The diocotron instability in a pulsar cylindrical electrosphere

Max-Planck-Institut für Kernphysik, Saupfercheckweg 1, 69117 Heidelberg, Germany e-mail: jerome.Petri@mpi-hd.mpg.de

Received: 25 August 2006
Accepted: 1 November 2006

Abstract

Context.The physics of the pulsar inner magnetosphere remains poorly constrained by observations. Although about 2000 pulsars have been discovered to date, little is known about their emission mechanism. Large vacuum gaps probably exist and a non-neutral plasma made of electrons in some regions and of positrons in some other regions fills space to form an electrosphere.

Aims.The purpose of this work is to study the stability properties of the differentially rotating equatorial disk in the pulsar's electrosphere for which the magnetic field is assumed to be dipolar. In contrast to previous studies, the magnetic field is not restricted to being uniform.

Methods.A pseudo-spectral Galerkin method using Tchebyshev-polynomial expansion is developed to compute the spectrum of the diocotron instability in a non-neutral plasma column confined between two cylindrically conducting walls. Moreover, the inner wall carries a given charge per unit length in order to account for the presence of a charged neutron star at the centre of the electrosphere.

Results.We show several eigenfunctions and eigenspectra obtained for different initial density profiles and electromagnetic field configurations useful for laboratory plasmas. The algorithm is very efficient in computing the fastest growing modes. Applications to a “cylindrical” electrosphere are also shown for several differential rotation profiles. It is found that the growth rates of the diocotron instability have the same order of magnitude as the rotation rate.

Conclusions.The instability develops on a very short timescale and can account for very efficient particle diffusion across the magnetic field lines, as already claimed in a previous work. The exact geometry of the confined plasma, whether a thin disk or a cylinder, does not significantly affect the spectrum of the diocotron instability.