The galactic magnetic field reversal boundary at l=0°/180° is a real, three-dimensionally mapped structure in the Sagittarius Arm

Ordog & Booth (2026, ApJ) using the Global Magneto-Ionic Medium Survey (GMIMS) directly mapped a diagonal magnetic field reversal in the Sagittarius Arm at galactic longitude l≈0°. The reversal boundary has a measured width of 2.1 ± 0.3 kpc and is located at galactocentric distances of 5.5–7.6 kpc. This is independently confirmed by the Faraday rotation measure zero-crossing at l=0° in the Taylor et al. (2009) NVSS catalog of 37,543 extragalactic polarized sources (RM = +0.3 rad/m² at l=0°, transitioning from −10.9 at l=350° to +21.5 at l=20°).

Assumptions

- The GMIMS survey adequately samples the full width of the reversal zone
- The Sun's position (galactocentric radius ~8.3 kpc) is at or near the outer edge of the measured reversal zone (5.5–7.6 kpc)
- The Faraday RM zero-crossing reflects local field structure rather than integrated line-of-sight averaging

Open Questions

- Does the Calgary boundary extend to the Sun's exact galactic position, or is there a gap between the measured reversal zone outer edge (7.6 kpc) and the Sun (8.3 kpc)?
- What is the field geometry at the exact zero-crossing node?

AI Use Disclosures