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We analyse dispersion measure (DM) variations in six years of radioobservations of more than 160 young pulsars, all gamma ray candidates for theFermi gamma ray telescope mostly located close to the Galactic plane. DMs werefit across 256 MHz of bandwidth for observations centred at 1.4 GHz and acrossthree frequencies -- 0.7 GHz, 1.4 GHz, and 3.1 GHz -- where multifrequencyobservations were available. Changes in dispersion measure, dDM/dt, werecalculated using a weighted linear fit across all epochs of available DMs. DMvariations were detected at a 3\sigma level in 11 pulsars, four of which wereabove 5\sigma: PSRs J0835-4510, J0908-4913, J1824-1945, and J1833-0827. We findthat after 28 years of gradual decline, the DM of PSR J0835-4510 is nowincreasing. The magnitude of variations in three of the four (PSRs J0835-4510,J0908-4913, and J1833-0827) are above what models would predict for an ISMdominated by Kolmogorov turbulence. We attribute this excess as likely due tothe pulsar's local environment - the supernova remnants near PSRs J0835-4510and J1833-0827 and the pulsar wind nebula around PSR J0908-4913. Upper limitswere determined for all pulsars without detectable values of dDM/dt, mostlimits were found to lie above the levels of variations predicted by ISMtheory. We find our results to be consistent with scattering estimates from theNE2001 model along these lines of sight.

Dispersion measure variations in a sample of 168 pulsars