Electrostatic ion-cyclotron waves in a currentless, anisotropic plasma with inhomogeneous flow

Abstract

The linearized dispersion relation describing waves in a plasma having a uniform magnetic field, uniform density, inhomogeneous parallel (to the magnetic field) flow, and thermal anisotropy (Ti⊥/Ti∥) is used to determine the threshold condition for growth of an electrostatic ion cyclotron wave and its harmonics. The inclusion of moderate ion thermal anisotropy (Ti⊥/Ti∥∼3) and parallel-flow shear (dVd/dx∼0.05Ωci), values typical of many space and laboratory plasmas, reduces the critical current necessary for instability growth to nearly zero. That an electrostatic instability conventionally associated with strong field aligned currents can be excited in the absence of any field aligned current and only by moderate parallel-flow shear suggests that ion cyclotron instabilities may play a larger role in many plasma environments than previously believed. The decrease in the instability threshold for increasing thermal anisotropy suggests that ion heating due to ion cyclotron waves may result in a positive feedback process absent in homogeneous plasmas.