Imagine a sunny day at the pool. Looking down at the bottom of the pool
reveals a network of ridges of bright light that is constantly in motion.
These structures, known as optical caustics, are an effect of the sunlight
being focused to a single point as it is refracted by the wavy surface of
the water. The rippling surface causes light to "pile up" in certain regions
at the bottom of the pool instead of filling all the space equally.
What does this have to do with astrophysics? Recently, Michael Bell, Henrik
Junklewitz and Torsten Enßlin have shown that similar features, which they
are calling "Faraday caustics", can be seen in images of polarized radio
emission produced using next generation radio telescopes. Just as the
caustics at the bottom of the pool trace conditions at the surface of the
water, Faraday caustics trace specific properties of magnetic fields in the
universe. In the same way that one might study the properties of the pool's
surface by observing the network of light patterns at the bottom, the
authors propose that Faraday caustics may be very useful for learning about
the distribution of magnetic fields in the universe and to help shed light
on their yet unknown origins.
Magnetic fields can be found everywhere in the cosmos. They are generated by
planets, like the Earth, stars or other celestial objects, and permeate the
vast space of the largest structures in the universe, such as galaxy
clusters. Nevertheless, although we know of their existence, it is often
difficult to measure their exact properties. For those observing radio
waves, an effect known as Faraday rotation can be a good tracer of some of
the magnetic fields' properties. The effect of Faraday rotation is that the
plane of polarization of a radio wave is rotated as it passes through a
magnetized plasma. The amount of rotation depends, among other things, on
the properties of the magnetic field and the observed frequency. Since this
rotation can be calculated from polarization sensitive observations at
different frequencies, Faraday rotation has been a very useful tool for
studying cosmic magnetism.
Astronomers face the problem that the radiation from a single direction
might have been emitted by two different radio sources. The radiation from
each source would have traveled through different magnetic fields and been
rotated by different amounts. How can astronomers distinguish between these
sources? To overcome this problem a new measurement technique was devised in
recent years called "Rotation Measure Synthesis". This technique uses the
same mathematical approach used to analyze the different frequency
components that produce a complicated acoustic signal, like a song. After
measuring polarized radio emission at many different frequencies, we can
reconstruct the 'Faraday spectrum', i.e. separate the polarized emission
into components that are rotated by different amounts due to Faraday
rotation. In this spectrum, Faraday caustics are predicted to leave a
The scientists of the Max Planck Institute have shown that Faraday caustics
are caused by reversals of the magnetic field orientation along a line of
sight. Such reversals are quite common in turbulent astrophysical
environments, therefore Faraday caustics are predicted to appear in many
observations. The behavior and statistics of the Faraday caustics will
reveal structural and statistical properties of cosmic magnetic fields.
The authors show that the new European radio telescope LOFAR, in whose
construction the Max Planck Institute for Astrophysics is participating,
will be ideally suited to observing Faraday caustics. Using LOFAR and other
telescopes, future observations specifically designed to look at Faraday
caustics will greatly improve our understanding of the magnetic fields in
our own and other galaxies, and help to unravel their yet unknown origins.
This is the aim of the German research unit "Magnetisierung des
interstellaren und intergalaktischen Mediums“ (Magnetization of the
interstellar and intergalactic medium) funded by the German Society of
Research (DFG) of which Michael Bell, Henrik Junklewitz and Torsten Ensslin
Michael Bell, Henrik Junklewitz, and Torsten Enßlin
LOFAR Telescope Array
LOFAR in Deutschland
MPA LOFAR Project
Research group about "Magnetisation of Interstellar and Intergalactic Media"
M. R. Bell, H. Junklewitz, T. A. Enßlin,
"Faraday caustics: Singularities in the Faraday spectrum and their utility as probes of magnetic field properties",
submitted to A&A.