We have measured variations in scattering time scales in the Crab Pulsar over a 30-year period , using observations made at 610 MHz with the 42-ft telescope at Jodrell Bank Observatory .
Over more recent years , where regular Lovell Telescope observations at frequencies around 1400 MHz were available , we have also determined the dispersion measure variations , after disentangling the scattering delay from the dispersive delay .
We demonstrate a relationship between scattering and dispersion measure variations , with a correlation coefficient of 0.56 \pm 0.01 .
The short time scales over which these quantities vary , the size of the variations , and the close correlation between scattering and dispersion measure all suggest that the effects are due to discrete structures within the Crab Nebula , with size scales of \sim 6 AU ( corresponding to an angular size of \sim 2 mas at an assumed distance of 2200 pc ) .
We mitigate the effects of scattering on the observed pulse shape by using the measured scattering information to modify the template used for generating the pulse arrival times , thus improving the precision to which the pulsar can be timed .
We test this on timing data taken during periods of high scattering , and obtain a factor of two improvement in the root mean square of the timing residuals .