Horizon Instability of Extremal Charged Black Holes: Enhanced Growth for Charged Scalar Perturbations

We investigate the stability of highly charged Reissner-Nordström black holes to charged scalar perturbations. We show that the near-horizon region exhibits a transient instability which becomes the Aretakis instability in the extremal limit. The growth rates obtained match the enhanced rates of nonaxisymmetric perturbations to the near-extremal and extremal Kerr solutions. The agreement is shown to arise from a shared near-horizon symmetry of the two scenarios.