The Alfvén ion-cyclotron instability. Simulation theory and techniques

The numerical properties of a particle-ion, fluid-electron computer simulation code, used in the study of the parallel-propagating electromagnetic Alfven ion-cyclotron (AIC) instability, are examined. A numerical odd-even mode is suppressed by means of a two-timestep averaging method. Excellent energy conservation is obtained by using a method similar to the Boris particle mover to advance the transverse fields. Linear growth rates obtained' from the code differ substantially from those predicted by uniform Vlasov theory, here derived using a muitifluid model. Short wavelengths in particular show substantial growth rates when damping is predicted, and additionally show strong linear mode coupling. Positive growth rates are even observed in the case of a Maxwellian ion distribution. Disagreement is also generally found among short-wavelength mode frequencies. These inconsistencies are resolved by taking into consideration general grid and discrete-particle effects of the simulation model. A theoretical study reveals a real, physical process by which an ion distribution may collisionlessly relax via short-wavelength AIC instabilities acting resonantly on small portions of the distribution function. This process is combined with a linear mode coupling theory and other characteristics of the AIC instability to explain all observed differences. Nonlinear short-wavelength saturation levels are also obtained and their relevance to other field-aligned, electromagnetic simulations is discussed.