Structure in the Rotation Measure Sky

TL;DR

This paper analyzes the structure of the rotation measure sky, identifying features linked to local interstellar medium structures and examining the statistical properties of RM variations across the sky.

Contribution

It provides the first detailed analysis of RM structure functions across the sky, linking them to local ISM features and Galactic asymmetries.

Findings

RM structures correlate with local ISM features like radio loops and nebulae.

RM variance increases with line-of-sight length and shows hemispheric asymmetry.

RM structure functions suggest a predominantly local origin for large-scale RM variations.

Abstract

An analysis of structure in rotation measure (RM) across the sky based on the RM catalog of Taylor et al. (2009) is presented. Several resolved RM structures are identified with structure in the local ISM, including radio loops I, II, and III, the Gum nebula, and the Orion-Eridanus super bubble. Structure functions (SFs) of RM are presented for selected areas, and maps of SF amplitude and slope across the sky are compared with H-alpha intensity and diffuse polarized intensity. RM variance on an angular scale of 1 degree is correlated with length of the line of sight through the Galaxy, with a contribution from local structures. The slope of the SFs is less concentrated to the Galactic plane and less correlated with length of the line of sight through the Galaxy, suggesting a more local origin for RM structure on angular scales ~ 10 degrees. The RM variance is a factor ~2 higher towards the SGP than towards the NGP, reflecting a more wide-spread asymmetry between the northern and southern Galactic hemispheres. Depolarization of diffuse Galactic synchrotron emission at latitudes < 30 degrees can be explained largely by Faraday dispersion related to small-scale variance in RM, but the errors allow a significant contribution from differential Faraday rotation along the line of sight.