Abstract
Abstract The Zeeman effect is of limited utility for probing the magnetism of the quiet solar chromosphere. The Hanle effect in some spectral lines is sensitive to such magnetism, but the interpretation of the scattering polarization signals requires taking into account that the chromospheric plasma is highly inhomogeneous and dynamic (i.e., that the magnetic field is not the only cause of symmetry breaking). Here we investigate the reliability of a well-known formula for mapping the azimuth of chromospheric magnetic fields directly from the scattering polarization observed in the Ca ii 8542 Å line, which is typically in the saturation regime of the Hanle effect. To this end, we use the Stokes profiles of the Ca ii 8542 Å line computed with the PORTA radiative transfer code in a three-dimensional (3D) model of the solar chromosphere, degrading them to mimic spectropolarimetric observations for a range of telescope apertures and noise levels. The simulated observations are used to obtain the magnetic field azimuth at each point of the field of view, which we compare with the actual values within the 3D model. We show that, apart from intrinsic ambiguities, the method provides solid results. Their accuracy depends more on the noise level than on the telescope diameter. Large-aperture solar telescopes, like DKIST and EST, are needed to achieve the required polarimetric sensitivity using reasonable exposure times.
Highlights
With the upcoming new generation of solar telescopes, like DKIST and EST, there is an urgent need for suitable methods to infer the magnetic field information from the unprecedented spectropolarimetric data that these telescopes will provide
The Hanle effect in some spectral lines is sensitive to such magnetism, but the interpretation of the scattering polarization signals requires taking into account that the chromospheric plasma is highly inhomogeneous and dynamic
The solar chromosphere is more complex than any present 3D model, we think that the Stokes Q/I and U/I profiles calculated by Stepan & Trujillo Bueno (2016) in the 3D snapshot model of Carlsson et al (2016) include all the key physical ingredients needed to reach solid conclusions concerning the reliability of the basic formula of Eq (4) for mapping the azimuth of the magnetic field in quiet regions of the solar chromosphere
Summary
To determine the magnetic field in the solar atmosphere, we need to observe the intensity (I), linear polarization (Q and U ) and circular polarization (V ) in spectral lines –that is, we need to measure the Stokes profiles I(λ), Q(λ), U (λ) and V (λ) as as function of wavelength (e.g., Stenflo 1994; del Toro Iniesta 2003). In this regime, which occurs when the Zeeman splitting in frequency units is much larger than the inverse lifetime of the relevant atomic levels, the linear polarization that results from scattering processes is sensitive only to the orientation of the magnetic field, but not to its strength (e.g., Section 13.5 in Landi Degl’Innocenti & Landolfi 2004) For such spectral lines, like the forbidden lines of the solar corona (e.g., Judge 2007), Eq (4) holds if the magnetic field azimuth is constant along the LOS. The solar chromosphere is more complex than any present 3D model, we think that the Stokes Q/I and U/I profiles calculated by Stepan & Trujillo Bueno (2016) in the 3D snapshot model of Carlsson et al (2016) include all the key physical ingredients needed to reach solid conclusions concerning the reliability of the basic formula of Eq (4) for mapping the azimuth of the magnetic field in quiet regions of the solar chromosphere
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