Published online by Cambridge University Press: 12 April 2016
We report on six months of dual-frequency, precision timing of PSR B1937+21 and the Crab pulsar. Variations in the column density of electrons – interstellar weather – are studied. The column is not unique in the presence of frequency-dependent multipath scattering, and this may explain the non-square law dependence of systematic timing residuals on B1937+21. The electron column density variations of the Crab pulsar are summarized in a structure function that ranges from 1012 cm to 1015 cm. These density fluctuations are located in the thermal shell surrounding the synchrotron nebula.