Everybody knows that the Earth is magnetic. The Earth’s magnetic field acts as a shield which protects us against energetic solar and interstellar particles; the Earth’s magnetism is vital for navigation, both for humans and for many other species too.
It has been well-established that stars, planets, galaxies and even diffuse interstellar gas are all also magnetic. This cosmic magnetism cannot be ascribed to permanent magnets like the ones which come in a science kit, but to motions of huge, thin clouds of gas which are electrically charged.
Cosmic magnetism spans an enormous range in its strength, varying by a factor of a hundred billion billion between the weak magnetic fields in interstellar space and the extreme magnetism found on the surface of collapsed stars. Because these cosmic magnetic fields are all-pervasive, they play a vital role in controlling how celestial sources form, age and evolve.
The challenge in studying cosmic magnetism is that while stars and galaxies can be seen directly by the light they emit, magnetic fields are invisible even to the largest telescopes. Astronomers thus need to employ a variety of indirect methods to study magnetism. For example, we know that synchrotron emission is produced when fast-moving electrons are trapped in magnetic fields, like planets caught by the Sun’s gravity. If we see a heavenly body emitting synchrotron emission, we know that this object must be magnetic, and we can use its properties to determine how strong its magnetism is and what direction a compass might point if we were near it.
One problem with this approach is that many magnetic objects in space are not energetic enough to produce detectable synchrotron emission. But we can study their magnetism using a remarkable
effect known as “Faraday rotation”, in which polarized light from a background star is changed when it passes through object in which significant magnetism is present. The change is subtle, involving the angle at which the vibrating light waves are inclined, but can be measured with radio telescopes, and can be used to calculate the strength of the magnetic field in the foreground object.
Studying cosmic magnetism in this way is comparatively easy. However, it is often difficult to apply this technique, because only rarely does a random galaxy or gas cloud happen to lie in line with a bright background object, so that we can detect the consequent Faraday rotation and thus measure the magnetic properties of the foreground object. Trying to study cosmic magnetism through this approach is like making an environmental study of a large lake, just by dipping one’s finger in the water at one location! Clearly this approach is never going to give us the big picture.
Since the Square Kilometre Array (SKA) will be so much more sensitive than current telescopes, we can use it to revolutionise the study of magnetic fields in space. If we point the SKA at any part of the sky, we will detect the radio emission from thousands of distant faint galaxies, spread like grains of sand all over the sky. These galaxies will be so closely spaced that we can use the Faraday rotation of their polarized radio emission to make detailed studies of the magnetism from all sorts of foreground objects.
Even if we want to study a relatively small cloud of gas, there will be hundreds of background galaxies whose light shines through it, allowing us to build up a detailed picture of the cloud’s magnetism.
This new technique will allow us to address many important unanswered questions. What is the shape and strength of the magnetic field in our Milky Way, and how does this compare to the magnetism in other galaxies? Is the overall Universe magnetic? If so, has the Universe’s magnetism affected the way in which individual stars and galaxies form? And ultimately, where has all this magnetism come from?
These are all questions we can hope to address with the unique and fascinating capabilities of the SKA. We know that there are magnets everywhere in space. But with the SKA, we will understand what these magnets look like, where they came from, and what role they have played in the evolving Universe.
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