Ion acceleration by wave-particle interaction

Ions in the auroral zone are observed to have pitch angle distributions peaked at large angles to the magnetic field. The most likely energization mechanism for such ions is an interaction between the ions and waves at the ion gyroresonance. This paper will review the theory of perpendicular ion heating by waves, considering electrostatic ion cyclotron waves, as well as waves on the whistler resonance cone. When the wave amplitudes are weak and the spectrum broadbanded, the quasi-linear theory can predict the magnitude of heating, if the wave spectrum and distribution in space are known. For large amplitude, narrow-banded waves, stochastic heating mechanisms can rapidly accelerate ions transverse to the magnetic field. These theoretical models can be compared with results found in plasma simulations as well as with observations in space. In particular, the relative heating of different ion species will be considered and compared with observations in the auroral zone. 55 references