A three-dimensional model for the reversal in the local large-scale interstellar magnetic field

Abstract: We probe the three-dimensional geometry of the large-scale Galactic magnetic field within 1 kpc of the Sun using the Dominion Radio Astrophysical Observatory (DRAO) Global Magneto-Ionic Medium Survey (GMIMS) of the Northern Sky (DRAGONS). DRAGONS is a new full polarization survey of the Northern sky from 350 to 1030 MHz covering declinations -20° < $δ$ < 90° and a component of GMIMS. The first moment of the Faraday depth spectra produced from DRAGONS above 500 MHz reveals large-angular-scale Faraday depth structures with signs that alternate only once in the Southern Galactic hemisphere and twice in the Northern hemisphere, patterns shared by other Faraday rotation datasets. DRAGONS is the first survey to achieve high Faraday depth resolution while maintaining sensitivity to broad Faraday depth structures, enabling the first use of Galactic longitude-Faraday depth plots. These plots reveal Faraday-complex structures across the sky, indicating a slab-like scenario in which emission and Faraday rotation are mixed. This complexity is overlaid on the same large-scale Faraday depth patterns that appear in the first moment map. We model these patterns as a magnetic reversal slicing through the disk on a diagonal and passing above the Sun in Galactic coordinates. We describe this reversal as a plane with a normal vector parallel to the line directed along ($\ell$, b) = (168.5°, -60°) and estimate its distance to be between 0.25 and 0.55 kpc. Our results show that much of the observed Faraday sky may be dominated by the local magnetic field configuration.

• The paper introduces a 3D parametric model that quantifies the reversal of the local interstellar magnetic field using advanced Faraday tomography.
• It employs combined DRAGONS and STAPS survey data with 3D dust extinction maps to model the field geometry and spatial reversal features.
• The model successfully reproduces observed sinℓ and sin2ℓ patterns, supporting a planar magnetic reversal inclined relative to the Galactic mid-plane.

Three-Dimensional Modeling of the Local Interstellar Magnetic Field Reversal

Introduction and Context

The paper "A three-dimensional model for the reversal in the local large-scale interstellar magnetic field" (2512.03332) presents a quantitative analysis and parametric modeling of the magnetic field reversal within 1 kpc of the Sun using advanced Faraday tomography. The authors exploit data from the Dominion Radio Astrophysical Observatory (DRAO) Global Magneto-Ionic Medium Survey (GMIMS) — specifically, the DRAGONS survey — and supplement the incomplete sky coverage with the Southern Twenty-centimeter All-sky Polarization Survey (STAPS). This work leverages the first-moment analysis of Faraday depth (FD) spectra to capture large-scale sign changes in Faraday rotation, which encode magnetic field geometry and reversals. The research targets the longstanding question of the spatial structure and position of local magnetic field reversals and their reproducibility in a global framework.

The study is highly data-driven, combining new FD cubes with three-dimensional extinction maps and rotation measure (RM) catalogs. The novelty of the approach lies in: (a) the use of DRAGONS for Faraday depth analysis with both high FD resolution and sensitivity to broad features, (b) direct morphological cross-matching with 3D ISM dust maps to infer distances, and (c) a formal parametric model that describes the reversal as a plane inclined with respect to the Galactic mid-plane.

Data Products and Methodology

The DRAGONS survey provides full polarization data across 500–1030 MHz, with an angular resolution of $2.45^\circ$ (after convolution) and FD resolution of $14$ rad m$^{-2}$ (Figure 1).

Figure 1: (a) and (b) show the frequency and $\lambda^2$ coverage of DRAGONS, respectively, while (c) displays the RMSF, which dictates FD resolution and sensitivity to broad features.

Missing southern coverage is filled with STAPS cubes, albeit at lower FD resolution. Faraday synthesis is applied using the RM-Tools package, including RM-CLEAN. The authors restrict the sky to $|b| > 5^\circ$ to avoid polarized intensity leakage near the plane.

The fundamental observable is the first moment (M1) of the FD spectrum per line-of-sight:

$${\rm M1} = \frac{\sum |\tilde{P}_i| \phi_i}{\sum |\tilde{P}_i|},$$

where $\tilde{P}_i$ is complex polarized intensity at $\phi_i$. This quantity collapses FD cubes to 2D maps while encoding the dominant sign and scale of Faraday rotation. The moment maps from DRAGONS and STAPS are merged to provide (nearly) full-sky coverage.

Observational Results: FD Sign Patterns

Analysis of the M1 maps recapitulates previously observed longitude-sign patterns: at $b < 0^\circ$, there is a single sign reversal (a $\sin \ell$ behavior), while at $b > 0^\circ$, there are two reversals (a $\sin 2\ell$ relationship) (Figure 2).

Figure 2: Peaks of $\sin\ell$ and $\sin 2\ell$ patterns, denoted by $+$ and $-$, overlaid on (a) the Hutschenreuter et al. map and (b) DRAGONS/STAPS M1.

Sinusoidal fitting in longitude confirms that the Southern hemisphere is $\sin\ell$-dominated while the Northern is $\sin 2\ell$-dominated (Figure 3).

Figure 3: DRAGONS (blue) and STAPS (orange) M1 values, with best-fit sinusoids, plotted along lines of constant latitude, validating the $\sin\ell$ and $\sin 2\ell$ decomposition.

Notably, these sign patterns persist across datasets with different polarization horizons, hinting that they reflect physical field structures rather than distance-dependent sampling artifacts.

