Measuring the Size of the Vela Pulsar ’ s Radio Emission Region

We describe the expected distribution of intensity for a scintillating source of finite size observed through a scattering medium, including systematic and instrumental effects. We describe measurements of the size of the Vela pulsar, using this technique. 1. Theoretical Background Waves from a pointlike source observed through a scattering medium will suffer random phase changes. If the phase changes are much larger than 1 radian, the observer will receive radiation from many Fresnel zones, and the scattering is said to be " strong ". In this case the electric field at the plane of the observer is the sum of the electric field from many lines of sight, differing random phases (Goodman 1985). The net electric field is the result of a random walk. The electric field is thus drawn from a Gaussian distribution. Its square modulus, the intensity, is drawn from an exponential distribution (Scheuer 1968).