Timing Observations of 27 Pulsars at the Pushchino Observatory from 1978 to 2012
We present results from timing observations of 27 pulsars made at the Pushchino Observatory over 33.5 yr between 1978 July and 2012 February. We also analyze archival Jet Propulsion Laboratory data of 10 pulsars to extend individual data span to 43.5 yr. We detected a new phenomenon in the timing behavior of two pulsars, B0823+26 and B1929+10, that demonstrates a rapid change of pulsar rotation parameters such that the sign of the second derivative $\ddotν$ is reversed. An analysis of the $\ddotν$ changes showed that this process can be considered as a modulation process in $\ddotν$. We showed that the process of rapidly changing of pulsar rotation parameters represents a new type of rotational irregularity that, together with three other types of rotational irregularities (discrete glitches, slow glitches and quasi-periodic oscillations), forms a large-scale structure of timing noise. These effects are all the cause of the deviation of the timing behavior of most ordinary pulsars from a simple $ν,\,\dotν$ spin-down model. We found that all four types of observed rotational irregularities have evolving nature. Irregularities in pulsar rotation rate pass through three evolutional stages that show that a certain type of rotational irregularity can occur only at a certain stage of pulsar rotation evolution. The age boundaries between different evolutionary stages are indistinct and diffusive. This fact is because different pulsars having similar properties evolve along different paths. The evolutionary scenario of the occurrence of rotational irregularities explains well many of the observed properties of pulsar rotation.