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

We present a distance-resolved reconstruction of the local line-of-sight Galactic magnetic field, B||, by combining a 3D electron density (ne) 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 ne 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○) 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± 0.16 μ 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 ± 0.19. We validate our B|| reconstruction with Galactic pulsars. Predicted RMs (computed by integrating neB|| to each pulsar's distance) correlates with observed RMs, and predicted dispersion measures (DMs) from the ne map also correlate with measured DMs, albeit with significant scatter.