The magnetic field is a major source of pressure in the ISM and controls the flow of charged particles in and out of the Galactic disk. Radio continuum polarization data carry important, and often unique, information about the strength and topology of the large scale Galactic magnetic field, plasma turbulence and the ionized components through the measurement of Faraday rotation. As such, these observations complement the line-of-sight measurements of interstellar scattering. Moreover, additional observational constraints would help motivate further MHD modelling, particularly now that computational power is approaching what is needed to do the complex simulations. Polarization studies could provide insight in the following areas: (1) Disk/halo emission (Parker instabilities, bubbles); (2) Cosmic ray origin and propagation; (3) Particle acceleration via reconnection processes; (4) Origin of the Galactic magnetic field (wound up by differential rotation from a primordial seed field?); (5) The existence of large-scale currents (e.g., those thought to be required by the Galactic center filaments); and (6) Distribution and temperature of ionized gas (103–107 K).
A recent additional motivation for the study of the diffuse polarized emission of disk galaxies is the recognition of its importance for high-redshift galaxies. The microJansky population of radio sources (at a few GHz) appears to be dominated by starforming galaxies at high redshift. The radio luminosity of these galaxies is generated probably by synchrotron emission from their disks. The sources of particles responsible for this emission seem clear (massive star formation, OB stars, SNe and pulsars). However, it is less clear how the magnetic field was formed and how strong it is. Understanding the field in our Galaxy (i.e., a prototypical z = 0 galaxy) is essential to infer correctly the properties of high-z, young disk galaxies.