Pulsar emission patterns seen as evidence for magnetospheric interactions

Abstract

ABSTRACT This paper seeks to understand the origin of the multiple complex patterns present in the emission of many pulsars. Previous attempts have often required the observed signal to be aliased, especially in pulsars exhibiting stationary subpulses or drift-mode changes. However, such mathematics places the observer in an improbably special position. It is therefore proposed that patterns are intrinsic to the magnetosphere and arise through beats between the magnetospheric drift and the time-delayed interaction of widely separated regions of the magnetosphere. The beat equation is modelled geometrically by two turning carousels of ‘footprints’ linked by a time-delay of approximately a pulse period, with the carousels adopting different speeds according to the number of footprints. Comparison with observed drift-changing modes suggests that footprints are separated at a minimum distance of about 95m on polar caps wider than those conventionally defined by the last closed fieldline touching the light cylinder. Applying this picture to a pulsar lifetime defines three stages: (1) young pulsars whose magnetosphere drift is only slightly slower than the pulsar but generating footprints rotating slowly in space; (2) a Nyquist stage where the magnetosphere and footprints rotate in tandem, observed as on-off pulses; and (3) older pulsars with relatively wide polar caps on which footprints are near-stationary. Explanations for chaotic mode-switching, nulling, and pulsar death are offered in the same context. A physical interpretation of the model is kept to a minimum, although it clearly suggests that pulsar emission is driven by magnetospheric rather than polar cap effects.