Blandford-Znajek process as Alfv\'enic superradiance

TL;DR

This paper demonstrates that the Blandford-Znajek process can be understood as a long wavelength limit of Alfvénic superradiance in a forcefree magnetosphere, revealing a deep connection between black hole energy extraction and superradiant scattering.

Contribution

It establishes a theoretical link between the Blandford-Znajek process and Alfvénic superradiance using a simplified black string spacetime model.

Findings

Superradiant scattering condition matches BZ process criteria.

Wave amplification can be significantly large due to resonance.

BZ process is the long wavelength limit of Alfvénic superradiance.

Abstract

The superradiant scattering of Alfv\'en waves (Alfv\'enic superradiance) in a forcefree magnetosphere is discussed to reveal the relationship between the Blandford-Znajek (BZ) process and superradiance. For simplicity, we consider a four-dimensional rotating black string spacetime of which each $z=\text{const}$ slice is a Ba\~nados-Teitelboim-Zanelli solution as an analogy of the equatorial plane of the Kerr spacetime. Then, it is confirmed that the condition for Alfv\'enic superradiance coincides with that for the BZ process, and the wave amplification can be very large due to a resonant scattering for some parameter sets of the wave frequency and the angular velocity of the magnetic field line. Moreover, by analysis of the Poynting flux, we first show that the BZ process can be interpreted as the long wavelength limit of Alfv\'enic superradiance.