Together with Henrik Junklewitz and Torsten Enßlin, I've just submitted a new paper to A&A introducing Faraday caustics to the world. Never heard of them? I don't blame you, we just coined the phrase. So what are they? They are singularities that appear in the Faraday spectrum, i.e. the distribution of polarized intensity as a function of Faraday depth (analogous to rotation measure). These singularities are caused by reversals of the magnetic field direction along the line of sight.
Sounds a pretty obscure, I know, but there are two reasons why they are important to understand. First, the RM Synthesis technique (used to measure Faraday spectra) is becoming very popular. Anyone looking at the polarized emission from diffuse polarized sources, e.g. the synchrotron emission from the interstellar medium in the Milky Way, is bound to find these Faraday caustics in their data. It's important to understand what these features are and where they come from in order to properly understand the results. Furthermore we show how Faraday caustics can be used as a powerful tool for studying the structural and statistical properties of magnetic fields. In this way, observations of Faraday caustics could be used to greatly improve our understanding of both the large and small-scale properties of the magnetic field in the Milky Way, for example.
If you are interested in learning more, our paper entitled "Faraday caustics: Singularities in the Faraday spectrum and their utility as probes of magnetic field properties" is available in pre-print now.