Refractive interstellar scintillation effects in pulsar observations

Propagation of radio waves through the interstellar medium (ISM) produces fluctuations in the observed intensity of a source because of the scattering caused by the irregular electron density distribution in the interstellar plasma. Under conditions of strong scintillations, the intensity fluctuations produced exhibit two predominant scales--the small scale (diffractive) fluctuations which are modulated by the large scale (refractive) fluctuations. Several aspects of the effect of refractive interstellar scintillation (RISS) in pulsar data are investigated. The results help to constrain the power spectrum of electron density fluctuations in the ISM.; A first order modelling of the refractive phase fluctuations as a linear phase front in a thin screen geometry is used to describe the modulation of the decorrelation widths in time and frequency and the random change of "drift" of the intensity features with epoch. This model is tested using some actual pulsar dynamic spectra data at 408 MHz. It is found to provide a fairly good first order description. The refractive shift values inferred from the data agree quite well with predictions. The model, however, fails to explain the fluctuation of the temporal decorrelation width. More sophisticated models for the refractive phase fluctuations--which can explain these second order effects--are discussed. Periodic intensity modulations observed in pulsar dynamic spectra can not be explained by the above models and are described in terms of interference between different images of the source. Their rather rare occurrence in our data set is consistent with a moderate (10{dollar}sp7 le{dollar} s{dollar}sb{lcub}rm i{rcub} le{dollar} 10{dollar}sp{lcub}10{rcub}{dollar}) inner scale.; Some of these aspects of RISS are tested using numerical simulations of the scattering from a thin screen, for moderate ({dollar}approx{dollar}20) values of strength of scattering (U). The refractive models are not found to work as well as for the real data. For inner scale values larger than the diffractive scale, the concepts of sub-images and caustics are found to provide a valid description of the periodic intensity modulation patterns. This allows investigation of the effect of varying inner scale on the nature of periodic patterns. The simulation results show enhanced frequency of occurrence of such patterns with increasing inner scale and/or decreasing U.; Accurate measurements of the modulation index and time scale of the refractive process can help discriminate between different possible values for the inner scale. Long term fluctuations of nine pulsars were observed far fourteen months at 74 MHz using the Fallbrook antenna. The inferred modulation indices (or lower bounds thereof) for many of these pulsars are found to be higher than those predicted by the simple Kolmogorov spectrum, supporting an inner scale of the order of 10{dollar}sp7-10sp8{dollar} m.