Canadian Radio Telescope Maps Surprise Magnetic-Field Reversal Cutting Diagonally Through the Milky Way’s Sagittarius Arm, Breaking the Century-Old Smooth-Spiral Galaxy Model

PENTICTON, British Columbia — Astronomers using a purpose-built radio telescope at Canada's Dominion Radio Astrophysical Observatory have produced the most detailed map ever made of the Milky Way's magnetic field — and discovered a previously unknown reversal cutting diagonally through the Sagittarius Arm in which the field's direction flips from clockwise to counterclockwise. The finding, announced this week and published in The Astrophysical Journal and The Astrophysical Journal Supplement Series, could reshape how astrophysicists model the formation of the galaxy itself.

The project, called the Global Magneto-Ionic Medium Survey, or GMIMS, is the first international effort to chart the polarization of radio emission across the entire northern sky at hundreds of frequencies simultaneously. "For nearly a century, our picture of the galactic magnetic field has been a smooth spiral that wraps around the Milky Way the same way the stars and gas do," Dr. Jo-Anne Brown, the University of Calgary astrophysicist who led the survey, said in a statement Tuesday. "What we are seeing now is something much more chaotic — a diagonal slash through one of the major spiral arms where the field flips entirely. Nothing in our existing dynamo models predicts that."

Magnetic fields permeate the interstellar medium at strengths of only a few microgauss — roughly one-millionth the strength of Earth's surface field — but they regulate everything from the formation of stars in dense molecular clouds to the propagation of cosmic rays from supernova remnants. Until the GMIMS data, astronomers had been forced to reconstruct the galactic field from sparse pulsar measurements and the Faraday rotation of extragalactic radio sources, leaving large gaps and contradictions in the picture. The DRAO instrument, a purpose-built array near Penticton, B.C., operated by the National Research Council Canada, allowed the team to image the full northern sky in continuous frequency coverage from 300 megahertz to 1.8 gigahertz, isolating the contribution of the Milky Way's own ionized gas.

The reversal in the Sagittarius Arm, located roughly 5,000 to 13,000 light-years from Earth on the inner side of our own Local Arm, is geometrically inconsistent with the alpha-omega dynamo models that have dominated galactic magnetism research since the 1980s. Those models predict smooth reversals at the boundaries between arms, not diagonally across them. "The most exciting possibility is that we are watching a fossil of an ancient galactic merger or a relic of differential rotation that the dynamo has not yet erased," said co-author Anna Ordog, a postdoctoral researcher at Cal Poly who was lead author on the second paper. "Either way, the textbook picture is wrong."

The GMIMS consortium, which includes astronomers from Canada, Australia, South Africa, the Netherlands and the United States, is now extending the survey to the southern sky using South Africa's HartRAO and Australia's Parkes telescopes. A full all-sky map is expected by 2028. The team has also begun comparing the Milky Way reversal to similar features observed in nearby spiral galaxies M83 and NGC 6946 by the Karl G. Jansky Very Large Array, in hopes of determining whether the Sagittarius flip is a feature unique to our galaxy or a generic property of barred spirals.