Fast Wave Polarization, Charge Horizons and the Time Evolution of Force-Free Magnetospheres

Abstract

Numerical simulations of force-free, degenerate (ffde)pulsar and black hole magnetospheres are often based on 1-D characteristics. In particular, the plasma wave polarizations that can be propagated along the 1-D characteristics determine the time evolution of the entire system. There are two sets of characteristics,corresponding to the fast and Alfven modes. The fast wave is generally considered to be a transverse light wave, however recently it has been claimed that light-like fast waves can transport a longitudinal electric polarization, $E_{\parallel }$, at the speed of light. The implication is quite profound if true, namely that the wrong information has been propagated along the fast characteristics in all previous simulations of force-free magnetospheres. It is shown in this Letter that the light-like fast waves must be transverse and previous simulations are valid. This result is demonstrated by means of a fundamental physical principle (associated with the fact that particles cannot flow faster than the speed of light), there exists a charge horizon in ffde magnetospheres. It is shown that the Alfven critical surfaces in a ffde magnetosphere are both charge and particle horizons, i.e. one way membranes that do not permit traversal by charges nor particles anti-directed to the bulk flow. Since the propagation of a discontinuous change in $E_{\parallel }$ requires a physical surface charge on the wave-face, it is also a one-way membrane for longitudinally polarized waves. Besides justifying previous ffde simulations this result also invalidates previous claims that fast waves can radiate $E_{\parallel }$ from the event horizon of a black hole.

Astrophysical Journal Letters in press

References in corpus (7)

• Electrodynamics of black hole magnetospheres
• On the Properties of Time-Dependent, Force-Free, Degenerate Electrodynamics
• Force-Free Magnetosphere of an Accretion Disk -- Black Hole System. I. Schwarzschild Geometry
• Some Properties of an Axisymmetric Pulsar Magnetosphere Constructed by Numerical Calculation
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• On time evolution and causality of force-free black hole magnetospheres
• The origins of Causality Violations in Force Free Simulations of Black Hole Magnetospheres

Cited by in corpus (2)

• Force-Free Magnetosphere of an Accretion Disk -- Black Hole System. II. Kerr Geometry
• A New Solution to the Plasma Starved Event Horizon Magnetosphere: Application to the Forked Jet in M87