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

Magnetic fields are fundamental to the dynamics of the interstellar medium (ISM) in spiral galaxies and are often separated into large-scale, regular (B) and small-scale, random (b) components. The thermal electron density, ne, can also be divided into large-scale, diffuse, 〈ne〉, and small-scale, clumpy, δne, components. Estimating the properties of b and δne 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 (DM) and rotation (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, 〈B∥〉[μG] = 1.232 RM [rad m−2]/DM [pc cm−3], and pulsar distance. For pulsars within 20 kpc, we estimate |B| ≈ 1.2 μG and 〈ne〉 ≈ 0.05 cm−3. More importantly, we determine correlation lengths of small-scale components, ℓb ≈ 20 – 30 pc and _δ nₑ ≈ 250 – 300 pc. At smaller distances, B remains roughly constant, while 〈ne〉 increases and both length scales decrease. These results refine our understanding of fundamental scales in the magneto-ionic medium, aiding the interpretation of extragalactic RMs and providing insights into the role of magnetic fields in galaxies.