A team of astronomers led by Dr. Anna Ordog at the University of Calgary has identified a magnetic-field reversal slicing diagonally through the Milky Way’s disk near the Sagittarius Arm, passing just above our solar system. The finding, described as a “magnetic flip” and “diagonal reversal,” challenges conventional models of how magnetic fields behave in spiral galaxies. Published in The Astrophysical Journal, the research draws on an extensive radio survey of the northern sky and offers the first three-dimensional geometry of this local distortion.
A Magnetic Flip Hiding in Plain Sight
The Milky Way’s magnetic field acts as an invisible scaffold, shaping how cosmic rays travel, where stars form, and how gas and dust distribute themselves across the galaxy’s spiral arms. For decades, astronomers assumed this field ran more or less parallel to the galactic plane, with reversals occurring at predictable boundaries between spiral arms. The new research upends that picture. According to a three-dimensional model developed by Ordog and collaborators, the reversal is not neatly aligned with the disk. Instead, it cuts diagonally through the galactic plane as a flat surface, or “reversal plane,” at an estimated distance of roughly 0.2 kiloparsecs from the Sun.
That proximity matters. At about 650 light-years away, this is not some distant anomaly on the far side of the galaxy. It sits in our local interstellar neighborhood, which means its effects on cosmic-ray propagation and the polarization of background radio sources are measurable from Earth. The diagonal tilt also means the reversal passes above the Sun rather than running alongside it, a geometry that existing two-dimensional models could not capture. Dr. Ordog, the lead author, emphasized how the survey’s scope enabled this result, noting that the broad sky coverage “really lets you get at the details about the magnetic field structure,” especially in the region of the Sagittarius Arm.
How Faraday Rotation Exposed the Twist
The discovery rests on a technique called Faraday rotation, which measures how a magnetic field along a line of sight rotates the polarization angle of radio waves passing through it. By mapping these rotations across the sky, astronomers can reconstruct the three-dimensional structure of the intervening magnetic field. The data came from the GMIMS-DRAGONS survey, a dedicated Faraday depth survey of the northern sky conducted with the DRAO 15-meter telescope and covering frequencies from 350 to 1030 MHz. That broad frequency range is what allowed the team to resolve fine details in the Faraday depth spectrum that narrower surveys would miss, turning subtle polarization twists into a full 3D map of the local field.
A large fraction of the survey’s sight lines showed complex Faraday depth structures, meaning the magnetic field along those paths was not simple or uniform. When the team modeled these patterns in three dimensions, the best fit was not a smooth, disk-parallel field but a field that flips direction across a tilted plane cutting through the Sagittarius Arm region. The peer-reviewed version of the GMIMS-DRAGONS analysis, published in The Astrophysical Journal Supplement Series and accessible via its digital object identifier, includes the full FITS data products and rotation-measure cubes. That level of transparency means other research groups can independently verify the geometry, test competing models of the reversal, and cross-match the Faraday features with other tracers of interstellar structure.
Why Existing Models Missed It
Previous maps of the Milky Way’s magnetic field relied heavily on two-dimensional projections, which compress all the structure along a line of sight into a single average value. That approach works well for detecting large-scale patterns, such as the general clockwise or counterclockwise direction of the field in different spiral arms, but it washes out diagonal features. The reversal identified by Ordog’s team only becomes visible when the data are analyzed in three dimensions, with depth information extracted from the frequency-dependent behavior of Faraday rotation. In this framework, each frequency band probes a different effective depth, allowing the researchers to separate overlapping structures and isolate the plane where the field actually flips direction.
There is also a question of scale and environment. Most galactic magnetic-field models treat reversals as occurring at arm-to-interarm boundaries, where gas density changes sharply and large-scale dynamo processes can switch sign. A diagonal reversal that slices through a spiral arm rather than running between arms does not fit that template. One possibility, which the published work notes as speculative, is that a localized turbulent event, such as the expanding shell of a past supernova, could have distorted the field into its current geometry. Testing that idea would require correlating the reversal’s position with catalogs of known supernova remnants and bubbles in the Sagittarius Arm, a project that future high-resolution Faraday surveys and complementary optical and X-ray observations could tackle in a more targeted way.
Broader Context: Magnetic Fields Across the Galaxy
This local reversal is not the only recent surprise involving the Milky Way’s magnetic architecture. Observations near the galactic center, for example, have revealed strongly ordered fields in regions once thought to be dominated by turbulence. Using polarized light from hot gas swirling around Sagittarius A*, the Event Horizon Telescope collaboration has shown that magnetic field lines there can form coherent, spiral-like structures rather than the chaotic tangles many theorists expected. As one researcher involved in that effort explained, by imaging polarized emission from material near the black hole, astronomers can directly infer both the orientation and relative strength of the central field, providing a striking contrast to the more diffuse magnetism in the galactic disk.
Taken together, these findings suggest that the Milky Way’s magnetic field is far more structured, and more variable, than standard models predict. The galactic center appears to host tightly wound, helical fields, while the disk contains a diagonal reversal that cuts across a spiral arm instead of simply tracing it. In between, large-scale surveys of synchrotron emission and starlight polarization reveal filaments, loops, and bubbles that further complicate the picture. The newly mapped reversal near the Sagittarius Arm adds an important local piece to this puzzle, highlighting how features only a few hundred light-years away can still challenge global theories of galactic magnetism.
Open Data, Community Tools, and What Comes Next
Behind the scenes, the infrastructure that enabled this discovery is as important as the result itself. The 3D modeling work and its companion survey papers build on open-access preprints and data releases hosted by platforms like arXiv’s repository, which has become a central hub for sharing astrophysics research before and after journal publication. The GMIMS-DRAGONS team’s decision to release full-resolution Faraday depth cubes and processing details ensures that other astronomers can reanalyze the same sky region, refine the geometry of the reversal, or apply alternative statistical techniques without starting from scratch.
That ecosystem depends on a combination of institutional backing and community support. Organizations that help maintain services such as arXiv rely on a network of member institutions and research libraries that underwrite operating costs, as well as individual users who choose to contribute financially. In turn, these platforms provide documentation, submission guidelines, and technical assistance through resources like the arXiv help pages, lowering the barrier for teams to share complex data products such as Faraday spectra and rotation-measure catalogs. For studies of the Milky Way’s magnetic field, this means that each new survey (whether radio, optical, or X-ray) can be rapidly combined with existing public data, accelerating the process of turning local anomalies like the Sagittarius Arm reversal into a coherent, galaxy-wide magnetic map.
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*This article was researched with the help of AI, with human editors creating the final content.
