Refractive interstellar scintillations of pulsars

A theory for refractive scintillations of pulsars is developed for two type s of spectra for electron-density fluctuations in the interstellar medium: power-law and piecewise power-law with a break. Expressions for the refract ive-scintillation parameters are obtained for the cases of a uniform distri bution of inhomogeneities along the line of sight, a phase screen, and a me dium localized near the source or the observer. Comparison of the theory an d five-year observations of flux variations for 21 pulsars at 610 MHz shows that the interstellar medium is highly nonuniform at dispersion measures D M > 30 pc/cm(3). Variations of scintillation parameters are determined main ly by electron-density variations, rather than the distance to the source. We present a way to estimate the location and thickness of the effective la yer of turbulent medium that gives rise to the observed scintillations. Sci ntillation parameters are estimated for the observed pulsars. Two types of spectra are realized in the interstellar medium: pure power-law and piecewi se power-law with a break. The spectral break corresponds to the characteri stic scale l similar or equal to 3 x 10(10) cm. Pure power-law spectra are realized when there is a diffuse medium filling nearly uniformly all of spa ce along the line of sight, while spectra with a break are typical when the re is a medium with compact features localized near the source.