Accurate Modeling of the Forward-Scattering Hanle Effect in the Chromospheric Ca I 4227 Å Line

Luca Belluzzi,Simone Riva,Gioele Janett,Nuno Guerreiro,Fabio Riva,Pietro Benedusi,Tanausú del Pino Alemán,Ernest Alsina Ballester,Javier Trujillo Bueno,Jiří Štěpán

ASTRONOMY & ASTROPHYSICS（2024）

Università della Svizzera italiana Institut für Sonnenphysik (KIS) | Università della Svizzera italiana Euler Institute | Università della Svizzera italiana Istituto ricerche solari Aldo e Cele Daccò (IRSOL) | Universidad de La Laguna Departamento de Astrofísica | Consejo Superior de Investigaciones Científicas Departamento de Astrofísica | Astronomical Institute ASCR

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Abstract

Measurable linear scattering polarization signals have been predicted anddetected at the solar disk center in the core of chromospheric lines. Theseforward-scattering polarization signals, which are of high interest formagnetic field diagnostics, have always been modeled either under theassumption of complete frequency redistribution (CRD), or taking partialfrequency redistribution (PRD) effects into account under the angle-averaged(AA) approximation. This work aims at assessing the suitability of the CRD andPRD-AA approximations for modeling the forward-scattering polarization signalsproduced by the presence of an inclined magnetic field, the so-calledforward-scattering Hanle effect, in the chromospheric CaI 4227 A line.Radiative transfer calculations are performed in semi-empirical 1D solaratmospheres, out of local thermodynamic equilibrium (LTE). A two-step solutionstrategy is applied: the non-LTE RT problem is first solved considering amultilevel atom and neglecting polarization phenomena. The same problem is thensolved including polarization, considering a two-level atom and keeping fixedthe lower-level population calculated at the previous step. The emergent linearpolarization signals calculated under the CRD and PRD-AA approximations areanalyzed and compared to those obtained by modeling PRD effects in theirgeneral angle-dependent (AD) formulation. With respect to the PRD-AD case, theCRD and PRD-AA calculations significantly underestimate the amplitude of theline-center polarization signals produced by the forward-scattering Hanleeffect. The results of this work suggest that a PRD-AD modeling is required inorder to develop reliable diagnostic techniques exploiting theforward-scattering polarization signals observed in the CaI 4227 A line. Theseresults need to be confirmed by full 3D calculations including non-magneticsymmetry-breaking effects.

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Key words

polarization,radiative transfer,scattering,Sun: chromosphere,Sun: magnetic fields

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