Universal Instability in Complex Field Geometries

An inhomogeneous collisionless plasma in a magnetic field B is unstable to an electrostatic oscillation propagating nearly perpendicular to B but with a large phase velocity parallel to B. We study the stability of this mode in a more complicated plasma equilibrium, including magnetic shear, finite plasma length, currents parallel to B, temperature gradients, and gravity-simulated magnetic cusp or mirror curvature. We find that many of these equilibria are stable to this mode, which has been misnamed a ``universal instability.''