Abstract
Measuring the coronal vector magnetic field is still a major challenge in solar physics. This is due to the intrinsic weakness of the field (e.g. ~4G at a height of 0.1Rsun above an active region) and the large thermal broadening of coronal emission lines. We propose using concurrent linear polarization measurements of near-infrared forbidden and permitted lines together with Hanle effect models to calculate the coronal vector magnetic field. In the unsaturated Hanle regime both the direction and strength of the magnetic field affect the linear polarization, while in the saturated regime the polarization is insensitive to the strength of the field. The relatively long radiative lifetimes of coronal forbidden atomic transitions implies that the emission lines are formed in the saturated Hanle regime and the linear polarization is insensitive to the strength of the field. By combining measurements of both forbidden and permitted lines, the direction and strength of the field can be obtained. For example, the SiX 1.4301 um line shows strong linear polarization and has been observed in emission over a large field-of-view (out to elongations of 0.5 Rsun. Here we describe an algorithm that combines linear polarization measurements of the SiX 1.4301 um forbidden line with linear polarization observations of the HeI 1.0830 um permitted coronal line to obtain the vector magnetic field. To illustrate the concept we assume the emitting gas for both atomic transitions is located in the plane of the sky. The further development of this method and associated tools will be a critical step towards interpreting the high spectral, spatial and temporal infrared spectro-polarimetric measurements that will be possible when the Daniel K. Inouye Solar Telescope (DKIST) is completed in 2019.
Highlights
Magnetometry using optical spectropolarimetry has yielded some of the most precise direct measurements of coronal magnetic fields (Kuhn, 1995; Lin et al, 2000, 2004; Tomczyk et al, 2008)
The method proposed provides important constraints on the coronal magnetic field and shows promise as a detailed magnetic field diagnostic, since it drastically constrains the coronal source region local magnetic field to four independent solutions using potentially high signal-to-noise IR linear polarization measurements. This is achieved without knowledge of polarization amplitudes for the forbidden lines that depends on the coronal electron density
It is interesting to note that the method obtains four degenerate solutions for magnetic fields located inside or outside the Van Vleck degeneracy region
Summary
Magnetometry using optical spectropolarimetry has yielded some of the most precise direct measurements of coronal magnetic fields (Kuhn, 1995; Lin et al, 2000, 2004; Tomczyk et al, 2008). Up until now attempts from the ground to measure the magnetic field strength have depended on the ability to detect very weak Zeeman splitting through Stokes-V (circular) polarization observations. A powerful coronal field diagnostic follows from simultaneous measurements of the optical scattering linear polarization of combined forbidden and permitted spectral lines. Some earlier measurements revealed diffuse coronal neutral triplet-state Helium associated with streamers (Kuhn et al, 1996). This initial measurement was eventually confirmed to have solar origin through groundbased spectro-polarimetric observations using the Scatter-free Observatory for Limb, Active Regions, and Coronae (SOLARC) telescope on Haleakala (Kuhn et al, 2007; Moise et al, 2010).
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