What we learn in this chapter

Radio waves traversing the interstellar medium (ISM) reveal a great deal about the medium because the waves are modified by plasmas and magnetic fields during their transit through the Galaxy. Dispersion is the variation with frequency of the group velocity of a radiation pulse. The measured spread of arrival times is directly related to the dispersion measure (DM), which is the integral of the electron density ne along the line of sight.

Faraday rotation is the rotational displacement of the electric vector of linearly polarized (LP) radiation about the propagation direction. The measured quantity, the rotation measure (RM), is the integral along the propagation path of the product neBz, where Bz is the component of the galactic magnetic field B parallel to the propagation direction.

The relations between the interstellar quantities and these two phenomena follow from Maxwell's equations as they apply to a dilute plasma. Their solution leads to a frequency-dependent phase velocity in terms of the dielectric constant of the medium. The square root of the latter is the frequency-dependent index of refraction. This in turn leads to the dispersion relation – a relation between the index of refraction, frequency, and wavelength. […]