Temporal variations of pulsar dispersion measures

Abstract

We present observations of dispersion measure (DM) variability in the directions of pulsars PSR 1821-24, PSR 1855+09, PSR 1937+21, 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 exp 13 to 10 exp 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 exp 9 to 10 exp 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.