Measuring interstellar magnetic fields by radio synchrotron emission

TL;DR

Radio synchrotron emission, polarization, and Faraday rotation are essential tools for studying the strength, structure, and dynamics of interstellar magnetic fields across different galaxies and the Milky Way.

Contribution

This paper reviews how radio synchrotron emission and Faraday rotation are used to measure and analyze interstellar magnetic fields, highlighting recent observational findings.

Findings

Total magnetic fields in spiral arms reach 20-30 μG.

Ordered fields often form spiral structures and magnetic arms.

Halo fields exhibit X-shaped configurations.

Abstract

Radio synchrotron emission, its polarization and its Faraday rotation are powerful tools to study the strength and structure of interstellar magnetic fields. The total intensity traces the strength and distribution of total magnetic fields. Total fields in gas-rich spiral arms and bars of nearby galaxies have strengths of 20-30 $\mu$Gauss, due to the amplification of turbulent fields, and are dynamically important. In the Milky Way, the total field strength is about 6 $\mu$G near the Sun and several 100 $\mu$G in filaments near the Galactic Center. -- The polarized intensity measures ordered fields with a preferred orientation, which can be regular or anisotropic fields. Ordered fields with spiral structure exist in grand-design, barred, flocculent and even in irregular galaxies. The strongest ordered fields are found in interarm regions, sometimes forming "magnetic spiral arms" between the optical arms. Halo fields are X-shaped, probably due to outflows. -- The Faraday rotation of the polarization vectors traces coherent regular fields which have a preferred direction. In some galaxies Faraday rotation reveals large-scale patterns which are signatures of dynamo fields. However, in most galaxies the field has a complicated structure and interacts with local gas flows. In the Milky Way, diffuse polarized radio emission and Faraday rotation of the polarized emission from pulsars and background sources show many small-scale and large-scale magnetic features, but the overall field structure in our Galaxy is still under debate.