Resonant Plasma Heating by Large Amplitude Sub-Cyclotron Waves

Resonant heating of charged particles by electrostatic and Alfven waves propagating in a confining uniform magnetic field is examined. It is shown that, with a sufficiently large wave amplitude, significant perpendicular stochastic heating can be obtained with wave frequency at a fraction of the cyclotron frequency. The simplest model problem is that of a particle gyrating in a constant magnetic field acted upon by an electrostatic plane wave propagating perpendicularly. Phase space Poincare plots show that resonances exist at fractional frequencies associated with the unstable domains of driven Mathieu equation. As wave amplitude increases, nonlinear generation of many fixed points produce chaos which permit stochastic heating of the plasma. Similarly, for a sufficiently large-amplitude, obliquely propagating Alfven wave, there exists efficient stochastic ion pitch angle scattering and heating. Test particle simulations with broadband Alfven turbulence show that heating results from stochastic diffusion in velocity space. These results may have relevance for the heating of ions in the solar corona, and is a generic phenomenon, independent of the type of wave considered.