Something strange is happening in the Milky Way’s magnetic field

"Without a magnetic field, the galaxy would collapse in on itself due to gravity," says Brown, a professor in the Department of Physics and Astronomy at the University of Calgary.

"We need to know what the magnetic field of the galaxy looks like now, so we can create accurate models that predict how it will evolve."

New Milky Way Magnetic Field Data and Models

This month, Brown and her colleagues published two new studies in The Astrophysical Journal and The Astrophysical Journal Supplement Series. Together, the papers introduce a complete dataset that astronomers around the world can use, along with a new model designed to improve understanding of how the Milky Way's magnetic field developed over time.

To gather the data, the team relied on a new radio telescope at the Dominion Radio Astrophysical Observatory in B.C., a National Research Council Canada facility. The instrument allowed them to scan the northern sky at multiple radio frequencies, offering a detailed look at the structure of the galaxy's magnetic field.

"The broad coverage really lets you get at the details about the magnetic field structure," says Dr. Anna Ordog, PhD, lead author of the first study.

The result is a high quality, wide ranging dataset collected as part of the Global Magneto-Ionic Medium Survey (GMIMS), an international effort to chart the Milky Way's magnetic field.

Tracking Faraday Rotation Across the Galaxy

The researchers measured a phenomenon known as Faraday rotation to trace the magnetic field. This effect occurs when radio waves pass through regions filled with electrons and magnetic fields, causing the waves to shift.

"You can think of it like refraction. A straw in a glass of water looks bent because of how light interacts with matter," says 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 these subtle changes in radio signals, the team was able to map how the magnetic field is arranged across vast stretches of the galaxy.

A Diagonal Magnetic Reversal in the Sagittarius Arm

In the second study, Booth focused on a striking feature within the Milky Way known as the Sagittarius Arm, where the magnetic field runs in the opposite direction compared to the rest of the galaxy.

"If you could look at the galaxy from above, the overall magnetic field is going clockwise," says Brown. "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 findings, Booth used the newly assembled dataset to construct a three dimensional model explaining this 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 explains.