The Galactic Magnetic Field

Abstract

Radio astronomical observations of such widely differing types of objects as extra-galactic nonthermal radio emitters, both quasars and galaxies, galactic supernova remnants, pulsars, and neutral hydrogen clouds, as well as observations of the non-thermal galactic background radiation, give information about the magnetic fields in interstellar space. This information is complemented by data on the optical polarization of starlight. Since rigorous models of the galactic magnetic field are difficult to formulate, our discussion is necessarily qualitative.All the various types of observations pertaining to the galactic magnetic field involve the existence of polarization of electromagnetic radiation. The continuum radiation from the so-called galactic “background,” presumably synchrotron emission, is intrinsically linearly polarized. The polarized signal propagating through the interstellar medium, which contains both thermal electrons and magnetic fields, can be decomposed into two oppositely circularly polarized components having different phase velocities. On emergence from the medium the waves can be recombined, but with relative phases which may be different from those which they had on entering the medium. The effect of this is a rotation of the plane of polarization, the amount of rotation varying as the square of the wavelength, and, as will be shown below, it is possible to estimate the magnetic field strength and the density ofthermal electrons in the line of sight by measuring this rotation. Furthermore, by extrapolating measurements at several wavelengths to zero wavelength we can derive the orientation of the magnetic field component transverse to the line of sight in the region in which the radiation originates. This rotation of the plane of polarization, known as Faraday rotation, also affects the polarized emission from pulsars and extra-galactic radio sources. The amount of Faraday rotation is given in terms of a parameter, known as the rotation measure, which is one of the most important tools for studying the galactic magnetic field.