Comment on 'Particle Simulation of Ion Heating in the Ring Current' by S. Qian, M.K. Hudson, and I. Roth

In a recent paper, Qian et al. [1990] investigated an ion cyclotron wave (ICW) instability and heating of thermal ions in a plasma with a loss cone distribution of hot ions. This study is an extension of a simulation study [Omura et al., 1985] of helium heating in an ICW instability driven by anisotropic hot protons with a bi-Maxwellian distribution. The dependence of the ion heating on the thermal H+/He+ density ratio is studied by simulation runs with better spatial resolution. In the simulations of the ICW instability, a small percentage of the thermal helium ions are accelerated beyond the theoretical maximum velocity. This is because the excited waves are not exactly monochromatic as assumed in the theory, and a stochastic acceleration takes place in the presence of waves with different wave numbers and frequencies. To confirm that the parallel electric field [Qian et al., 1990, equation (10)] is not operative in the process of parallel heating, the author has performed a computer experiment where the parallel electric field is artificially eliminated from the equation of motion in the hybrid code. He assumed the same parameters of the simulation in the work by Omura et al. [1988]. The results shows that the dynamics of the thermal helium ions in the acceleration phase is the same and that the time history of the thermal energy of helium ions is almost identical with the results of Omura et al. [1988]. The computer experiment evidently demonstrates that the parallel electric field is not responsible for the parallel heating. As a summary, the interaction of the forward and backward traveling waves plays an important and essential role in the process of parallel heating found in the ICW instability driven by a temperature anisotropy