Before describing the theory of something (not of everything) it is highly desirable, in natural sciences, to understand in some detail the empirical evidence of the subject under discussion. While technical accounts of the various experimental techniques exist already [12, 13, 14, 15, 16, 17, 18] it seems useful, in the present context, to give an account of the experimental situation by emphasizing the physical principles of the various measurements. There are valuable books dealing directly with large-scale magnetism [19, 20, 21]. Furthermore excellent reviews of the morphological features of large scale magnetism can be found in [14, 15, 16, 17]. Even if the present review will not be concerned with planetary magnetic fields, analogies with physical situations arising closer to the earth are often useful and, along this perspective, magnetic field observations in the solar system have been recently reviewed .
Magnetic fields observed in the Universe have a homogeneous (or uniform) component and a non-uniform component. It is then useful, for the purposes of this Section, to write, in a schematic notation, that
where is the uniform component and B the non-uniform component. Both components are phenomenologically very relevant. Different experimental techniques can probe different components of large-scale magnetic fields (i.e. either the total magnetic field or its homogeneous component). Another important distinction is between B and B||: there are measurements (like synchrotron emission) which are sensitive to B i.e. the transverse component of the magnetic field; there are other measurements (like the Faraday Rotation measure or the Zeeman splitting of spectral lines) which are sensitive to B|| i.e. the magnetic field along the line of sight.