Radio pulsar drifting sub-pulses and diocotron instability

¹ Astronomical Institute, Utrecht University, PO Box 80000, 3508 TA Utrecht, The Netherlands e-mail: fung@astro.uu.nl
² Department of Astrophysics, IMAPP, Radboud University Nijmegen, PO Box 9010, 6500 GL Nijmegen, The Netherlands
³ Centre for Plasma Astrophysics, K. U. Leuven, Celestijnenlaan 200B, 3001 Leuven, Belgium; on leave for Institute of Astronomy, University of Zielena Góra, Lubuska 2, 65-265, Zielena Góra, Poland / Astrophysical Observatory, Al. Kazbegi ave. 2a, Tbilisi 0160, Georgia

Received: 10 March 2005
Accepted: 12 July 2005

Abstract

The potential role of a diocotron instability in causing drifting sub-pulses in radio pulsar emission is investigated for aligned magnetic rotators. It is assumed that the out-flowing plasma above a pulsar polar cap consists of an initially axially symmetric, hollow beam of relativistic electron positron pair plasma which carries an electric charge as well as a current. The occurrence of instability depends on shear in the angular velocity distribution of the beam as a function of axial distance. Instability occurs under typical pulsar conditions at mode numbers 40. It destroys the symmetry of the equilibrium configuration and leads to a carousel of density columns which rotates at fixed angular pattern speed. The process is applied to two pulsars with observed carousels of drifting sub-pulses, and the diocotron instability at corresponding mode number and axial distance is used as a diagnostic for the charge and current density of the polar flow.