Solenoidal Improvements for the JF12 Galactic Magnetic Field Model

TL;DR

This paper improves the JF12 Galactic magnetic field model by ensuring it is divergence-free and addressing structural issues, thereby enhancing its physical accuracy and utility in cosmic-ray simulations.

Contribution

It introduces a solenoidal correction to the JF12 model and proposes strategies to fix structural kinks, improving the model's physical consistency and simulation performance.

Findings

The corrected model is truly divergence-free.

The improved model reduces structural kinks in the magnetic field.

Test simulations show enhanced cosmic-ray propagation accuracy.

Abstract

The popular JF12 analytic model by Jansson & Farrar provides a quantitative description of the Galaxy's large-scale magnetic field, which is widely used in various astrophysical applications. However, both the poloidal X-type component and the spiral disk component of JF12 exhibit regions in which the magnetic divergence constraint is violated. We first propose a cure for this problem, resulting in a truly solenoidal large-scale spiral field. Second, the otherwise straight field lines of the X-type component exhibit kinks in the Galactic plane that, in addition to implying the presence of a singular current sheet, may pose difficulties for e.g., numerical tracing of cosmic-ray particles. We propose and discuss two possible strategies to mitigate this problem. Although all corrections are kept as minimal as possible, the extended set of model parameters will have to be carefully readjusted in order to fully restore the agreement to observational data that the unmodified JF12 field is based on. Furthermore, the performance of our improved version of the field model is quantitatively assessed by test simulations using the CRPropa Galactic cosmic-ray propagation code.