Observations of Nonradial Pulsations in Radio Pulsars

We introduce a model for pulsars in which nonradial oscillations of high spherical degree (l) aligned to the magnetic axis of a spinning neutron star reproduce the morphological features of pulsar beams. In our model, rotation of the pulsar carries a pattern of pulsation nodes underneath our sight line, reproducing the longitude-stationary structure seen in average pulse profiles, while the associated timelike oscillations reproduce "drifting subpulses"—features that change their longitude between successive pulsar spins. We show that the presence of nodal lines can account for observed 180° phase jumps in drifting subpulses and their otherwise poor phase stability, even if the timelike oscillations are strictly periodic. Our model can also account for the "mode changes" and "nulls" observed in some pulsars as quasi-periodic changes between pulsation modes of different l or radial overtone n, analogous to pulsation mode changes observed in oscillating white dwarf stars. We discuss other definitive and testable requirements of our model and show that they are qualitatively supported by existing data. While reserving judgment until the completion of quantitative tests, we are inspired enough by the existing observational support for our model to speculate about the excitation mechanism of the nonradial pulsations, the physics we can learn from them, and their relationship to the period evolution of pulsars.