Magnetic structure of our Galaxy: A review of observations

The magnetic structure in the Galactic disk, the Galactic center and the Galactic halo can be delineated more clearly than ever before. In the Galactic disk, the magnetic structure has been revealed by starlight polarization within 2 or 3 kpc of the Solar vicinity, by the distribution of the Zeeman splitting of OH masers in two or three nearby spiral arms, and by pulsar dispersion measures and rotation measures in nearly half of the disk. The polarized thermal dust emission of clouds at infrared, mm and submm wavelengths and the diffuse synchrotron emission are also related to the large-scale magnetic field in the disk. The rotation measures of extragalactic radio sources at low Galactic latitudes can be modeled by electron distributions and large-scale magnetic fields. The statistical properties of the magnetized interstellar medium at various scales have been studied using rotation measure data and polarization data. In the Galactic center, the non-thermal filaments indicate poloidal fields. There is no consensus on the field strength, maybe mG, maybe tens of uG. The polarized dust emission and much enhanced rotation measures of background radio sources are probably related to toroidal fields. In the Galactic halo, the antisymmetric RM sky reveals large-scale toroidal fields with reversed directions above and below the Galactic plane. Magnetic fields from all parts of our Galaxy are connected to form a global field structure. More observations are needed to explore the untouched regions and delineate how fields in different parts are connected.

💡 Research Summary

The paper provides a comprehensive synthesis of the current observational picture of the Milky Way’s magnetic field, dividing the discussion into three principal Galactic components: the disk, the central region, and the halo. In the disk, the authors draw on a suite of complementary techniques. Starlight polarization within roughly 2–3 kpc of the Sun reveals a coherent spiral‑like field lying largely in the Galactic plane with an average strength of a few microgauss. Zeeman splitting measurements of OH masers in nearby spiral arms (Perseus, Sagittarius, Carina, etc.) extend this picture to larger radii, showing field strengths ranging from a few to several tens of microgauss and indicating possible phase shifts between arm and inter‑arm regions. Pulsar dispersion measures (DM) and rotation measures (RM) cover nearly half of the Galactic disk; when combined with electron density models such as NE2001, they confirm a large‑scale toroidal component that follows the spiral pattern and produce an “antisymmetric” RM signature—alternating positive and negative values on opposite sides of the Galactic plane—suggestive of a dipolar or A0 dynamo mode.

Thermal dust emission polarized at infrared, millimeter, and sub‑millimeter wavelengths provides an independent probe of magnetic orientation within dense molecular clouds. These data show that the large‑scale spiral field coexists with localized flux‑tube structures, and the diffuse synchrotron emission’s polarization aligns with the toroidal component, reinforcing the global picture. Low‑latitude extragalactic source RMs can be modeled successfully by coupling the large‑scale field geometry with the electron distribution, further validating the disk field model.

In the Galactic center, the presence of non‑thermal filaments (NTFs) that run nearly perpendicular to the plane is taken as evidence for a strong poloidal field. However, the exact field strength remains debated: estimates range from tens of microgauss up to milligauss levels. Polarized dust emission in the central molecular zone and the extremely high rotation measures (up to several thousand rad m⁻²) observed toward background radio sources point to an additional toroidal component, indicating that both vertical and azimuthal fields coexist in the central few hundred parsecs.

The halo is characterized by an “antisymmetric” RM sky: rotation measures at high Galactic latitudes show a systematic sign reversal above versus below the plane. This pattern is interpreted as a large‑scale toroidal field that flips direction across the mid‑plane, consistent with an A0 dynamo mode operating in the thick disk/halo. The authors argue that the halo field is linked to the disk field, forming a continuous magnetic circuit that extends several kiloparsecs above and below the Galactic plane.

Statistical analyses of RM and polarization data across a wide range of spatial scales reveal a power‑law behavior indicative of magnetohydrodynamic turbulence extending from parsec‑scale clouds up to kiloparsec‑scale structures. This suggests that both turbulent cascade processes and large‑scale dynamo action shape the observed magnetic geometry.

The review concludes that, while a coherent global magnetic architecture is emerging—comprising a spiral toroidal component in the disk, a mixed poloidal‑toroidal system in the central region, and a reversed‑direction toroidal halo—the picture remains incomplete. Significant observational gaps persist in the far outer Galaxy (>15 kpc), in the precise measurement of the central field strength, and in the low‑density high‑latitude halo. The authors stress that forthcoming facilities such as the Square Kilometre Array (SKA), the next‑generation VLA (ngVLA), JWST, and upgraded sub‑millimeter polarimeters (e.g., ALMA upgrades) will be essential to fill these gaps. Moreover, they call for sophisticated MHD simulations that incorporate realistic electron density distributions and dynamo physics to compare directly with the expanding RM and polarization datasets. In sum, the paper maps the state‑of‑the‑art observational constraints, highlights the remaining uncertainties, and outlines a clear roadmap for future research aimed at fully characterizing the Milky Way’s magnetic field as a unified, multi‑scale phenomenon.