Intensity variations of the Crab pulsar and refractive interstellar scintillation

New measurements of the slow amplitude variations of the crab pulsar at 610 Mhz are presented. Both the rms depth of variation and the typical time-scale confirm the interpretation that the variability is not intrinsic but is refractive interstellar scintillation. By combining our results with earlier pulsar measurements, we conclude that the spectrum of the interstellar electron density is not as steep a power-law function of wavenumber as has sometimes been proposed. It is, however, consistent with a Kolmogorov spectrum, modified by an inner scale in the region of 109 m (or with a variety of other models in which the ratio of power at the typical 'diffractive wave number' and 'refractive wavenumber' is somewhat enhanced over that in the Kolmogorov spectrum). The physical significance lies in supporting the notion of interstellar plasma turbulence, with a possible dissipation scale. (author)