Refractive interstellar scintillation of pulsar intensities at 74 MHz

Results of daily observations of the flux of nine pulsars made with a large phased-array at 74 MHz from January 24, 1989 for 400 d are presented. Modulation indices and time scales are estimated for the variations. The results are only in partial agreement with the variability expected from refractive interstellar scintillation. The observed modulation indices are above the expectations from a simple Kolmogorov model of interstellar plasma turbulence, particularly for the nearby pulsars with only moderate levels of scattering. These results are in reasonable agreement with earlier observations, but disagree with recent measurements at 610 MHz by Kaspi & Stinebring (1992). A tentative explanation is proposed in terms of a Kolmogorov spectral model with an inner scale cutoff, in which there is an inverse relation between inner scale and local strength of turbulence.