A kinetic ballooning/interchange instability in the magnetotail

Three‐dimensional electromagnetic particle‐in‐cell simulations are used to investigate the stability properties of a plasma sheet equilibrium with a minimum in the magnetic normal ( B z ) component. Such a configuration is found to be unstable to a ballooning/interchange type mode that is localized tailward of the B z minimum. The mode has a relatively short dawn‐dusk wavelength of the order of the ion Larmor radius in the B z field (∼2000 km) and a phase velocity in the direction of the ion diamagnetic drift with a magnitude about one‐fifth of the drift speed. The real frequency is about 60% of the midplane ion cyclotron frequency. The dominant mode polarization is δ ϕ and δB ∥ . A linear kinetic analysis including bounce and drift resonance interactions for the electrons and an orbit average over the flux tube volume for the Boltzmann term in the ion density perturbation produces agreement with the simulation mode properties and permits identification of the mode as the low‐frequency extension of the lower hybrid drift instability in straight magnetic geometry. In its nonlinear evolution, the mode develops Rayleigh‐Taylor fingers that extend across the B z minimum and into the near‐Earth dipole region. These fingers transport magnetic flux earthward, and the flux is redistributed by electron Hall currents that flow around the fingers. This mode is likely to contribute to the nearly continuous presence of turbulence in the center of the plasma sheet.