Long-term scintillation studies of pulsars. II. Refractive effects and the spectrum of plasma density fluctuations

Abstract

Refractive scintillation effects in pulsars are powerful techniques for discriminating between different models proposed for the electron density fluctuation spectrum in the interstellar medium. Data from our long-term scintillation study of 18 pulsars in the dispersion measure range 3-35 pc cm^-3 (Paper I) are used to investigate two important observable effects of refractive scintillation, viz., (1) modulations of diffractive scintillation observables and flux density, and (2) drifting bands in dynamic scintillation spectra. Our data provide simultaneous measurements of decorrelation bandwidth, scintillation timescale, flux density, and drift rate of patterns. The observed modulations of the first three are compared with the available theoretical predictions, and constraints are placed on the power spectrum of plasma density fluctuations. The measured modulation indices are found to be larger than predicted by a Kolmogorov form of density spectrum. The properties of the drift rate of patterns along with the diffractive scintillation parameters have been used to estimate independently the slope of the density power spectrum, which is found to be consistent with a Kolmogorov form for several pulsars. The contradictory results from these two independent methods of constraining the electron density spectrum are not reconcilable with the simple theoretical models based on power-law forms of density spectrum. Our observations show anomalous scintillation behavior such as persistent drifting bands for some pulsars. This can be interpreted as an excess power in the low wavenumber range (~10^-12 to 10^-13 m^-1) compared to the Kolmogorov expectations, or the existence of localized density structures. The results from our observations are discussed in combination with those from earlier studies in an attempt to understand the overall nature of the density spectrum. The emerging picture is a Kolmogorov-like spectrum (α≈11/3) in the wavenumber range ~10^-6 m^-1 to ~10^-11 m^-1, which either steepens or has a bump near ~10^-12 to 10^-13 m^-1. The accumulated data also suggest the existence of discrete density structures along some lines of sight. We also discuss the possible implications of our results for the theoretical models.