Quasi-periodic scintillation patterns of the pulsars PSR 1133+16 and PSR 1642-03

A new series of interstellar scintillation measurements of the pulsars PSR 1133+16 and PSR 1642-03 at 408 and 1426 MHz made in 1982 and 1983 is presented. Quasi-periodic patterns are seen in the dynamic spectra of pulsar scintillation which change on time-scales ranging from one day to several months, but the coherence of these periodic modulations is maintained only over small portions of the time-frequency plane. These effects can be explained in terms of a theoretical model combining random diffraction from a number of small-scale electron density fluctuations in the interstellar medium with systematic refraction caused by larger irregularities. (author)