Probing the magneto-ionic medium of the Milky Way using pulsars

Abstract

Magnetic fields are fundamental to the dynamics of the interstellar medium (ISM) in spiral galaxies and are often separated into large-scale, regular ($\boldsymbol{B}$) and small-scale, random ($\boldsymbol{b}$) components. The thermal electron density, $n_{\rm e}$, can also be divided into large-scale, diffuse, $\langle n_{\rm e} \rangle$, and small-scale, clumpy, $\delta n_{\rm e}$, components. Estimating the properties of $b$ and $\delta n_{\rm e}$ from observations, even within the Milky Way, has long been challenging. This work addresses the challenge using pulsars, which probe the Milky Way's magneto-ionic medium. Using data of more than 1200 pulsars from the Australia Telescope National Facility pulsar catalogue, we combine dispersion (${\rm DM}$) and rotation (${\rm RM}$) measures with theoretical models to estimate both small- and large-scale properties of the Galactic magnetic field and thermal electron density. We find no significant correlation between the average parallel magnetic field strength, $\langle B_{\parallel} \rangle [\mu{\rm G}] = 1.232\,{\rm RM}\,[{\rm rad\,m^{-2}}]/{\rm DM}\,[{\rm pc\,cm^{-3}}]$, and pulsar distance. For pulsars within $20\,{\rm kpc}$, we estimate $|B| \approx 1.2\,\mu{\rm G}$ and $\langle n_{\rm e} \rangle \approx 0.05\,{\rm cm}^{-3}$. More importantly, we determine correlation lengths of small-scale components, $\ell_{b} \approx 20$ -- $30\,{\rm pc}$ and $\ell_{\delta n_{\rm e}} \approx 250$ -- $300\,{\rm pc}$. At smaller distances, $B$ remains roughly constant, while $\langle n_{\rm e} \rangle$ increases and both length scales decrease. These results refine our understanding of fundamental scales in the magneto-ionic medium, aiding the interpretation of extragalactic ${\rm RM}$s and providing insights into the role of magnetic fields in galaxies.