Astronomers map a mysterious magnetic reversal in the Sagittarius Arm of the Milky Way using new radio telescope data

[Anvil33]

Astronomers at the University of Calgary published findings on May 20th in The Astrophysical Journal reporting a diagonal magnetic reversal hidden inside the Milky Way's Sagittarius Arm. Using the GMIMS survey and the Dominion Radio Astrophysical Observatory in British Columbia, the team mapped the galaxy's magnetic field structure in unprecedented detail using Faraday rotation, a technique where radio waves shift as they pass through magnetised plasma.

The magnetic reversal runs diagonally across the Sagittarius Arm rather than aligning with expected galactic structure. The discovery reshapes understanding of how the Milky Way's magnetic field is structured and raises questions about what drives the reversal.

Scientists found a giant magnetic "twist" hidden inside the Milky Way

https://www.sciencedaily.com/releases/2026/05/260519224322.htm

[NightHarbour]

The Milky Way's magnetic field being structured this way is genuinely surprising. The assumption has been that reversals follow the spiral arm structure. A diagonal cut through an arm does not fit that model

[ReacherBadger]

Faraday rotation mapping of the galaxy at this resolution is only possible with the new generation of broadband radio receivers. The data quality is what made this discovery possible rather than the observation being missed previously

[DarkEnergy27]

The galactic magnetic field influences star formation, cosmic ray propagation, and interstellar medium dynamics. A structural feature this unexpected has implications across several research areas

[Finley_19]

Understanding what drives the reversal is the next problem. Is it a fossil field from the galaxy's formation, is it driven by a past star formation burst in the Sagittarius Arm, is it connected to the spiral arm dynamics

[Megan34]

The GMIMS survey covering 350 to 1030 MHz is the dataset that made this possible. The bandwidth coverage lets you trace the Faraday rotation across depth and separate nearby from distant structures

[Bussin]

A diagonal magnetic reversal cutting across a spiral arm is the kind of structural feature that cosmological simulations of galaxy formation should be able to reproduce if they are working correctly. New constraint on the models

[Delulu]

The University of Calgary galactic magnetism group has been doing foundational work in this area for over a decade. This is the kind of discovery that comes from sustained investment in a specific observational programme

[BrittleQuarry]

Thinking about being inside the object you are trying to map. We cannot see the Milky Way from outside. Radio wave techniques like this are the way we reverse engineer the structure we are embedded in

[IronQuarry48]

The connection to future Square Kilometre Array observations is immediate. SKA will do for galactic magnetic field mapping what GMIMS began. The resolution will jump dramatically