An efficient decomposition technique to solve angle-dependent Hanle scattering problems

TL;DR

This paper introduces an efficient Fourier-based decomposition technique for solving angle-dependent Hanle scattering problems, improving accuracy in modeling polarization profiles influenced by weak magnetic fields.

Contribution

The authors develop a Fourier expansion method combined with a new numerical approach to accurately solve angle-dependent line transfer problems in Hanle scattering.

Findings

Angle-dependent frequency domains are essential for accurate Hanle transfer modeling.

The proposed method outperforms angle-averaged approaches in accuracy.

Micro-turbulent magnetic fields amplify differences between angle-dependent and angle-averaged solutions.

Abstract

Hanle scattering is an important diagnostic tool to study weak solar magnetic fields. Partial frequency redistribution (PRD) is necessary to interpret the linear polarization observed in strong resonance lines. Usually angle-averaged PRD functions are used to analyze linear polarization. However it is established that angle-dependent PRD functions are often necessary to interpret polarization profiles formed in the presence of weak magnetic fields. Our aim is to present an efficient decomposition technique, and the numerical method to solve the concerned angle-dependent line transfer problem. Together with the standard Stokes decomposition technique we employ Fourier expansion over the outgoing azimuth angle to express in a more convenient form, the angle-dependent PRD function for the Hanle effect. It allows the use of angle-dependent frequency domains of Bommier to solve the Hanle transfer problem. Such an approach is self-consistent and accurate compared to a recent approach where angle-averaged frequency domains were used to solve the same problem. We show that it is necessary to incorporate angle-dependent frequency domains instead of angle-averaged frequency domains to solve the Hanle transfer problem accurately, especially for the Stokes U parameter. The importance of using angle-dependent domains has been highlighted by taking the example of Hanle effect in the case of line transfer with vertical magnetic fields in a slab atmosphere. We have also studied the case of polarized line formation when micro-turbulent magnetic fields are present. The difference between angle-averaged and angle-dependent solutions is enhanced by the presence of micro-turbulent fields.