Five years of pulsar flux density monitoring: Refractive scintillation andthe interstellar medium

We have monitored the radio flux density of 21 pulsars on a daily basis for five years. The 610 MHz flux density time series for these pulsars range f rom nearly constant for the most distant and heavily scattered pulsars to r apidly varying, saturated time series for more nearby pulsars. The measured stability of the flux density from the most distant pulsars (variations le ss than 5%) implies that the average radio emission from pulsars, before it has been affected by propagation through the interstellar medium, is const ant in strength on timescales of a few hours to several years. The modulati on index of the flux density variations never exceeds 0.5, ruling out a den sity inhomogeneity spectrum with a steep power-law exponent (beta > 4). The flux density variations for 15 of the pulsars are consistent with a Kolmog orov turbulence spectrum over a range of more than 5 orders of magnitude in scattering strength, with no detectable presence of an inner scale. For th ese lines of sight we constrain the inhomogeneity slope to be in the range 3.5 less than or equal to beta less than or equal to 3.7, which brackets th e Kolmogorov value of beta = 3.67, The flux density variations are greater than predicted by this model for six pulsars-including the Crab and Vela-bu t this group is consistent with a Kolmogorov spectrum and an inner scale of approximate to 10(10) cm. The lines of sight to three of the other pulsars in this group pass through H II regions around young, hot stars. For six p ulsars we have found a change in the slope of the intensity structure funct ion, which could be connected with a change in the slope of the inhomogenei ty power spectrum at a scale of approximate to 10(10) cm.