The two largest-magnitude M1 regions, at $(\ell, b) \sim (130^\circ, 35^\circ)$ (negative) and $(40^\circ, 30^\circ)$ (positive), coincide with discrete ISM features. Comparison to the Edenhofer et al. 3D dust map using AstroHOG establishes morphological correspondence at distances $\sim400$–500 pc (Figure 4).

Figure 4: Projected Rayleigh and Pearson statistics showing strong DRAGONS–dust map correlation at $\sim$400–500 pc, with overlaid M1 contours on 3D dust structures.

This result locates the main contributors to the Northern $\sin 2\ell$ pattern within the local ISM, at $d \lesssim 500$ pc, i.e., clearly on the near side of the Sagittarius Arm. The FD sign in these regions is spatially uniform, supporting their association with the underlying large-scale field.

The Planar Reversal Model

The core model posits the field reversal as a plane slicing the Galactic disk, inclined with respect to the mid-plane. The plane’s normal vector points to $(\ell, b) = (168.5^\circ, -60^\circ)$ in Galactic coordinates. The intersection with the $x$-axis (toward the Galactic center) is parametrized as $x_0 = 0.25$–$0.55$ kpc.

Figure 5: Schematic of the 3D geometry; the reversal plane’s normal is parameterized by $(\ell_n, b_n)$ and intercept $x_0$.

Below the plane, the field is azimuthally clockwise (as viewed from the North Galactic Pole), above counterclockwise. The M1 value for each LOS is computed analytically for both Faraday screen (all emission behind the medium) and slab (emission and rotation mixed) geometries, incorporating two different vertical tilt angles ($\beta_{CW}$, $\beta_{CCW}$) for the field’s inclination above and below the plane.

Varying the model parameters, the simulated M1 maps robustly reproduce the observed large-scale $\sin 2\ell$ pattern for sensible choices, without requiring fine-tuning (Figure 6).

Figure 6: Simulated M1 longitude profiles at $b=30^\circ$; horizontal and vertical offsets illustrate the effects of varying path length and tilt angles.

Model Fitting and Quantitative Results

Fitting the model to the DRAGONS M1 map yields best-fit parameters with a path length $R/x_0 \approx 3$, vertical tilt angles $\beta_{CW} \approx +18^\circ$, $\beta_{CCW} \approx -23^\circ$, and $n_e |B| R \approx 0.02$–0.03 (screen), achieving a Pearson correlation of 0.6 across the sky (Figure 7).

Figure 7: (a) Model-predicted M1; (b) comparison of predicted M1 to DRAGONS/STAPS observations.

Latitude-dependent fitting demonstrates robust agreement in both peak positions and zero crossings for all mid-latitude bins (Figure 8).

Figure 8: Model fits (screen and slab) for M1 as a function of longitude within several latitude bins; black (screen) and red (slab) lines nearly overlap on data.

The vertical tilt angles agree with Planck dust polarization results, reinforcing their physical significance.

When the path length is increased as appropriate for extragalactic RM maps (Hutschenreuter et al.), the model’s M1=0 locus shifts and aligns with the Ordog et al. diagonal, showing the model’s adaptability across path length (Figure 9).

Figure 9: (a) Model-predicted RM map for Hutschenreuter; (b) model overlay on data, with RM=0 line matching the Ordog diagonal.

Implications for Large-Scale Galactic Field Structure

The model establishes that a single local, inclined planar reversal, positioned $\sim$0.25–0.55 kpc from the Sun, can account for the Northern $\sin 2\ell$ FD sign geometry observed across both limited and extragalactic sightlines. The Southern hemisphere’s lack of a $\sin 2\ell$ signature — and absence of a corresponding planar crossing for most lines-of-sight — emerges naturally. The inferred large-scale magnetic field topology is one with a local switch in azimuthal field sense above and below a planar interface passing above the Sun.

The findings strongly suggest that the observed Faraday rotation at mid/high latitude is dominated by local field topology, not by superpositions of more distant or halo field components. This locality is consistent with results from cosmological MHD simulations, which also show local reversals can imprint global Faraday structures. The deduced planar geometry diverges from models assuming either infinitely thin shears or numerous arm-by-arm reversals, instead supporting a single, spatially coherent feature controlling FD morphology within 1 kpc. The paper provides detailed analytic formalism that will be directly usable in forward models of Milky Way Faraday rotation and future comparisons to ISM simulations or tomographic Galactic magnetic field reconstructions.

Mechanistically, the reversal is interpretable as the result of either dynamo modes (odd and even parity) or spiral arm–interstellar medium interactions, both shown in simulations to yield local plane-like reversals. The vertical field tilt (positive below, negative above) is in quantitative agreement with the Planck analysis of magnetically aligned dust, suggesting a robust link between the global field and local ISM morphology.

Conclusion

This work delivers a formal three-dimensional framework for describing the large-scale, local magnetic field reversal in the Milky Way, putting the $\sin 2\ell$ Faraday structure on a rigorous geometric footing. The model is tightly constrained by observations in both Faraday and dust channels and matches the well-known extragalactic RM sign transition. The result has strong implications for Galactic magnetism studies, suggesting that much of the observable Faraday sky (outside the inner plane) can be modeled as the superposition of emission and rotation through a locally reversed field, without resort to complex or multi-arm reversals.

Future progress will likely focus on detailed incorporation of non-uniform electron density and turbulent field components, extending the model to spiral curvature, and leveraging higher-resolution FD grids. Practical applications include improved propagation models for precision CMB foregrounds, cosmic ray anisotropy modeling, and deeper understanding of dynamo mode excitation signatures.

The formalism and observational methodology established here will be foundational for the interpretation of forthcoming large-sky Faraday tomography experiments, and for the quantitative modeling of the Milky Way’s magneto-ionic medium.