The coherent (large-scale ordered) magnetic field of the Galactic disk is known to follow the spiral arms, and decades of pulsar and extragalactic rotation-measure data establish at least one reversal of its direction — but the total number of reversals, their Galactocentric locations, and their three-dimensional geometry remain unsettled. Curtin, Weisberg and Rankin (2024) introduced a geometrically corrected, arm-by-arm technique that segments each pulsar line of sight into spiral-arm and interarm zones and solves for the field strength and direction zone by zone; applied to 313 new Arecibo rotation measures it finds a single reversal near the Local–Sagittarius arm boundary whose location depends on Galactic latitude, hinting at a tilted three-dimensional structure, while leaving the Norma–Outer Arm, the Perseus–Norma interarm, the region interior to the Scutum Arm, and the region beyond the Perseus Arm unmapped and the reversal geometry unmodeled. The open problem is to complete this distance-resolved map: determine the zone-by-zone field in the unsampled regions, establish how many reversals exist and where, and determine whether the Local–Sagittarius reversal is a tilted or curved three-dimensional structure, with quantified robustness to electron-density-model, distance, random-field, and local-structure systematics.

Background

Pulsars are highly magnetized rotating neutron stars that emit polarized radio beams. Linearly polarized radio emission propagating through the magnetized, ionized interstellar medium undergoes Faraday rotation: the polarization plane rotates by an angle proportional to wavelength squared, with proportionality constant RM=0.812 ∫neB∥ dlRM = 0.812\,\int n_e B_\parallel\,dl (in rad m−2\mathrm{rad\ m^{-2}}, with electron density nen_e in cm−3\mathrm{cm^{-3}}, line-of-sight magnetic field component B∥B_\parallel in μG\mu\mathrm{G}, and path length in pc). The rotation measure (RM) is therefore a path integral of the electron density weighted by the magnetic field component along the line of sight; its sign records the net field direction along the path. Pulsars also provide a dispersion measure (DM=∫ne dlDM = \int n_e\,dl), which, combined with a model of the Galactic free-electron density — most commonly YMW17 (Yao, Manchester and Wang 2017) — yields a distance estimate, and differences in DM between pulsars at different distances isolate the electron content of the zone between them.

The coherent disk field roughly follows the Milky Way's spiral arms — from the Sun outward across the Local (Orion) Arm and the Perseus Arm; inward across the Sagittarius–Carina Arm, the Sagittarius-to-Scutum interarm, and the Scutum–Centaurus Arm (called the Crux–Scutum Arm by Curtin et al.); and the Norma–Outer Arm, a single logarithmic-spiral structure in the four-arm model of Hou & Han (2014), as slightly modified by YMW17, whose inner (Norma) segment lies interior to the Scutum arm, while its outer segment (the Outer Arm) lies beyond the Perseus Arm toward the Galactic anticenter and is the segment probed by the new Arecibo rotation measures at Galactic longitudes of roughly 175°–210°. By convention the field direction is called clockwise (CW) or counterclockwise (CCW) as viewed from the North Galactic pole. A field reversal is a change of this sign between adjacent zones. Global model classes are distinguished by reversal structure: an axisymmetric spiral (ASS) has no reversals with Galactocentric radius, a bisymmetric spiral (BSS) has one azimuthal reversal, and ASS-plus-ring models add a narrow reversed annulus; a bisymmetric field has never been conclusively confirmed in any spiral galaxy. A persistent complication is that the random (turbulent) magnetic field is comparable to or larger than the coherent field, adding scatter to every RM, and that localized objects such as the Gum Nebula perturb both DMs and RMs so strongly that affected sightlines must usually be excised.

Using 313 new low-latitude pulsar RMs from the Arecibo radio telescope (Rankin et al. 2023) plus archival RMs, Curtin, Weisberg and Rankin (2024) developed an arm-by-arm technique that accounts for the dot-product nature of Faraday rotation and splits the observed RM and DM of each pulsar into segments corresponding to the zones crossed by its line of sight: solving first for the nearest zone's mean field, they subtract its modeled RM contribution from pulsars farther away and proceed recursively outward. Under the assumption that each zone has a uniform field magnitude ∣B∣|B| directed parallel to the zone's outer spiral boundary, they obtained strengths and directions for the Local Arm, Sagittarius Arm, Sagittarius-to-Scutum interarm, Scutum Arm, and Perseus Arm, above and below the Galactic plane. They confirmed the long-standing reversal between the Sagittarius (CCW) and Local/Perseus (CW) fields, found no reversal within the Perseus Arm itself (disagreeing with earlier analyses such as Han et al. 2018, which attributed a counterclockwise field to the Perseus Arm, and Xu et al. 2022, which found reversals in the Perseus Arm and beyond), and discovered that the reversal location varies with Galactic latitude: at positive latitudes it lies at the Local–Sagittarius boundary, at negative latitudes inside the Sagittarius Arm itself. This latitude dependence matches a diagonal sign boundary in the total-RM sky found by Ordog et al. (2017) near the Sagittarius arm tangent, and suggests — but does not establish — that the reversal is a plane-like structure tilted with respect to the Galactic plane rather than a purely axisymmetric feature in the disk.

Problem Statement

Complete the distance-resolved, arm-by-arm determination of the coherent magnetic field of the Galactic disk and settle the structure of its reversals, in the formulation left open by Curtin, Weisberg & Rankin (2024). Specifically: (a) determine the strength ∣B∣|B| and direction (clockwise or counterclockwise as viewed from the North Galactic pole) of the coherent field in the zones their technique could not sample — the Norma–Outer Arm and the Perseus–Norma interarm — using distance-resolved pulsar rotation and dispersion measures; (b) establish whether reversals of the coherent field exist interior to the Scutum Arm and beyond the Perseus Arm — the two segments of the Norma–Outer Arm whose zone-by-zone fields are determined in (a) — and thereby fix the total number of reversals within the studied region and their Galactocentric locations, resolving the existing conflict with the reversal census of Han et al. (2018) and Xu et al. (2022); and (c) determine the three-dimensional geometry of the established reversal near the Local–Sagittarius arm boundary — whether it is a tilted or curved quasi-planar structure as suggested by its latitude dependence and by local models such as Booth et al. (2026), or a projection of a different topology such as odd-parity disk–halo coupling (a halo field antisymmetric with respect to the Galactic plane) — together with its physical origin. The zone-by-zone field strengths and directions of (a), the reversal count and Galactocentric locations of (b), and the geometry and physical origin of (c) rest on one and the same internally consistent, distance-resolved arm-by-arm map of the disk field — covering both the zones already sampled by Curtin, Weisberg & Rankin (2024) and the previously unsampled zones named in (a) — and each holds at a stated statistical significance and only in so far as it survives the systematic effects the source study identified: electron-density-model (YMW17) and distance-assignment errors propagating through the zone-segmented dispersion measures, intra-zone spatial field variation beyond the uniform-field assumption, random-field contamination at strengths comparable to the systematic field, and localized perturbing structures (e.g., the Gum Nebula), whether the affected sightlines are modeled or excised.

References

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