Exploring the Magnetic Field Structure of the Milky Way with Pulsars in the SKA Era

Abstract: The magnetic field structure of the Milky Way can offer critical insights into the origin of galactic magnetic fields. Measurements of magnetic structures of the Milky Way are still sparse in far regions of the Galactic disk and halo. Pulsars are the best probes for the three-dimensional structure of the Galactic magnetic field, primarily owing to their highly polarized short-duration radio pulses, negligible intrinsic Faraday rotation compared to the contribution from the medium in front, and their widespread distribution throughout the Galaxy across the thin disk, spiral arms, and extended halo. In this article, we give an overview of Galactic magnetic field investigation using pulsars. The sensitive SKA1 design baseline (AA4) will increase the number of known pulsars by a factor of around three, and the initial staged delivery array (AA*) will probably double the total number of the current pulsar population. Polarization observations of pulsars with the AA* telescopes will give rotation measures along several thousand lines of sight, enabling detailed exploration of the magnetic structure of both the Galactic disk and the Galactic halo.

• The paper demonstrates that SKA-era pulsar surveys can accurately reconstruct the Milky Way’s 3D magnetic field using high-precision RM and DM data.
• It employs advanced simulations and sensitivity projections to predict discoveries of over 10,000 pulsars, unveiling detailed disk and halo magnetic features.
• This comprehensive mapping supports improved models for cosmic ray propagation, star formation, and foreground removal in CMB polarization studies.

Probing the Milky Way's Magnetic Field Structure with Pulsars in the SKA Era

Introduction

The detailed morphology and strength of the Milky Way’s Galactic magnetic field (GMF) are foundational parameters for astrophysical models of the ISM, star formation, cosmic ray propagation, and CMB polarization measurements. Despite decades of study, mapping the 3D large-scale magnetic field remains an open challenge due to the sparsity of tracers, particularly at large Galactocentric distances and in the Galactic halo. Pulsars, as luminous, highly-polarized, and widely distributed radio sources, provide unique access to Faraday rotation and dispersion along diverse lines of sight, thereby offering a direct probe of the GMF with both spatial and distance information. The new capabilities delivered by the Square Kilometre Array (SKA) will dramatically expand the pulsar RM (Rotation Measure) dataset, enabling unprecedented advances in the mapping and physical understanding of Galactic magnetism (2607.03314).

Pulsars as Magnetometric Probes

Pulsar RM and DM measurements are physically orthogonal, encoding line-of-sight-weighted means of the magnetic field and electron density, respectively. For a pulsar at distance $D$:

• The DM yields the integrated thermal electron density: $\mathrm{DM} = \int_0^D n_e\, dl$.
• The RM delivers the Faraday rotation from the ISM: $\mathrm{RM} = 0.81 \int_0^D n_e B_\parallel\, dl$.
• Taking the ratio, one obtains the average $B_\parallel$ along the path, robust to electron density modeling on scales above the outer turbulence scale.

With several thousand catalogued pulsars and RMs for roughly half, present maps cover the local disk and inner halo. However, the spatial distribution is highly anisotropic; sight lines toward the far disk and remote halo are still poorly sampled. Existing surveys with FAST, Parkes, and MeerKAT have improved statistics, but sensitivity limits restrict RM sampling, particularly for faint and highly-dispersed sources deep in the disk or at high latitudes.

SKA Capabilities and Expected Pulsar Yields

The SKA Phase 1 (SKA1) is comprised of SKA1-Low (50–350 MHz) and SKA1-Mid (350 MHz–15 GHz). Even with hardware de-scoping, SKA1-Low and SKA1-Mid (AA4 and AA* configurations) will outperform current facilities by orders of magnitude in survey speed, bandwidth, and polarimetric accuracy.

• Population Synthesis: Simulations show that SKA1-Low will dominate discoveries at high latitudes and low DMs, while SKA1-Mid will probe the densest disk regions and reach distant, faint, and highly-dispersed pulsars inaccessible to current instrumentation.
• Yield Projections: SKA1 should discover $\approx$10,000–13,000 normal pulsars and $\sim$1,000 millisecond pulsars, at a minimum tripling the known population (2607.03314). The SKA2 census will increase the total to $\mathcal{O}(30,000)$, providing probe lines well beyond the Galactic center.
• Polarimetric Sensitivity: With wide fractional bandwidths and high spectral resolution (64k frequency channels for SKA1-Mid), high-precision RMs can be extracted for even weakly polarized sources at an SNR $>50$, enabling even faint pulsars to contribute to the RM grid.

Mapping the Magnetic Field: Disk and Halo

Disk Structure

The GMF in the disk displays a superposition of large-scale regular (coherent over kpc) and turbulent ($\lesssim$100 pc) components. Pulsar RMs across the local and inner disk identify field reversals at arm and interarm boundaries, supporting a bisymmetric or spirally symmetric disk field. However, present RM statistics outside the Solar circle are grossly insufficient.

SKA1’s sensitivity will:

• Substantially increase RM density in regions now poorly sampled, notably the fourth quadrant and the outer disk.
• Enable detection of field reversals and structural correlations between quadrants, and examine field geometry continuity across arm/interarm zones $>\!10$ kpc from the Sun.
• Allow robust reconstruction of the large-scale field using gradient and delta-methods between closely spaced pulsars distributed over a range of DMs and distances.
• Provide high-resolution Faraday tomography to disentangle depth-dependent field components.

Halo Structure

Observations of extragalactic sources reveal an antisymmetric RM sky, interpreted as a toroidal halo field with opposite signs above and below the plane in the inner Galaxy. Pulsar RMs at $\mathrm{DM} = \int_0^D n_e\, dl$0 provide independent, local probes, crucial for decomposing line-of-sight RM into disk, halo, and local contributions.

Key SKA-driven outcomes:

• Thousands of new halo pulsar RMs will enable direct measurement of halo field strength and scale height, and establish the radial and vertical profiles of the toroidal halo field.
• By combining RMs with extragalactic source data and using local subtraction schemes, SKA will produce the most detailed 3D map of the Galactic halo’s magnetized structure, constraining the symmetry, size, and possible reversals of the halo field.
• Enhanced sampling in the fourth quadrant and the outer Galaxy with SKA1-Mid and SKA1-Low will rectify longstanding hemispherical biases.

Implications and Future Prospects

SKA-driven pulsar RM datasets will create the reference RM grid for 3D magneto-ionic models of the Galaxy. Consequences span:

• Validation or falsification of large-scale dynamo models and pitch angle measurement.
• Improved modeling of cosmic ray propagation and diffusion, crucial for high-energy astrophysics and particle astronomy.
• Subtraction of Galactic foregrounds from CMB $\mathrm{DM} = \int_0^D n_e\, dl$1-mode, E-mode separation, and accurate mapping of primordial polarization signatures.
• Astrophysical studies of ISM turbulence, magneto-thermal instabilities, and the effect of magnetic fields on Galactic ecology.

Furthermore, combined with advances in parallax and timing measurements for precise distance scaling, the SKA-era pulsar population will underpin tomographic reconstructions of the GMF over all accessible spatial scales.

Conclusion

SKA-Era pulsar surveys will revolutionize the spatial and distance coverage of rotation and dispersion measures, establishing pulsar RMs as the chief probe of the Galactic magnetic field’s 3D structure. The resulting datasets will enable precise mapping of both disk and halo fields, test GMF dynamo models, and impact astrophysics from star formation to cosmic rays. The anticipated scientific return positions SKA as the essential facility for Galactic magnetism studies (2607.03314).