The radial component of the local Galactic magnetic field in 3D

TL;DR

This paper reconstructs a 3D map of the local Galactic magnetic field component along the line of sight by combining electron density models and Faraday rotation data, revealing complex magnetic structures within 1.25 kpc.

Contribution

It introduces a novel distance-resolved reconstruction method of the local Galactic magnetic field using Gaussian-process priors and geometric variational inference.

Findings

Reconstructed magnetic field exhibits coherent patches with alternating signs.

The field shows kpc-scale modulations and structures on 100 pc scales.

The average line-of-sight magnetic field strength is approximately 1.63 μG.

Abstract

We present a distance-resolved reconstruction of the local line-of-sight Galactic magnetic field, $B_{||}$, by combining a 3D electron density ($n_{e}$) map derived from dust map-informed simulations and a full-sky map of Faraday rotation measure (RM). The forward model evaluates RM on the same 3D grid as the $n_{e}$ map and compares to the Galactic Faraday rotation sky. We infer $B_{||}$ with a Gaussian-process prior whose power spectrum is inferred from the data using geometric variational inference. The result is a local (within 1.25 kpc where $|b|>5^{\circ}$) map of $B_{||}$ with uncertainties. The reconstructed RM sky reproduces prominent features of Faraday rotation sky, with a root mean square average strength of $B_{||}$ of $1.63\pm 0.16$ $\mu$G. In face-on views, the magnetic field exhibits coherent patches with alternating sign and hints of kpc-scale modulations, but with significant structure seen on scales of order 100 pc. The $B_{||}$ field is seen to exhibit a 3D power spectrum with an average slope of $-2.73 \pm 0.19$. We validate our $B_{||}$ reconstruction with Galactic pulsars. Predicted RMs (computed by integrating $n_{e}B_{||}$ to each pulsar's distance) correlates with observed RMs, and predicted dispersion measures (DMs) from the $n_{e}$ map also correlate with measured DMs, albeit with significant scatter.