Abstract
Recent observations at radio frequencies revealed that pulsars exhibit variations in emission properties of time-dependent nature. Phenomena such as drifting subpulses of multiple drift modes, Quasi-periodic switching in emission between on and off states, and the changes in spin-down rate all presents difficulties to the traditional understanding of pulsar emission. Furthermore, studies on the timing noises from radio pulsars suggest that these variations may all be the result of changes in magnetospheric configurations. This also implies the universality of the phenomenon in pulsars. These unusual evolutionary features and complicated characteristics strongly suggest that multiple quasi-stable rotation states are allowed in the magnetospheres and that pulsars can jump abruptly between different states, as oppose to the conventional understanding of single magnetospheric configuration for each pulsars.
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