Short‐wavelength modes on expanding plasma clouds

Publisher Website
Google Scholar
Abstract
It is proposed that the short‐wavelength (less than an ion gyroradius) flute modes observed on the surface of expanding plasma clouds, such as the Active Magnetospheric Particle Tracer Explorers (AMPTE) barium releases in the magnetotail, can be explained in terms of the lower hybrid drift instability, which in its general form includes an electron‐ion drift induced by the deceleration of the plasma across a magnetic field. Similar types of structure are also seen in laser and theta pinch experiments and in recent simulations modeling these phenomena. Analytical and numerical solutions of the linear dispersion relation are given for a number of parameters. The linear theory reproduces the growth rate scaling of Hassam and Huba in the long‐wavelength limit and indicates that maximum growth occurs at shorter wavelengths. The growth of the modes is illustrated by particle simulations showing that the instability is excited at early times, remains locked in at a fixed mode number, and because it continues to be driven by the deceleration, grows to amplitudes far larger than expected from usual ion trapping arguments. Issues related to the linear mode structure and the nonlinear evolution of the instability in the context of AMPTE are also discussed.