Magnetism is one of the fundamental forces of nature, and it's all around us. There are magnets in your microwaves, fridges, and headphones. Powerful magnets are used to perform some medical procedures like MRIs, and the fastest trains in the world use huge magnets on the bottom side of the train and on the tracks to make the trains float! These magnetic forces - the same ones that keep your favourite pictures stuck to your fridge - also surround the Earth, sun and every galaxy. Although these forces are invisible, we can still detect them.
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When it comes to Earth's magnetic field, one of the simplest ways is with a compass. The Earth is like a huge magnet and like all magnets, it has a north and a south pole. Compasses work by magnetising the rotating pin so it aligns with the magnetic poles of the planet.
Earth's magnetic field is important because it shields us from the sun's radiation. You can see this effect happening if you watch the beautiful northern and southern lights at the Earth's poles. These natural light shows are called aurorae, and they are caused by sprays of energy from the sun colliding with the magnetic field of the Earth.
When it comes to magnetic fields around galaxies, detecting them gets much trickier. We can't travel to other galaxies and use a compass! What we can do is observe them with telescopes. But just like we can't see Earth's magnetic field with our eyes, we can't observe the magnetic fields of galaxies with optical telescopes. We have to find another way.
We can use radio telescopes to allow us to study magnetic fields. When gas particles like electrons get close to a magnetic field line, they start to spiral around it. Magnetic field lines are imaginary lines that astronomers use to describe the direction that particles move in when they're near a magnetic field. As the particles spiral around the field lines, they start moving really fast (close to the speed of light), and they send out special kinds of radio waves called synchrotron radiation. Using radio telescopes like the Australian Square Kilometre Array Pathfinder (ASKAP), astronomers can measure synchrotron radiation and find out about the magnetic field that caused it.
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The Polarisation Sky Survey of the Universe's Magnetism (POSSUM) is a project that will use ASKAP, which is located in remote Western Australia. POSSUM will study magnetic fields in various environments across the Universe, through synchrotron radiation. The unique capabilities of ASKAP will allow astronomers to form a magnetic picture of the Universe in extraordinary detail.
A key component of this is the measurement of Faraday rotation measures. Rotation measures are a property of radio-emitting sources like galaxies, that tell us the strength and direction of the magnetic fields between us and the source. POSSUM will measure 1 million radio sources with rotation measures that astronomers will use to study magnetism in our own Milky Way, in other galaxies, and even in the cosmic web that forms the backbone of the universe.
Astronomers have found that magnetic fields play a crucial role in many processes including star formation, the evolution of galaxies and the structure of black holes. But there are still so many mysteries to be uncovered. How did the universe become magnetic in the first place? How has it evolved to become the magnetic universe that we know today? These are some of the mysteries that the new generation of astronomy surveys (like POSSUM) hope to resolve.
- Sarah Bradbury is a PhD student specialising in cosmic magnetism at Mt Stromlo Observatory, The Australian National University.
