Normal Modes for Electrostatic Ion Waves in an Inhomogeneous Plasma

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Abstract
The propagation of low‐frequency electrostatic oscillations perpendicular to a density gradient in a low‐density, fully ionized plasma in a strong magnetic field B is studied from the point of view of the linearized macroscopic equations. Two sets of waves are found in the limits where the electrons move freely along B or not at all along B. Among these are waves which travel at the electron and ion diamagnetic drift velocities, and a physical description of this phenomenon is given in terms of microscopic motions. The transition between these two sets of modes occurs at angles of propagation very close to 90° to B; to treat this region of angles one must take into account either the electron inertia or the plasma resistivity. Dispersion curves are given for this transition region. In addition, it is found that in a small range of angles one of the cyclotron waves and the wave traveling near the electron drift velocity are unstable and can be excited by the pressure gradient, even if no longitudinal currents exist in equilibrium.

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