# The geometry of a radio pulsar beam

## TL;DR

This paper proposes a new geometric model for radio pulsar beams, combining cone and fan beam features into a flaring spiral that explains observed profile diversity and bifurcations.

## Contribution

It introduces a natural spiral geometry model that unifies existing pulsar beam concepts and accounts for observed profile features and bifurcations.

## Key findings

- The spiral geometry explains the size of bifurcated features.
- The model aligns with the properties of curvature radiation.
- It provides a unified framework for pulsar beam profiles.

## Abstract

Taxonomy of radio pulsar profiles is mostly based on a system of Ptolemaic artificiality, consisting of separated rings and a core, arbitrarily located at disparate altitudes in the magnetosphere. Diversity of observed profile shapes clearly exceeds the interpretive capability of such conal model. Moreover, bifurcated features observed in pulsar profiles imply a system of fan beams radially extending away from the dipole axis. The bifurcations can be understood as the imprint of the elementary radiation pattern of the long-sought radio emission mechanism, thus identifying the latter. Their size, however, is several times larger than implied by the curvature of magnetic dipole lines. Here I show that the illusion of disconnected rings and the size of bifurcated features can be explained through a natural geometry which combines the properties of both the cone and the fan beam. It is a flaring spiral which makes several revolutions around the dipole axis on its way to leave the magnetosphere. Such geometry is consistent with a stream of outflowing and laterally drifting plasma. The bifurcated components are so wide, because the curvature on such a spiral is larger than that of the dipolar magnetic field, hence they are consistent with the extraordinary mode curvature radiation.