Scattering polarization in the CaII Infrared Triplet with Velocity Gradients

TL;DR

This paper investigates how vertical velocity gradients in the solar atmosphere influence scattering polarization signals in the CaII infrared triplet, highlighting their importance for accurate magnetic field measurements.

Contribution

It provides a theoretical analysis of the effects of velocity gradients on scattering polarization, emphasizing their role in interpreting spectropolarimetric data.

Findings

Velocity gradients significantly affect polarization signals.

Gradients influence radiation field anisotropy and level alignment.

Implications for magnetic field inference via Hanle effect.

Abstract

Magnetic field topology, thermal structure and plasma motions are the three main factors affecting the polarization signals used to understand our star. In this theoretical investigation, we focus on the effect that gradients in the macroscopic vertical velocity field have on the non-magnetic scattering polarization signals, establishing the basis for general cases. We demonstrate that the solar plasma velocity gradients have a significant effect on the linear polarization produced by scattering in chromospheric spectral lines. In particular, we show the impact of velocity gradients on the anisotropy of the radiation field and on the ensuing fractional alignment of the CaII levels, and how they can lead to an enhancement of the zero-field linear polarization signals. This investigation remarks the importance of knowing the dynamical state of the solar atmosphere in order to correctly interpret spectropolarimetric measurements, which is important, among other things, for establishing a suitable zero field reference case to infer magnetic fields via the Hanle effect.