Relativistic stabilisation of the diocotron instability in a pulsar "cylindrical" electrosphere

Abstract

In a previous work, we showed that the differentially rotating equatorial disk in the pulsar's electrosphere is diocotron unstable in the non-relativistic regime. In this paper, we extend these results and study the relativistic and electromagnetic stabilisation effects by including the magnetic field perturbation and allow for relativistic speeds of the guiding centre, in a self-consistent manner. We use the electric drift approximation, valid for low-density plasmas. We linearise the coupled relativistic cold-fluid and Maxwell equations in the electric drift approximation. The non-linear eigenvalue problem for the perturbed azimuthal electric field is solved numerically with standard technics for boundary value problems like the shooting method. The spectrum of the relativistic diocotron instability in a non-neutral plasma column confined between two cylindrically conducting walls is computed. For low-speed motions, we recover the eigenfunctions and eigenspectra of the non-relativistic diocotron instability. Our algorithm is also checked in the relativistic planar diode geometry for which an analytical expression of the dispersion relation is known. As expected, when the relativistic and electromagnetic effects become significant, the diocotron instability tends to stabilise. In cylindrical geometry, for some special rotation profile, all azimuthal modes $l$ are completely suppressed for sufficiently relativistic flows. However, for the profile relevant to the electrosphere, depending on the exact rotation curves, the growth rates can either significantly decrease till they vanish or persist for moderate $l$.