The role of multipolar magnetic fields in pulsar magnetospheres

Abstract

We explore the role of complex multipolar magnetic fields in determining physical processes near the surface of rotation powered pulsars. We model the actual magnetic field as the sum of global dipolar and star-centered multipolar fields. In configurations involving axially symmetric and uniform multipolar fields, 'neutral points' and 'neutral lines' exist close to the stellar surface. Also, the curvature radii of magnetic field lines near the stellar surface can never be smaller than the stellar radius, even for very high order multipoles. Consequently, such configurations are unable to provide an efficient pair creation process above pulsar polar caps, necessary for plasma mechanisms of generation of pulsar radiation. In configurations involving axially symmetric and non-uniform multipoles, the periphery of the pulsar polar cap becomes fragmented into symmetrically distributed narrow sub-regions where curvature radii of complex magnetic field lines are less than the radius of the star. The pair production process is only possible just above these 'favourable' sub-regions. As a result, the pair plasma flow is confined within narrow filaments regularly distributed around the margin of the open magnetic flux tube. Such a magnetic topology allows us to model the system of twenty isolated sub-beams observed in PSR B0943+10 by Deshpande & Rankin (1999, 2001). We suggest a physical mechanism for the generation of pulsar radio emission in the ensemble of finite sub-beams, based on specific instabilities. We propose an explanation for the subpulse drift phenomenon observed in some long-period pulsars.