An unstable superfluid Stewartson layer in a differentially rotating neutron star

TL;DR

This paper reviews evidence for Stewartson layers in superfluid neutron star cores, presents first numerical simulations of their instability, and relates these findings to pulsar glitch observations.

Contribution

It introduces the first numerical simulations of superfluid Stewartson layers and analyzes their stability in the context of neutron star dynamics.

Findings

Superfluid Stewartson layers are disrupted by nonaxisymmetric instabilities.

Glitching pulsars are on the stable side of the instability boundary.

Differential rotation can build up before pulsar glitches.

Abstract

Experimental and numerical evidence is reviewed for the existence of a Stewartson layer in spherical Couette flow at small Ekman and Rossby numbers ($\Ek \lsim 10^{-3}$, $\Ro \lsim 10^{-2}$), the relevant hydrodynamic regime in the superfluid outer core of a neutron star. Numerical simulations of a superfluid Stewartson layer are presented for the first time, showing how the layer is disrupted by nonaxisymmetric instabilities. The unstable ranges of $\Ek$ and $\Ro$ are compared with estimates of these quantities in radio pulsars that exhibit glitches. It is found that glitching pulsars lie on the stable side of the instability boundary, allowing differential rotation to build up before a glitch.