TEMPORAL VARIATIONS OF PULSAR DISPERSION MEASURES

We present observations of dispersion measure (DM) variability in the directions of pulsars PSR 1821 - 24, PSR 1855 + 09, PSR 1937 + 2 1, and PSR 1951 + 32, and relate these results to previously reported work. Temporal variations of pulsar dispersion measures provide a direct sample of the integrated spectrum of electron density fluctuations in the intervening plasma. The spectrum is sampled over a range of length scales from 10(13) to 10(15) cm owing to the relative peculiar motion of the Sun and the pulsar with respect to their local standards of rest. The amplitude of the variations over a fixed time interval appears to depend on the square root of the distance. This dependence is flatter than expected based on studies of diffractive scattering effects that are produced by density fluctuations on scales from 10(9) to 10(11) cm. This difference in statistics suggests a distinction in the physical properties, such as location or driving mechanism, of the fluctuations on the two scales. The square root dependence of the DM variations can be attributed to the random walk growth of the electron column density fluctuations through many independent cells. Observations over a wider range of dispersion measure are required to place the dependence on firmer statistical grounds. We interpret the cells as wedges of electron density excess, or deficit. Pulsars in supernova remnants and pulsar wind nebulae show enhanced scattering in comparison with isolated pulsars with similar electron column densities-a conclusion which has been assumed true, but which has lacked strong experimental evidence.