This is the Milky Way’s Magnetic Field

The Milky Way galaxy has its own magnetic field. It’s extremely weak compared to Earth’s; thousands of times weaker, in fact. But astronomers want to know more about it because of what it can tell us about star formation, cosmic rays, and a host of other astrophysical processes.

A team of astronomers from Curtin University in Australia, and CSIRO (Commonwealth Scientific and Industrial Research Organization) have been studying the Milky Way’s magnetic field, and they’ve published the most comprehensive catalogue of measurements of the Milky Way’s magnetic field in 3D.

The paper is titled “Low-frequency Faraday rotation measures towards pulsars using LOFAR: probing the 3D Galactic halo magnetic field.” It was published in Monthly Notices of the Royal Astronomical Society in April 2019. The lead author is Dr. Charlotte Sobey, a university associate at Curtin University. The team includes scientists from Canada, Europe, and South Africa.

The team worked with LOFAR, or the Low-Frequency Array, a European radio telescope. LOFAR works in radio frequencies below 250 MHz and consists of many antennae spread over a 1500 km area in Europe, with its core in the Netherlands.

<Click to Enlarge> LOFAR sites are spread around Europe, with the concentrated central core in the Netherlands. Image Credit: LOFAR

The team assembled the largest catalogue to date of magnetic field strengths and directions towards pulsars. With that data in hand, they were able to estimate the Milky Way’s decreasing field strength with distance from the plane of the galaxy, where the spiral arms are.

In a press release, lead author Sobey said “We used pulsars to efficiently probe the Galaxy’s magnetic field in 3-D. Pulsars are distributed throughout the Milky Way, and the intervening material in the Galaxy affects their radio-wave emission.”

Free electrons and the magnetic field in our Galaxy between the pulsar and us affect the radio waves emitted by the pulsars. In an email interview with Dr. Sobey she told us, “Although these effects need to be corrected in order to study the pulsars’ signals, they are really useful for providing information about our Galaxy that would not be possible to obtain otherwise.”

An illustration of a pulsar. Pulsars emit electromagnetic energy along the magnetic axis. Image Credit: NASA/Goddard Space Flight Center Conceptual Image Lab

As the pulsar’s radio waves travel through the galaxy, they’re subject to an effect called dispersion, due to intervening free electrons. This means that higher frequency radio waves arrive sooner than lower frequency waves. Data from LOFAR allows astronomers to measure this difference, called the “dispersion measure” or DM. DM tells astronomers how many free electrons are in between us and the pulsar. If the DM is higher, that means either the pulsar is farther away, or the interstellar medium is denser.

That’s just one of the factors in the measurement of the Milky Way’s magnetic field. The other involves electron density and the magnetic field of the interstellar medium.

Pulsar emissions are often polarized, and when polarized light travels through a plasma with a magnetic field, the plane of rotation rotates. That’s called Faraday Rotation or the Faraday Effect. Radio telescopes can measure that rotation, and it’s called the Faraday Rotation Measure (RM). According to Dr. Sobey, “This tells us the number of free electrons and the strength of the magnetic field parallel to the line of sight, as well as the net direction. The larger the absolute RM means more electrons and/or greater field strengths, due to larger distances or towards the plane of the Galaxy.”

With that data in hand, the researchers then estimated the average magnetic field strength of the Milky Way towards each pulsar in the catalogue, by dividing the Rotation Measure by the Dispersion Measure. And that’s how they created the map. Each single pulsar measurement is one point on the map. As Dr. Sobey told Universe Today, “Obtaining these measurements for large numbers of pulsars (which have distance measurements or estimates) allows us to reconstruct a map of the structure of the Galactic electron density and magnetic field in 3-D.”

A representation of how our Galaxy would look in the sky if we could see magnetic fields. The plane of the Galaxy runs horizontally through the middle, and the Galactic centre direction is the middle of the map. Red–pink colours show increasing Galactic magnetic field strengths where the direction is pointing towards the Earth. Blue–purple colours show increasing Galactic magnetic field strengths where the direction is pointing away from the Earth. The background shows the signal reconstructed using sources outside our Galaxy. The points show the current measurements for pulsars. The squares show the measurements from this work using LOFAR pulsar observations. Image Credit: Sobey et al, 2019.

So what good does it do to have a map of the Milky Way’s magnetic structure in 3D?

The galaxy’s magnetic field affects all kinds of astrophysical processes across different strength and distance scales.

The magnetic field shapes the path that cosmic rays follow. So when astronomers are studying a distant source of cosmic rays, like an active galactic nucleus (AGN), knowing the strength of the magnetic field can help them understand their object of study.

The galaxy’s magnetic field also plays a role in star formation. Though the effect is not fully understood, the strength of a magnetic field may affect molecular clouds. As Dr. Sobey told UT, “At smaller scales (on the order of parsecs), magnetic fields play a role in star formation, with too weak or strong a field in a molecular cloud possibly inhibiting the collapse of a cloud into a stellar system.”

Dr. Sobey chilling in a telescope. Image Credit: CSIRO

This new catalogue is based on observations of 137 pulsars in the northern sky. The authors say that their catalogue “improves the precision of existing RM measurements on average by a factor of 20…” They also say “Overall, our initial low-frequency catalogue provides valuable information about the 3D structure of the Galactic magnetic field.”  

But Dr. Sobey isn’t finished mapping the Milky Way’s magnetic field strength yet. She’s now using Australia’s Murchison Widefield Array to map the magnetic field in the southern sky. And both of these mapping endeavours are leading up to something better.

Artist’s impression of the 5km diameter central core of Square Kilometre Array (SKA) antennas. Image Credit: SPDO/TDP/DRAO/Swinburne Astronomy Productions – SKA Project Development Office and Swinburne Astronomy Productions

The world’s largest radio telescope is now in the planning phase. It’s called the Square Kilometer Array (SKA) and it will be built in both Australia and South Africa. Its receiving stations will extend out to 3,000 kilometers (1900 miles) from its central core. Its massive size and distance between receivers will give us our highest resolution images in all of astronomy.

In a CSIRO blog post, Dr. Sobey said “My work in the future will focus on building towards doing science with the SKA telescope, which is currently entering the final stages of the planning phase. One long-term goal for SKA science is to revolutionize our understanding of our galaxy, including producing a detailed map of our galaxy’s structure (which is difficult because we’re located inside it!), particularly its magnetic field.”

The Milky Way’s magnetic field will have nowhere to hide.

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Evan Gough

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