Astronomers have mapped the Milky Way’s magnetic field in unprecedented detail, revealing a surprisingly chaotic structure and a striking reversal in the Sagittarius Arm. The findings, published this month in The Astrophysical Journal and The Astrophysical Journal Supplement Series, provide a comprehensive dataset for researchers worldwide and a new model to understand the galaxy’s magnetic evolution.
The perform is led by Dr. Jo-Anne Brown, PhD, a professor in the Department of Physics and Astronomy at the University of Calgary, who emphasizes the fundamental importance of galactic magnetic fields. “Without a magnetic field, the galaxy would collapse in on itself due to gravity,” she said.
The research team utilized a new radio telescope at the Dominion Radio Astrophysical Observatory in British Columbia, a facility operated by the National Research Council Canada. This instrument enabled a detailed scan of the northern sky across multiple radio frequencies, providing a high-resolution view of the magnetic field’s structure. “The broad coverage really lets you get at the details about the magnetic field structure,” explained Dr. Anna Ordog, PhD, lead author of the first study.
The data was collected as part of the Global Magneto-Ionic Medium Survey (GMIMS), an international collaboration dedicated to charting the Milky Way’s magnetic field. Researchers employed a technique called Faraday rotation to trace the field’s arrangement. This phenomenon measures the shift in radio waves as they pass through regions containing electrons and magnetic fields.
“You can consider of it like refraction. A straw in a glass of water looks bent since of how light interacts with matter,” said Rebecca Booth, a PhD candidate working with Brown and lead author of the second study. “Faraday rotation is a similar concept, but it’s electrons and magnetic fields in space interacting with radio waves.” By analyzing subtle changes in radio signals, the team constructed a map of the magnetic field across vast galactic distances.
A particularly noteworthy discovery centers on the Sagittarius Arm, where the magnetic field direction opposes that of the rest of the Milky Way. Brown described the initial surprise: “If you could look at the galaxy from above, the overall magnetic field is going clockwise. But, in the Sagittarius Arm, it’s going counterclockwise. We didn’t understand how the transition occurred. Then one day, Anna brought in some data and I went, ‘O.M.G., the reversal’s diagonal!’”
Building on Ordog’s initial findings, Booth developed a three-dimensional model to explain this diagonal reversal. “My work presents a new three-dimensional model for the magnetic field reversal. From Earth, this would appear as the diagonal that we observe in the data,” Booth explained.
The newly released dataset and model are now available to astronomers globally, offering a foundation for further research into the Milky Way’s magnetic field and its ongoing evolution. Researchers have noted the field is “far more chaotic than expected” according to SciTechDaily.