Spark Model for Pulsar Radiation Modulation Patterns

Abstract

A non-stationary polar gap model first proposed by Ruderman & Sutherland(1975) is modified and applied to spark-associated pulsar emission at radiowave-lengths. It is argued that under physical and geometrical conditionsprevailing above pulsar polar cap, highly non-stationary spark discharges donot occur at random positions. Instead, sparks should tend to operate in welldetermined preferred regions. At any instant the polar cap is populated asdensely as possible with a number of two-dimensional sparks with acharacteristic dimension as well as a typical distance between adjacent sparksbeing about the polar gap height. Our model differs, however, markedly from itsoriginal 'hollow cone' version. The key feature is the quasi-central sparkdriven by pair production process and anchored to the local pole of asunspot-like surface magnetic field. This fixed spark prevents the motion ofother sparks towards the pole, restricting it to slow circumferential driftacross the planes of field lines converging at the local pole. We argue thatthe polar spark constitutes the core pulsar emission, and that the annularrings of drifting sparks contribute to conal components of the pulsar beam. Wefound that the number of nested cones in the beam of typical pulsar should notexcced three; a number also found by Mitra & Deshpande (1999) using acompletely different analysis.