The Second Stellar Spectrum and the non-LTE Problem of the Second Kind

TL;DR

This paper reviews the non-LTE radiative transfer problem of the second kind in astrophysics, highlighting its role in understanding the solar spectrum and magnetic fields, and suggests new diagnostic possibilities involving magnetic field inclination.

Contribution

It provides an overview of the non-LTE problem of the second kind and reveals how magnetic field inclination can modulate atomic level populations, offering new diagnostic tools.

Findings

Population ratios can be modulated by magnetic field inclination.

High-sensitivity polarimeters could enhance understanding of solar magnetism.

The non-LTE problem of the second kind aids in interpreting the second solar spectrum.

Abstract

This paper presents an overview of the radiative transfer problem of calculating the spectral line intensity and polarization that emerges from a (generally magnetized) astrophysical plasma composed of atoms and molecules whose excitation state is significantly influenced by radiative transitions produced by an anisotropic radiation field. The numerical solution of this non-LTE problem of the 2nd kind is facilitating the physical understanding of the second solar spectrum and the exploration of the complex magnetism of the extended solar atmosphere, but much more could be learned if high-sensitivity polarimeters were developed also for the present generation of night-time telescopes. Interestingly, I find that the population ratio between the levels of some resonance line transitions can be efficiently modulated by the inclination of a weak magnetic field when the anisotropy of the incident radiation is significant, something that could provide a new diagnostic tool in astrophysics.