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

Rebecca A. Booth, Anna Ordog, Jo-Anne Brown, T. L. Landecker, Alex S. Hill, Jennifer L. West, Minjie Lei, S. E. Clark, Andrea Bracco, John M. Dickey, Ettore Carretti

TL;DR

This study tests a planar reversal model for the local Galactic magnetic field using DRAGONS GMIMS data (500–1030 MHz) and complementary STAPS observations, augmented by 3D dust maps and RM data. By computing the first FD moment M1 and analyzing ell-φ representations, the authors show that Northern sin2ℓ and Southern sinℓ patterns can arise from a single local planar reversal located at about 0.25–0.55 kpc, tilted toward the Northern hemisphere and inclined relative to the Galactic plane. The model reproduces the observed large-scale patterns across all-sky and latitude-binned DRAGONS data and remains consistent when tested against the Hutschenreuter2022 RM map, suggesting the local ISM dominates the observed Faraday structure. These results imply that the local magnetic field geometry, rather than distant halo fields, largely shapes the observed FD sky, with slab-like emission revealed by the ell-φ plots and 3D dust-based distance constraints supporting a nearby origin. The work provides a framework for incorporating three-dimensional ISM structure into Faraday-rotation studies and connects dynamical processes (e.g., dynamos and spiral-arm interactions) to the observed planar reversal.

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.

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

TL;DR

This study tests a planar reversal model for the local Galactic magnetic field using DRAGONS GMIMS data (500–1030 MHz) and complementary STAPS observations, augmented by 3D dust maps and RM data. By computing the first FD moment M1 and analyzing ell-φ representations, the authors show that Northern sin2ℓ and Southern sinℓ patterns can arise from a single local planar reversal located at about 0.25–0.55 kpc, tilted toward the Northern hemisphere and inclined relative to the Galactic plane. The model reproduces the observed large-scale patterns across all-sky and latitude-binned DRAGONS data and remains consistent when tested against the Hutschenreuter2022 RM map, suggesting the local ISM dominates the observed Faraday structure. These results imply that the local magnetic field geometry, rather than distant halo fields, largely shapes the observed FD sky, with slab-like emission revealed by the ell-φ plots and 3D dust-based distance constraints supporting a nearby origin. The work provides a framework for incorporating three-dimensional ISM structure into Faraday-rotation studies and connects dynamical processes (e.g., dynamos and spiral-arm interactions) to the observed planar reversal.

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° < < 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 (, 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.

Paper Structure

This paper contains 24 sections, 18 equations, 15 figures, 3 tables.

Table of Contents

  1. Introduction
  2. Data
  3. DRAGONS
  4. The DRAGONS Faraday depth cube
  5. STAPS
  6. Supporting datasets
  7. Edenhofer et al. 2024 dust map
  8. Hutschenreuter et al. 2022 rotation measure map
  9. The first moment of the DRAGONS and STAPS Faraday depth cubes
  10. The DRAGONS polarization horizon
  11. Analysis of Observations
  12. The Southern $\sin\ell$ and Northern $\sin 2\ell$ patterns
  13. The distance to the $\sin2\ell$ pattern in DRAGONS
  14. A model for the reversal in the local large-scale field
  15. The planar reversal model
  16. ...and 9 more sections

Figures (15)

  • Figure 1: Key DRAGONS parameters. (a) and (b) The DRAGONS frequency and $\lambda^2$ coverage respectively. White lines indicate the 302 frequency channels included in the survey. The dark gaps are due to channels omitted as a result of radio frequency interference. (c) The DRAGONS rotation measure spread function.
  • Figure 2: DRAGONS and STAPS FD moment 1 combined. The STAPS data were convolved to the DRAGONS spatial resolution of 2.45$^\circ$. The dashed line indicates the boundary between the two datasets. We have masked out the Galactic plane, within $|b| < 5^\circ$, where instrumental effects make the data unreliable.
  • Figure 3: Comparison of STAPS M1 (vertical axis) to DRAGONS M1 (horizontal axis) in the region where the two surveys overlap. The red line is the 1:1 line where STAPS M1 $=$ DRAGONS M1.
  • Figure 4: (a) The peaks of the $\sin\ell$ and $\sin 2\ell$ patterns marked with $+$ and $-$ signs plotted on the Hutschenreuter2022 map. (b) The same $+$ and $-$ peaks marked on the combined DRAGONS and STAPS moment 1 map.
  • Figure 5: DRAGONS (blue) and STAPS (orange) moment 1 values plotted along lines of constant latitude. Sinusoids fitted to the combined DRAGONS and STAPS moment 1 map (Figure \ref{['fig:STAPS_M1']}) along the corresponding latitudes are plotted in red.
  • ...and 10 more figures