Stability in Force-Free Electrodynamics

We show that flows of pure electromagnetic energy are subject to instabilities familiar from hydrodynamics and plasma physics—Kelvin-Helmholtz and screw instabilities. In the framework of force-free electrodynamics, it is found that electric-like tangential discontinuities are Kelvin-Helmholtz-unstable. All non-trivial cylindrically symmetrical magnetic configurations are screw-unstable. The Poynting jet of the Goldreich-Julian pulsar is screw-unstable if the current density in the polar cap region exceeds the charge density times the speed of light. This may be the process that determines the pulsar luminosity.