as plasma heating and acceleration, particle energization, and explosive activity, faces major hurdles due to the lack of reliable measurements of key parameters such as densities, temperatures, velocities, and magnetic fields
We focus on the diagnostics of coronal magnetic fields through the linear polarization of selected spectral lines (i.e., H I Ly-α and He I 10,830 Å) that are sensitive to the “Hanle effect.”
The Hanle effect (Hanle, 1924), which is the modification of the linear polarization of a spectral line by a local magnetic field, may provide strong diagnostics of regions of weak magnetic fields such as the solar corona, where a number of spectral lines with different but complementary sensitivity ranges are present
Summary
Our understanding of coronal phenomena, such as plasma heating and acceleration, particle energization, and explosive activity, faces major hurdles due to the lack of reliable measurements of key parameters such as densities, temperatures, velocities, and magnetic fields. Spectroscopic diagnostics in the ultraviolet (UV) and extreme ultraviolet (EUV) wavelength regimes provide measurements of plasma densities, temperatures, and partial information on the velocity They cannot provide any insight into the coronal magnetic field, which is the key player in the structuring of the solar corona and in dominating most (if not all) physical processes underlying the multi-scaled solar activity. Coronal magnetic fields are usually approximated through MHD modeling and extrapolation of photospheric measurements (e.g., Wiegelmann et al, 2014) We focus on the diagnostics of coronal magnetic fields through the linear polarization of selected spectral lines (i.e., H I Ly-α and He I 10,830 Å) that are sensitive to the “Hanle effect.” Other spectral lines are of interest, but the analysis of their polarization is left for future publications
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