Ground-state alignment of atoms and ions: New Diagnostics of Astrophysical Magnetic field in diffuse medium

TL;DR

This paper introduces a novel method for detecting and analyzing weak magnetic fields in diffuse astrophysical media using the polarization of light caused by ground-state atomic alignment, enabling 3D magnetic field mapping.

Contribution

The paper presents a new technique leveraging atomic ground-state alignment and polarization measurements to diagnose magnetic fields in various astrophysical environments, including the early universe.

Findings

Alignment-induced polarization can exceed 20%.

Polarization direction correlates with magnetic field orientation.

Method applicable to optical and UV polarimetry for 3D magnetic field mapping.

Abstract

We discuss a new technique of studying magnetic fields in diffuse astrophysical media, e.g. interstellar and intergalactic gas/plasma. This technique is based on the angular momentum alignment of atoms and ions in their ground or metastable states. As the life-time of atoms in such states is long, the alignment induced by anisotropic radiation is susceptible to weak magnetic fields ($1{\rm G}\gtrsim B\gtrsim0.1\mu$G). The alignment reveals itself in terms of the polarization of the absorbed and emitted light. A variety of atoms with fine or hyperfine splitting of the ground or metastable states exhibit the alignment and the resulting polarization degree in some cases exceeds 20%. We show that in the case of absorption the polarization direction is either parallel or perpendicular to magnetic field, while more complex dependencies emerge for the case of emission of aligned atoms. We show that the corresponding studies of magnetic fields can be performed with optical and UV polarimetry. A unique feature of these studies is that they can reveal the 3D orientation of magnetic field. In addition, we point out that the polarization of the radiation arising from the transitions between fine and hyperfine states of the ground level can provide yet another promising diagnostics of magnetic fields, including the magnetic fields in the Early Universe. We mention several cases of interplanetary, circumstellar and interstellar magnetic fields for which the studies of magnetic fields using ground state atomic alignment effect are promising.