Chapter 4

In Chapters 1 and 2 we saw that the pulsar signal is affected in a number of ways (frequency dispersion, Faraday rotation, scintillation and scattering) as it propagates through the interstellar medium (ISM). This chapter presents the basic theoretical background in those areas necessary for planning and interpreting observations. We begin with a discussion of dispersion and Faraday rotation which can be understood by propagation through a homogeneous medium. Then we move on to describe scattering and scintillation effects caused by propagation through a more realistic model of the ISM which includes turbulence on a variety of length scales. As essentially perfect point sources, pulsars show scintillation effects that are absent in extended radio sources for the same reason that `stars twinkle, and planets do not'. This has a number of important observational consequences that we discuss in detail.