On the thermodynmamics of diamagnetic plasma expansions

The expansion of a hot plasma into a weak uniform magnetic field in vacuum is perhaps the simplest possible plasma-physical system, yet its thermodynamic behavior is complex. Observational evidence for non-adiabatic expansion has recently come from a study of hot diamagnetic cavities detected near the earth's bow shock. These are localized regions of plasma characterized by high temperature, low density, and low magnetic field strength. Although the formation mechanism has not been established, it appears that some process may be necessary to maintain or increase the plasma temperature as the density decreases. Particle-in-cell simulations of diamagnetic plasma expansions are used to illustrate the interplay of energy among the ions, electrons, background magnetic field, and generated electrostatic and electromagnetic waves. Initially, the field is everywhere uniform and straight, and ions moving parallel to the field do no work and escape unhindered. Ions moving perpendicular to the field do work against the field and lose energy to it. Thus the perpendicular ion energy decreases faster than the parallel energy, and a large temperature anisotropy favoring Tparallel develops. The electrons, which are initially at the same temperature as the ions, follow along with the expanding ions without decreasing much in temperature